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<!DOCTYPE html>
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<title>Live-Listener API Migration Guide (v3 refactoring) — MantidProject main documentation</title>
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<section id="live-listener-api-migration-guide-v3-refactoring">
<span id="livelistenermigration"></span><h1>Live-Listener API Migration Guide (v3 refactoring)<a class="headerlink" href="#live-listener-api-migration-guide-v3-refactoring" title="Link to this heading">#</a></h1>
<p>Mantid’s live-listener interface was refactored to separate state-read
from state-transition, eliminate hidden side effects in <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code>,
and allow <a class="reference external" href="https://docs.mantidproject.org/nightly/algorithms/LoadLiveData-v1.html#algm-loadlivedata" title="(in MantidProject v6.16)"><span class="xref std std-ref">LoadLiveData</span></a> to run successfully as
a stand-alone algorithm without requiring
<a class="reference external" href="https://docs.mantidproject.org/nightly/algorithms/MonitorLiveData-v1.html#algm-monitorlivedata" title="(in MantidProject v6.16)"><span class="xref std std-ref">MonitorLiveData</span></a>.</p>
<p>This document covers the generic interface changes that apply to every
<code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code> subclass. For details specific to
<code class="docutils literal notranslate"><span class="pre">SNSLiveEventDataListener</span></code> (ADARA packet handling, the deferred
<code class="docutils literal notranslate"><span class="pre">RunStatusPkt</span></code> flow, the state-machine diagrams, and the pre-existing
<code class="docutils literal notranslate"><span class="pre">m_ignorePackets</span></code> defect) see <a class="reference internal" href="SNSLiveEventDataListenerRefactoring.html#snsliveeventdatalistenerrefactoring"><span class="std std-ref">SNSLiveEventDataListener refactoring</span></a>.</p>
<nav class="contents local" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#motivation" id="id1">Motivation</a></p></li>
<li><p><a class="reference internal" href="#new-api-use-these" id="id2">New API (use these)</a></p></li>
<li><p><a class="reference internal" href="#removed-api" id="id3">Removed API</a></p>
<ul>
<li><p><a class="reference internal" href="#runstatus" id="id4"><code class="docutils literal notranslate"><span class="pre">runStatus()</span></code></a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#extractdata-as-a-template-method" id="id5"><code class="docutils literal notranslate"><span class="pre">extractData()</span></code> as a template method</a></p></li>
<li><p><a class="reference internal" href="#state-transition-hooks" id="id6">State-transition hooks</a></p></li>
<li><p><a class="reference internal" href="#migration-recipe-for-listener-authors" id="id7">Migration recipe for listener authors</a></p>
<ul>
<li><p><a class="reference internal" href="#step-1-remove-your-runstatus-override" id="id8">Step 1 — Remove your <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> override</a></p></li>
<li><p><a class="reference internal" href="#step-2-add-a-listenerstate-const-override" id="id9">Step 2 — Add a <code class="docutils literal notranslate"><span class="pre">listenerState()</span> <span class="pre">const</span> <span class="pre">override</span></code></a></p></li>
<li><p><a class="reference internal" href="#step-3-override-runstate-const-recommended" id="id10">Step 3 — Override <code class="docutils literal notranslate"><span class="pre">runState()</span> <span class="pre">const</span></code> (recommended)</a></p></li>
<li><p><a class="reference internal" href="#step-4-optionally-override-ispaused-and-lasttransition" id="id11">Step 4 — Optionally override <code class="docutils literal notranslate"><span class="pre">isPaused()</span></code> and <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code></a></p></li>
<li><p><a class="reference internal" href="#step-5-override-doextractdata-not-extractdata" id="id12">Step 5 — Override <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code>, not <code class="docutils literal notranslate"><span class="pre">extractData()</span></code></a></p></li>
<li><p><a class="reference internal" href="#step-6-move-any-fsm-tick-code-out-of-runstatus" id="id13">Step 6 — Move any FSM-tick code out of <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code></a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#worked-examples" id="id14">Worked examples</a></p>
<ul>
<li><p><a class="reference internal" href="#pattern-a-constant-run-state" id="id15">Pattern A — constant run state</a></p></li>
<li><p><a class="reference internal" href="#pattern-b-edge-detected-run-state" id="id16">Pattern B — edge-detected run state</a></p></li>
<li><p><a class="reference internal" href="#pattern-c-fsm-tick-anti-pattern-fix" id="id17">Pattern C — FSM-tick anti-pattern fix</a></p></li>
<li><p><a class="reference internal" href="#pattern-d-test-mocks" id="id18">Pattern D — test mocks</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#behaviour-preservation-guarantees" id="id19">Behaviour preservation guarantees</a></p>
<ul>
<li><p><a class="reference internal" href="#critical-detail-c1" id="id20">Critical detail (C1)</a></p></li>
<li><p><a class="reference internal" href="#two-subtleties-to-flag-in-code-review" id="id21">Two subtleties to flag in code review</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#pitfalls-and-faq" id="id22">Pitfalls and FAQ</a></p>
<ul>
<li><p><a class="reference internal" href="#why-is-extractdata-final" id="id23">Why is <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> <code class="docutils literal notranslate"><span class="pre">final</span></code>?</a></p></li>
<li><p><a class="reference internal" href="#what-if-my-listener-never-had-a-runstatus-override" id="id24">What if my listener never had a <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> override?</a></p></li>
<li><p><a class="reference internal" href="#why-are-onbeginrun-onendrun-protected-rather-than-public" id="id25">Why are <code class="docutils literal notranslate"><span class="pre">onBeginRun</span></code> / <code class="docutils literal notranslate"><span class="pre">onEndRun</span></code> protected rather than public?</a></p></li>
<li><p><a class="reference internal" href="#how-do-i-drive-onrunpause-in-a-unit-test" id="id26">How do I drive <code class="docutils literal notranslate"><span class="pre">onRunPause</span></code> in a unit test?</a></p></li>
<li><p><a class="reference internal" href="#when-should-i-override-onafterextract-rather-than-onbeforeextract" id="id27">When should I override <code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> rather than <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code>?</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#further-reading" id="id28">Further reading</a></p></li>
</ul>
</nav>
<section id="motivation">
<h2><a class="toc-backref" href="#id1" role="doc-backlink">Motivation</a><a class="headerlink" href="#motivation" title="Link to this heading">#</a></h2>
<p>The pre-refactor interface had three coupled defects:</p>
<ol class="arabic simple">
<li><p><strong>Conflated state.</strong> <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> returned a value that was part
DAS state (<code class="docutils literal notranslate"><span class="pre">NoRun</span> <span class="pre">/</span> <span class="pre">BeginRun</span> <span class="pre">/</span> <span class="pre">Running</span> <span class="pre">/</span> <span class="pre">EndRun</span></code>) and part
listener-internal FSM — it mutated to <code class="docutils literal notranslate"><span class="pre">Running</span></code> or <code class="docutils literal notranslate"><span class="pre">NoRun</span></code> as a
side effect of being called. Callers could not ask either question
without paying the cost of the other.</p></li>
<li><p><strong>Hidden side effects on a “getter”.</strong> Calling <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code>
cleared the geometry cache, the name map, marked the workspace
uninitialised, ran <code class="docutils literal notranslate"><span class="pre">initWorkspacePart1()</span></code>, consumed any deferred
<code class="docutils literal notranslate"><span class="pre">RunStatusPkt</span></code>, and dropped the back-pressure flag
<code class="docutils literal notranslate"><span class="pre">m_pauseNetRead</span></code>. None of this was visible at the call site.</p></li>
<li><p><strong>External-control deadlock.</strong> Stand-alone <code class="docutils literal notranslate"><span class="pre">LoadLiveData</span></code> does not
call <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code>. After a <code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> / <code class="docutils literal notranslate"><span class="pre">EndRun</span></code> packet set
<code class="docutils literal notranslate"><span class="pre">m_pauseNetRead</span> <span class="pre">=</span> <span class="pre">true</span></code>, the background reader stopped, the
workspace could not be re-initialised, and the next
<code class="docutils literal notranslate"><span class="pre">extractData()</span></code> either hit the 10-second <code class="docutils literal notranslate"><span class="pre">NotYet</span></code> timeout or
spun indefinitely on a stale workspace.</p></li>
</ol>
<p>A fourth issue was an undocumented ordering contract: the legacy
<code class="docutils literal notranslate"><span class="pre">MonitorLiveData</span></code> polling loop relied on calling <code class="docutils literal notranslate"><span class="pre">extractData()</span></code>
first and <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> second within the same iteration; nothing in
the type system enforced this.</p>
<p>The v3 refactor has three objectives, in priority order:</p>
<ol class="arabic simple">
<li><p>Expose the listener state and the ADARA run state without conflating
them.</p></li>
<li><p>Separate state-read from state-transition; make every state mutation
explicit and named.</p></li>
<li><p>Move as much of the state machine as possible into <code class="docutils literal notranslate"><span class="pre">extractData()</span></code>,
the only method every consumer of <code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code> must call.</p></li>
</ol>
<p>The result is four pure-getter queries on <code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code> and a
template-method <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> on <code class="docutils literal notranslate"><span class="pre">LiveListener</span></code> that dispatches to
named protected hooks. The old <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> getter was kept for a
transitional period as a <code class="docutils literal notranslate"><span class="pre">[[deprecated]]</span></code> shim and has since been
removed; see <a class="reference internal" href="#removed-api">Removed API</a> below.</p>
</section>
<section id="new-api-use-these">
<h2><a class="toc-backref" href="#id2" role="doc-backlink">New API (use these)</a><a class="headerlink" href="#new-api-use-these" title="Link to this heading">#</a></h2>
<p>All four methods are <strong>pure getters</strong> — calling them never mutates
internal state. They are declared on <code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code>.</p>
<div class="pst-scrollable-table-container"><table class="table">
<colgroup>
<col style="width: 30.0%" />
<col style="width: 70.0%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Method</p></th>
<th class="head"><p>Meaning</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">runState()</span> <span class="pre">const</span></code></p></td>
<td><p>Current DAS run state: <code class="docutils literal notranslate"><span class="pre">NoRun</span></code>, <code class="docutils literal notranslate"><span class="pre">BeginRun</span></code>, <code class="docutils literal notranslate"><span class="pre">Running</span></code>, or
<code class="docutils literal notranslate"><span class="pre">EndRun</span></code>. Updated by the background thread; readable from the
foreground at any time. Default implementation returns
<code class="docutils literal notranslate"><span class="pre">NoRun</span></code> for listeners with no concept of run boundaries.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">isPaused()</span> <span class="pre">const</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">true</span></code> when the DAS has signalled a run pause (ADARA
annotation packet). Orthogonal to <code class="docutils literal notranslate"><span class="pre">runState()</span></code> — the run
state remains <code class="docutils literal notranslate"><span class="pre">Running</span></code> while paused. Default returns
<code class="docutils literal notranslate"><span class="pre">false</span></code>.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">listenerState()</span> <span class="pre">const</span></code></p></td>
<td><p>Connection / health: <code class="docutils literal notranslate"><span class="pre">Disconnected</span></code>, <code class="docutils literal notranslate"><span class="pre">ReadWait</span></code>,
<code class="docutils literal notranslate"><span class="pre">Connected</span></code>, or <code class="docutils literal notranslate"><span class="pre">Error</span></code>. <strong>Pure virtual</strong> — every concrete
listener must override it.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">lastTransition()</span> <span class="pre">const</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">std::optional<RunStatus></span></code> — the edge (<code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> or
<code class="docutils literal notranslate"><span class="pre">EndRun</span></code>) committed by the most recent <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> call,
or <code class="docutils literal notranslate"><span class="pre">std::nullopt</span></code> if none. Cleared automatically on the
<em>next</em> successful <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> cycle that commits a <em>new</em>
edge, so that <code class="docutils literal notranslate"><span class="pre">MonitorLiveData</span></code> does not re-process the same
edge. Default returns <code class="docutils literal notranslate"><span class="pre">std::nullopt</span></code>.</p></td>
</tr>
</tbody>
</table>
</div>
<p>The base-class declarations are:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">namespace</span><span class="w"> </span><span class="nn">Mantid</span><span class="o">::</span><span class="nn">API</span><span class="w"> </span><span class="p">{</span>
<span class="k">enum</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">ListenerState</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">Disconnected</span><span class="p">,</span><span class="w"> </span><span class="c1">///< Not connected</span>
<span class="w"> </span><span class="n">Connected</span><span class="p">,</span><span class="w"> </span><span class="c1">///< Connected and reading</span>
<span class="w"> </span><span class="n">ReadWait</span><span class="p">,</span><span class="w"> </span><span class="c1">///< Connected but paused at a run boundary</span>
<span class="w"> </span><span class="n">Error</span><span class="w"> </span><span class="c1">///< Background thread reported an exception</span>
<span class="p">};</span>
<span class="k">class</span><span class="w"> </span><span class="nc">MANTID_API_DLL</span><span class="w"> </span><span class="n">ILiveListener</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">Kernel</span><span class="o">::</span><span class="n">PropertyManager</span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">RunStatus</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">NoRun</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">JoiningRun</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">BeginRun</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="n">Running</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="n">EndRun</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">4</span><span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="n">RunStatus</span><span class="w"> </span><span class="nf">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">NoRun</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="nf">isPaused</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="nb">false</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">optional</span><span class="o"><</span><span class="n">RunStatus</span><span class="o">></span><span class="w"> </span><span class="n">lastTransition</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">nullopt</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Workspace</span><span class="o">></span><span class="w"> </span><span class="n">extractData</span><span class="p">()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// ... other existing methods unchanged ...</span>
<span class="p">};</span>
<span class="p">}</span><span class="w"> </span><span class="c1">// namespace Mantid::API</span>
</pre></div>
</div>
<p>Two questions are orthogonal and answered by independent getters:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">runState()</span></code> answers “what is the DAS doing?”</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">isPaused()</span></code> answers “is the current run paused?”</p></li>
</ul>
<p>A caller inspecting <code class="docutils literal notranslate"><span class="pre">runState()</span></code> sees <code class="docutils literal notranslate"><span class="pre">Running</span></code> whether the run is
paused or not; a caller inspecting <code class="docutils literal notranslate"><span class="pre">isPaused()</span></code> sees the pause flag
regardless of run phase. Neither query reads or mutates the other’s
backing field.</p>
</section>
<section id="removed-api">
<h2><a class="toc-backref" href="#id3" role="doc-backlink">Removed API</a><a class="headerlink" href="#removed-api" title="Link to this heading">#</a></h2>
<section id="runstatus">
<h3><a class="toc-backref" href="#id4" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">runStatus()</span></code></a><a class="headerlink" href="#runstatus" title="Link to this heading">#</a></h3>
<p>The old <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> method carried large hidden side effects:</p>
<ul class="simple">
<li><p>Cleared geometry and name-map caches.</p></li>
<li><p>Re-initialised the workspace.</p></li>
<li><p>Consumed the deferred <code class="docutils literal notranslate"><span class="pre">RunStatusPkt</span></code> (SNS only).</p></li>
<li><p>Cleared the <code class="docutils literal notranslate"><span class="pre">m_pauseNetRead</span></code> gate (causing deadlock in stand-alone
<code class="docutils literal notranslate"><span class="pre">LoadLiveData</span></code>).</p></li>
</ul>
<p>It has been <strong>removed</strong> in favour of the combination of
<code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> and <code class="docutils literal notranslate"><span class="pre">runState()</span></code>. For a transitional period it
was kept as a <code class="docutils literal notranslate"><span class="pre">[[deprecated]]</span></code> base-class shim equivalent to:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">RunStatus</span><span class="w"> </span><span class="nf">ILiveListener::runStatus</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">edge</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">lastTransition</span><span class="p">())</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="o">*</span><span class="n">edge</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">runState</span><span class="p">();</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Any remaining call site should be updated to call the two replacement
getters directly:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">const</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">status</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">listener</span><span class="o">-></span><span class="n">lastTransition</span><span class="p">().</span><span class="n">value_or</span><span class="p">(</span><span class="n">listener</span><span class="o">-></span><span class="n">runState</span><span class="p">());</span>
</pre></div>
</div>
<p>Call <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> first to commit any pending transition, then
read <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> / <code class="docutils literal notranslate"><span class="pre">runState()</span></code> — this reproduces the
historical <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> contract exactly (see below).</p>
<section id="how-the-former-shim-preserved-monitorlivedata-semantics">
<h4>How the former shim preserved <code class="docutils literal notranslate"><span class="pre">MonitorLiveData</span></code> semantics<a class="headerlink" href="#how-the-former-shim-preserved-monitorlivedata-semantics" title="Link to this heading">#</a></h4>
<p><code class="docutils literal notranslate"><span class="pre">MonitorLiveData</span></code> calls <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> first (via
<code class="docutils literal notranslate"><span class="pre">loadAlg->executeAsChildAlg()</span></code>) and then reads the run status second
within the same loop iteration. By that point, <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> has
already committed any pending transition and <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> is
populated. Reading <code class="docutils literal notranslate"><span class="pre">lastTransition().value_or(runState())</span></code> returns
the edge once across the boundary, then falls through to
<code class="docutils literal notranslate"><span class="pre">runState()</span></code> on subsequent calls (<code class="docutils literal notranslate"><span class="pre">Running</span></code> or <code class="docutils literal notranslate"><span class="pre">NoRun</span></code>) —
matching the historical return-once-then-settle behaviour of the old
<code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> exactly. See
<code class="docutils literal notranslate"><span class="pre">Framework/LiveData/src/MonitorLiveData.cpp</span></code> for the call site.</p>
</section>
</section>
</section>
<section id="extractdata-as-a-template-method">
<h2><a class="toc-backref" href="#id5" role="doc-backlink"><code class="docutils literal notranslate"><span class="pre">extractData()</span></code> as a template method</a><a class="headerlink" href="#extractdata-as-a-template-method" title="Link to this heading">#</a></h2>
<p>The base class <code class="docutils literal notranslate"><span class="pre">API::LiveListener</span></code> (from which every in-tree concrete
listener derives) <strong>finalises</strong> <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> and dispatches to three
protected hooks:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// In Framework/API/inc/MantidAPI/LiveListener.h</span>
<span class="k">class</span><span class="w"> </span><span class="nc">MANTID_API_DLL</span><span class="w"> </span><span class="n">LiveListener</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">ILiveListener</span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Workspace</span><span class="o">></span><span class="w"> </span><span class="n">extractData</span><span class="p">()</span><span class="w"> </span><span class="k">final</span><span class="p">;</span><span class="w"> </span><span class="c1">// template method</span>
<span class="k">protected</span><span class="o">:</span>
<span class="w"> </span><span class="c1">/// Foreground-thread hook called at the very start of extractData(),</span>
<span class="w"> </span><span class="c1">/// before any workspace construction. Default no-op.</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">onBeforeExtract</span><span class="p">()</span><span class="w"> </span><span class="p">{}</span>
<span class="w"> </span><span class="c1">/// The actual workspace construction step. Pure virtual.</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Workspace</span><span class="o">></span><span class="w"> </span><span class="n">doExtractData</span><span class="p">()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="c1">/// Success-only hook called after doExtractData() returns normally,</span>
<span class="w"> </span><span class="c1">/// before the workspace is returned to the caller. Default no-op.</span>
<span class="w"> </span><span class="c1">/// Not called if doExtractData() throws.</span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">onAfterExtract</span><span class="p">()</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
</pre></div>
</div>
<p>The template-method body is:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Workspace</span><span class="o">></span><span class="w"> </span><span class="n">LiveListener</span><span class="o">::</span><span class="n">extractData</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">onBeforeExtract</span><span class="p">();</span><span class="w"> </span><span class="c1">// Phase 1: pre-extract bookkeeping</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">ws</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">doExtractData</span><span class="p">();</span><span class="w"> </span><span class="c1">// Phase 2: build the workspace</span>
<span class="w"> </span><span class="n">onAfterExtract</span><span class="p">();</span><span class="w"> </span><span class="c1">// Phase 3: success-only post-extract work</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">ws</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This three-phase split has the following consequences for listener authors:</p>
<ol class="arabic simple">
<li><p><strong>Override ``doExtractData()``, not ``extractData()``.</strong>
<code class="docutils literal notranslate"><span class="pre">extractData()</span></code> is <code class="docutils literal notranslate"><span class="pre">final</span></code>; the compiler will reject any
override. Move your existing <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> body into
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> unchanged.</p></li>
<li><p><strong>Use ``onBeforeExtract()`` for pre-extract work; use
``onAfterExtract()`` for success-only post-extract work.</strong>
<code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> runs unconditionally at the start of every
<code class="docutils literal notranslate"><span class="pre">extractData()</span></code> call — this is where the SNS listener commits
queued <code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> transitions (see
<a class="reference internal" href="SNSLiveEventDataListenerRefactoring.html#snsliveeventdatalistenerrefactoring"><span class="std std-ref">SNSLiveEventDataListener refactoring</span></a>).
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> runs only when <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> returned
normally — this is where the SNS listener dispatches <code class="docutils literal notranslate"><span class="pre">EndRun</span></code>
and clears the delivered transition edge.</p></li>
<li><p><strong>Exception safety across the three-phase split.</strong> If
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> throws (e.g. <code class="docutils literal notranslate"><span class="pre">Exception::NotYet</span></code>),
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> is <strong>not</strong> called. The side effects of
<code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> — including any <code class="docutils literal notranslate"><span class="pre">m_lastTransition</span></code>
assignment — are preserved across the retry. Edge-clearing must
not run on a retry path; the SNS listener achieves this with a
<code class="docutils literal notranslate"><span class="pre">m_previousExtractCompleted</span></code> flag that gates the deferred clear
in the next <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> — the flag is only set at the
end of <code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code>, so a <code class="docutils literal notranslate"><span class="pre">NotYet</span></code> from any phase leaves
it <code class="docutils literal notranslate"><span class="pre">false</span></code> and the edge survives intact. See the
<a class="reference internal" href="#migration-c1-invariant"><span class="std std-ref">C1 invariant</span></a> for details.</p></li>
</ol>
</section>
<section id="state-transition-hooks">
<h2><a class="toc-backref" href="#id6" role="doc-backlink">State-transition hooks</a><a class="headerlink" href="#state-transition-hooks" title="Link to this heading">#</a></h2>
<p>Listeners that need to react to run boundaries should override the
<em>hook</em> methods rather than the getters:</p>
<div class="pst-scrollable-table-container"><table class="table">
<colgroup>
<col style="width: 22.0%" />
<col style="width: 18.0%" />
<col style="width: 60.0%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Hook</p></th>
<th class="head"><p>Thread</p></th>
<th class="head"><p>When called</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">onBeginRun()</span></code></p></td>
<td><p>foreground</p></td>
<td><p>Called from <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> when a <code class="docutils literal notranslate"><span class="pre">BeginRun</span></code>
transition is pending. The new run details are available at
this point.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">onEndRun()</span></code></p></td>
<td><p>foreground</p></td>
<td><p>Called from <code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> when an <code class="docutils literal notranslate"><span class="pre">EndRun</span></code>
transition is pending. Deferred to after <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code>
so the finishing run’s accumulated events are harvested before
the workspace buffer is reset.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">onRunPause(bool</span> <span class="pre">paused)</span></code></p></td>
<td><p>background</p></td>
<td><p>Called directly from the background reader when a DAS
pause/resume annotation is received. <code class="docutils literal notranslate"><span class="pre">paused</span> <span class="pre">=</span> <span class="pre">true</span></code> for
PAUSE, <code class="docutils literal notranslate"><span class="pre">false</span></code> for RESUME.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code></p></td>
<td><p>foreground</p></td>
<td><p>Called at the very start of <code class="docutils literal notranslate"><span class="pre">extractData()</span></code>, before
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code>. Override here for any per-extraction
bookkeeping that does not fit the run-boundary hook model.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code></p></td>
<td><p>foreground</p></td>
<td><p>Called at the end of <code class="docutils literal notranslate"><span class="pre">extractData()</span></code>, but <strong>only</strong> when
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> returned normally (i.e., did not throw).
Override here for success-only bookkeeping — state that
must not take effect when extraction is interrupted by
<code class="docutils literal notranslate"><span class="pre">Exception::NotYet</span></code>.</p></td>
</tr>
</tbody>
</table>
</div>
<p>The asymmetric dispatch is intentional:</p>
<ul class="simple">
<li><p><strong>Run-state transitions are deferred</strong> to the next <code class="docutils literal notranslate"><span class="pre">extractData()</span></code>
call because they have workspace-level side effects (cache clears,
re-init) that must run on the foreground thread.</p></li>
<li><p><strong>Pause/resume is applied immediately</strong> because it has no
workspace-level side effect — it only gates event appending in
<code class="docutils literal notranslate"><span class="pre">rxPacket(BankedEventPkt)</span></code>. Deferring pause/resume to
<code class="docutils literal notranslate"><span class="pre">extractData()</span></code> would retroactively mis-categorise events that
arrived between the PAUSE annotation and the next extraction.</p></li>
<li><p><strong>BeginRun and EndRun are committed at different phases.</strong>
<code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> is dispatched from <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> so the new
run’s workspace initialisation is in place before
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> snapshots it. <code class="docutils literal notranslate"><span class="pre">EndRun</span></code> is dispatched from
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> so <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> can first harvest the
finishing run’s accumulated events before <code class="docutils literal notranslate"><span class="pre">onEndRun()</span></code> resets the
buffer.</p></li>
</ul>
<p>All hooks have no-op default implementations in <code class="docutils literal notranslate"><span class="pre">LiveListener</span></code> and
may be selectively overridden.</p>
</section>
<section id="migration-recipe-for-listener-authors">
<h2><a class="toc-backref" href="#id7" role="doc-backlink">Migration recipe for listener authors</a><a class="headerlink" href="#migration-recipe-for-listener-authors" title="Link to this heading">#</a></h2>
<p>If you maintain a concrete <code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code> subclass, migrate as
follows.</p>
<section id="step-1-remove-your-runstatus-override">
<h3><a class="toc-backref" href="#id8" role="doc-backlink">Step 1 — Remove your <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> override</a><a class="headerlink" href="#step-1-remove-your-runstatus-override" title="Link to this heading">#</a></h3>
<p><code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> has been removed from <code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code>. Drop the
override entirely; call sites should read <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> /
<code class="docutils literal notranslate"><span class="pre">runState()</span></code> directly instead.</p>
</section>
<section id="step-2-add-a-listenerstate-const-override">
<h3><a class="toc-backref" href="#id9" role="doc-backlink">Step 2 — Add a <code class="docutils literal notranslate"><span class="pre">listenerState()</span> <span class="pre">const</span> <span class="pre">override</span></code></a><a class="headerlink" href="#step-2-add-a-listenerstate-const-override" title="Link to this heading">#</a></h3>
<p><code class="docutils literal notranslate"><span class="pre">listenerState()</span></code> is <strong>pure virtual</strong>. The minimal override maps
your existing connection flag to the enum:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">API</span><span class="o">::</span><span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_isConnected</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">API</span><span class="o">::</span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">API</span><span class="o">::</span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Disconnected</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>A listener that has back-pressure semantics (e.g. SNS’s
<code class="docutils literal notranslate"><span class="pre">m_pauseNetRead</span></code>) or that can report an error from the background
thread should expand the override to return <code class="docutils literal notranslate"><span class="pre">ReadWait</span></code> / <code class="docutils literal notranslate"><span class="pre">Error</span></code>
as appropriate.</p>
</section>
<section id="step-3-override-runstate-const-recommended">
<h3><a class="toc-backref" href="#id10" role="doc-backlink">Step 3 — Override <code class="docutils literal notranslate"><span class="pre">runState()</span> <span class="pre">const</span></code> (recommended)</a><a class="headerlink" href="#step-3-override-runstate-const-recommended" title="Link to this heading">#</a></h3>
<p>The base default of <code class="docutils literal notranslate"><span class="pre">NoRun</span></code> is rarely what you want. Provide an
explicit override even if it is a one-liner returning <code class="docutils literal notranslate"><span class="pre">Running</span></code>:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">RunStatus</span><span class="w"> </span><span class="nf">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">API</span><span class="o">::</span><span class="n">ILiveListener</span><span class="o">::</span><span class="n">Running</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
</pre></div>
</div>
</section>
<section id="step-4-optionally-override-ispaused-and-lasttransition">
<h3><a class="toc-backref" href="#id11" role="doc-backlink">Step 4 — Optionally override <code class="docutils literal notranslate"><span class="pre">isPaused()</span></code> and <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code></a><a class="headerlink" href="#step-4-optionally-override-ispaused-and-lasttransition" title="Link to this heading">#</a></h3>
<p>The base defaults (<code class="docutils literal notranslate"><span class="pre">false</span></code> / <code class="docutils literal notranslate"><span class="pre">nullopt</span></code>) are correct for listeners
without pause semantics or run-boundary edge detection.</p>
</section>
<section id="step-5-override-doextractdata-not-extractdata">
<h3><a class="toc-backref" href="#id12" role="doc-backlink">Step 5 — Override <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code>, not <code class="docutils literal notranslate"><span class="pre">extractData()</span></code></a><a class="headerlink" href="#step-5-override-doextractdata-not-extractdata" title="Link to this heading">#</a></h3>
<p>If your previous override was</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">Workspace</span><span class="o">></span><span class="w"> </span><span class="n">MyListener</span><span class="o">::</span><span class="n">extractData</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="p">}</span>
</pre></div>
</div>
<p>mechanically rename it to <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code>. <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> is
<code class="docutils literal notranslate"><span class="pre">final</span></code> on <code class="docutils literal notranslate"><span class="pre">API::LiveListener</span></code> and the compiler will reject the
old override.</p>
</section>
<section id="step-6-move-any-fsm-tick-code-out-of-runstatus">
<h3><a class="toc-backref" href="#id13" role="doc-backlink">Step 6 — Move any FSM-tick code out of <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code></a><a class="headerlink" href="#step-6-move-any-fsm-tick-code-out-of-runstatus" title="Link to this heading">#</a></h3>
<p>If your old <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> contained mutating logic (incrementing
counters, advancing timers, performing I/O), move it to a private
helper and call that helper from <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code>. If the
bookkeeping is success-only — state that must not take effect when
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> throws <code class="docutils literal notranslate"><span class="pre">Exception::NotYet</span></code> — use
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> instead. See the worked example for
<code class="docutils literal notranslate"><span class="pre">FakeEventDataListener</span></code> below.</p>
</section>
</section>
<section id="worked-examples">
<h2><a class="toc-backref" href="#id14" role="doc-backlink">Worked examples</a><a class="headerlink" href="#worked-examples" title="Link to this heading">#</a></h2>
<p>The following worked examples cover every concrete
<code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code> subclass in the tree. <code class="docutils literal notranslate"><span class="pre">SNSLiveEventDataListener</span></code>
is treated separately in <a class="reference internal" href="SNSLiveEventDataListenerRefactoring.html#snsliveeventdatalistenerrefactoring"><span class="std std-ref">SNSLiveEventDataListener refactoring</span></a>.</p>
<section id="pattern-a-constant-run-state">
<h3><a class="toc-backref" href="#id15" role="doc-backlink">Pattern A — constant run state</a><a class="headerlink" href="#pattern-a-constant-run-state" title="Link to this heading">#</a></h3>
<p>Used by <code class="docutils literal notranslate"><span class="pre">FileEventDataListener</span></code>, <code class="docutils literal notranslate"><span class="pre">ISISLiveEventDataListener</span></code>,
<code class="docutils literal notranslate"><span class="pre">ISISHistoDataListener</span></code>, <code class="docutils literal notranslate"><span class="pre">TestGroupDataListener</span></code>,
<code class="docutils literal notranslate"><span class="pre">TestDataListener</span></code>, and the unit-test <code class="docutils literal notranslate"><span class="pre">MockLiveListener</span></code>. These
listeners have no notion of run boundaries; their old <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code>
returned a constant.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// Header</span>
<span class="n">RunStatus</span><span class="w"> </span><span class="nf">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">Running</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_isConnected</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Disconnected</span><span class="p">;</span>
<span class="p">}</span>
<span class="c1">// Drop the existing runStatus() override; the base-class default is</span>
<span class="c1">// equivalent and reports the new edge-detection contract correctly</span>
<span class="c1">// (always nullopt for these listeners).</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">FileEventDataListener</span></code> additionally maps its chunk index to the
connection state:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_chunkNumber</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Disconnected</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</section>
<section id="pattern-b-edge-detected-run-state">
<h3><a class="toc-backref" href="#id16" role="doc-backlink">Pattern B — edge-detected run state</a><a class="headerlink" href="#pattern-b-edge-detected-run-state" title="Link to this heading">#</a></h3>
<p>Used by <code class="docutils literal notranslate"><span class="pre">KafkaEventListener</span></code> and <code class="docutils literal notranslate"><span class="pre">KafkaHistoListener</span></code>. These
listeners query an underlying decoder to determine whether an
end-of-run boundary has been observed, but they do not maintain a
<code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> / <code class="docutils literal notranslate"><span class="pre">EndRun</span></code> edge-detection contract — only “are we
inside a run right now?”.</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">RunStatus</span><span class="w"> </span><span class="nf">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_decoder</span><span class="o">-></span><span class="n">hasReachedEndOfRun</span><span class="p">()</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">EndRun</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">Running</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_decoder</span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="n">m_decoder</span><span class="o">-></span><span class="n">isCapturing</span><span class="p">()</span>
<span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Disconnected</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> is not overridden (returns <code class="docutils literal notranslate"><span class="pre">nullopt</span></code>); Kafka
does not have the <code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> / <code class="docutils literal notranslate"><span class="pre">EndRun</span></code> edge-detection contract
that ADARA does, so <code class="docutils literal notranslate"><span class="pre">lastTransition().value_or(runState())</span></code> falls
through to <code class="docutils literal notranslate"><span class="pre">runState()</span></code> and returns the same value the legacy
<code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> override would have.</p>
</section>
<section id="pattern-c-fsm-tick-anti-pattern-fix">
<h3><a class="toc-backref" href="#id17" role="doc-backlink">Pattern C — FSM-tick anti-pattern fix</a><a class="headerlink" href="#pattern-c-fsm-tick-anti-pattern-fix" title="Link to this heading">#</a></h3>
<p>Two listeners besides <code class="docutils literal notranslate"><span class="pre">SNSLiveEventDataListener</span></code> had real side
effects inside <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> — exactly the anti-pattern this
refactor exists to eliminate.</p>
<p><strong>``FakeEventDataListener``</strong> (pre-refactor):</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="n">ILiveListener</span><span class="o">::</span><span class="n">RunStatus</span><span class="w"> </span><span class="nf">FakeEventDataListener::runStatus</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">m_endRunEvery</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">&&</span>
<span class="w"> </span><span class="n">DateAndTime</span><span class="o">::</span><span class="n">getCurrentTime</span><span class="p">()</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="n">m_nextEndRunTime</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">m_nextEndRunTime</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">DateAndTime</span><span class="o">::</span><span class="n">getCurrentTime</span><span class="p">()</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">m_endRunEvery</span><span class="p">;</span>
<span class="w"> </span><span class="n">m_runNumber</span><span class="o">++</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">EndRun</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">Running</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This getter incremented <code class="docutils literal notranslate"><span class="pre">m_runNumber</span></code> and advanced
<code class="docutils literal notranslate"><span class="pre">m_nextEndRunTime</span></code> as a side effect of being polled.</p>
<p><strong>Post-refactor.</strong> All end-of-run side effects move to
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code>; <code class="docutils literal notranslate"><span class="pre">runState()</span></code> becomes a pure getter.
<code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> is not overridden (the base no-op suffices):</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// Header</span>
<span class="n">RunStatus</span><span class="w"> </span><span class="nf">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_runState</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_isConnected</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Disconnected</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">std</span><span class="o">::</span><span class="n">optional</span><span class="o"><</span><span class="n">RunStatus</span><span class="o">></span><span class="w"> </span><span class="n">lastTransition</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_lastTransition</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="k">protected</span><span class="o">:</span>
<span class="kt">void</span><span class="w"> </span><span class="n">onAfterExtract</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="p">;</span>
<span class="k">private</span><span class="o">:</span>
<span class="n">RunStatus</span><span class="w"> </span><span class="n">m_runState</span><span class="p">{</span><span class="n">Running</span><span class="p">};</span>
<span class="n">std</span><span class="o">::</span><span class="n">optional</span><span class="o"><</span><span class="n">RunStatus</span><span class="o">></span><span class="w"> </span><span class="n">m_lastTransition</span><span class="p">;</span>
<span class="c1">// Implementation</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">FakeEventDataListener::onAfterExtract</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// This listener only ever commits an EndRun edge (never BeginRun), so</span>
<span class="w"> </span><span class="c1">// the simple pattern — unconditional reset then conditional re-set in</span>
<span class="w"> </span><span class="c1">// the same onAfterExtract() — is correct here. Listeners that also</span>
<span class="w"> </span><span class="c1">// commit a BeginRun edge in onBeforeExtract() must use the gated-clear</span>
<span class="w"> </span><span class="c1">// pattern (m_previousExtractCompleted) described in the C1 invariant</span>
<span class="w"> </span><span class="c1">// section below, so that the BeginRun edge survives through the return</span>
<span class="w"> </span><span class="c1">// of the same extractData() call and is observable by the caller.</span>
<span class="w"> </span><span class="n">m_lastTransition</span><span class="p">.</span><span class="n">reset</span><span class="p">();</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">m_endRunEvery</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">&&</span>
<span class="w"> </span><span class="n">DateAndTime</span><span class="o">::</span><span class="n">getCurrentTime</span><span class="p">()</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="n">m_nextEndRunTime</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">m_nextEndRunTime</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">DateAndTime</span><span class="o">::</span><span class="n">getCurrentTime</span><span class="p">()</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">m_endRunEvery</span><span class="p">;</span>
<span class="w"> </span><span class="n">m_runNumber</span><span class="o">++</span><span class="p">;</span>
<span class="w"> </span><span class="n">m_runState</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">EndRun</span><span class="p">;</span>
<span class="w"> </span><span class="n">m_lastTransition</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">EndRun</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">m_runState</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Running</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The externally observable cadence of <code class="docutils literal notranslate"><span class="pre">EndRun</span></code> and the
<code class="docutils literal notranslate"><span class="pre">m_runNumber</span></code> increment is preserved; only the trigger moves from
<code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> polling to <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> invocation. Placing all
side effects in <code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> rather than <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code>
is required by the <a class="reference internal" href="#migration-c1-invariant"><span class="std std-ref">C1 invariant</span></a>:
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> runs only when <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> returns
normally, so a <code class="docutils literal notranslate"><span class="pre">NotYet</span></code> thrown from <code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> cannot erase
or advance state that the retry will need. Note that this pattern is
sufficient when the listener only commits edges in <code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code>
(as <code class="docutils literal notranslate"><span class="pre">FakeEventDataListener</span></code> does); see the C1 section for the
additional <code class="docutils literal notranslate"><span class="pre">m_previousExtractCompleted</span></code> flag needed when edges are
also committed in <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code>.</p>
<p><strong>``SINQHMListener``</strong> (pre-refactor): <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> performed an
HTTP request, parsed the response, wrote the <code class="docutils literal notranslate"><span class="pre">hmhost</span></code> member, set
<code class="docutils literal notranslate"><span class="pre">dimDirty</span></code> when transitioning <code class="docutils literal notranslate"><span class="pre">NoRun</span> <span class="pre">→</span> <span class="pre">Running</span></code>, and was <em>also</em>
invoked from inside <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> specifically to perform these
side effects.</p>
<p><strong>Post-refactor.</strong> Extract the HTTP poll into a private
<code class="docutils literal notranslate"><span class="pre">pollStatus()</span></code> helper; have <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> call it.
<code class="docutils literal notranslate"><span class="pre">runState()</span></code> returns the cached state:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="c1">// Header</span>
<span class="n">RunStatus</span><span class="w"> </span><span class="nf">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">m_cachedRunState</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="n">ListenerState</span><span class="w"> </span><span class="nf">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">connected</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span>
<span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Disconnected</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">protected</span><span class="o">:</span>
<span class="kt">void</span><span class="w"> </span><span class="n">onBeforeExtract</span><span class="p">()</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">pollStatus</span><span class="p">();</span><span class="w"> </span><span class="p">}</span>
<span class="k">private</span><span class="o">:</span>
<span class="kt">void</span><span class="w"> </span><span class="n">pollStatus</span><span class="p">();</span><span class="w"> </span><span class="c1">// contains the HTTP request and field updates</span>
<span class="n">RunStatus</span><span class="w"> </span><span class="n">m_cachedRunState</span><span class="p">{</span><span class="n">NoRun</span><span class="p">};</span>
<span class="c1">// pollStatus() body is the old runStatus() body, but writes</span>
<span class="c1">// m_cachedRunState instead of returning, and continues to write</span>
<span class="c1">// hmhost and dimDirty as before.</span>
</pre></div>
</div>
<p>Because <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> already triggered <code class="docutils literal notranslate"><span class="pre">pollStatus()</span></code> implicitly
in the old code (it called the side-effecting <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code>), callers
that only invoke <code class="docutils literal notranslate"><span class="pre">extractData()</span></code> see no behavioural change.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Unlike the <code class="docutils literal notranslate"><span class="pre">FakeEventDataListener</span></code> example above,
<code class="docutils literal notranslate"><span class="pre">SINQHMListener</span></code> writes its cached run-state and the <code class="docutils literal notranslate"><span class="pre">dimDirty</span></code>
flag inside <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code> (via <code class="docutils literal notranslate"><span class="pre">pollStatus()</span></code>) rather than
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code>. This is correct <strong>only</strong> because (a)
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> does not throw <code class="docutils literal notranslate"><span class="pre">Exception::NotYet</span></code> and (b) the
listener does not override <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code>.
<code class="docutils literal notranslate"><span class="pre">doExtractData()</span></code> <em>reads</em> <code class="docutils literal notranslate"><span class="pre">dimDirty</span></code> and <code class="docutils literal notranslate"><span class="pre">hmhost</span></code> to build the
workspace, so the poll must precede the build — that is the
load-bearing reason the call lives in <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code>. If a
future change to this listener adds a <code class="docutils literal notranslate"><span class="pre">NotYet</span></code> path <em>or</em> a
<code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> override, the <code class="docutils literal notranslate"><span class="pre">NoRun</span> <span class="pre">→</span> <span class="pre">Running</span></code> side effects
(<code class="docutils literal notranslate"><span class="pre">dimDirty</span></code> set, <code class="docutils literal notranslate"><span class="pre">oldStatus</span></code> advance) must move to
<code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code> to satisfy the
<a class="reference internal" href="#migration-c1-invariant"><span class="std std-ref">C1 invariant</span></a>.</p>
</div>
</section>
<section id="pattern-d-test-mocks">
<h3><a class="toc-backref" href="#id18" role="doc-backlink">Pattern D — test mocks</a><a class="headerlink" href="#pattern-d-test-mocks" title="Link to this heading">#</a></h3>
<p><code class="docutils literal notranslate"><span class="pre">MockLiveListener</span></code> and similar fixtures need only the two-line
addition:</p>
<div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">MockLiveListener</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">ILiveListener</span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="n">RunStatus</span><span class="w"> </span><span class="n">runState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">Running</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">ListenerState</span><span class="w"> </span><span class="n">listenerState</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">ListenerState</span><span class="o">::</span><span class="n">Connected</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// ... other mocked methods ...</span>
<span class="p">};</span>
</pre></div>
</div>
<p>Any algorithm-level mocks (in <code class="docutils literal notranslate"><span class="pre">LoadLiveDataTest</span></code>,
<code class="docutils literal notranslate"><span class="pre">MonitorLiveDataTest</span></code>, <code class="docutils literal notranslate"><span class="pre">StartLiveDataTest</span></code>) that derive from
<code class="docutils literal notranslate"><span class="pre">ILiveListener</span></code> directly need the same two-line addition.</p>
</section>
</section>
<section id="behaviour-preservation-guarantees">
<h2><a class="toc-backref" href="#id19" role="doc-backlink">Behaviour preservation guarantees</a><a class="headerlink" href="#behaviour-preservation-guarantees" title="Link to this heading">#</a></h2>
<p>The refactor preserves every observable algorithm-level behaviour
except one: stand-alone <code class="docutils literal notranslate"><span class="pre">LoadLiveData</span></code> previously deadlocked after a
run boundary and now succeeds. That is the bug the refactor exists to
fix.</p>
<div class="pst-scrollable-table-container"><table class="table">
<colgroup>
<col style="width: 55.0%" />
<col style="width: 22.0%" />
<col style="width: 23.0%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Behaviour</p></th>
<th class="head"><p>Pre-refactor</p></th>
<th class="head"><p>Post-refactor</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">runStatus()</span></code> returns <code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> / <code class="docutils literal notranslate"><span class="pre">EndRun</span></code> exactly once
at each boundary</p></td>
<td><p>yes, via mutation</p></td>
<td><p>yes, via <code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> populated by <code class="docutils literal notranslate"><span class="pre">extractData()</span></code></p></td>
</tr>
<tr class="row-odd"><td><p>Workspace re-initialised at run boundaries</p></td>
<td><p>inside <code class="docutils literal notranslate"><span class="pre">runStatus()</span></code></p></td>
<td><p><code class="docutils literal notranslate"><span class="pre">onBeginRun()</span></code> called from <code class="docutils literal notranslate"><span class="pre">onBeforeExtract()</span></code>;
<code class="docutils literal notranslate"><span class="pre">onEndRun()</span></code> called from <code class="docutils literal notranslate"><span class="pre">onAfterExtract()</span></code></p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">MonitorLiveData</span></code> workspace renaming triggers on
<code class="docutils literal notranslate"><span class="pre">BeginRun</span></code> / <code class="docutils literal notranslate"><span class="pre">EndRun</span></code></p></td>
<td><p>yes</p></td>
<td><p>yes (via <code class="docutils literal notranslate"><span class="pre">lastTransition().value_or(runState())</span></code>)</p></td>
</tr>
<tr class="row-odd"><td><p>Listener can be queried for its state without mutating it</p></td>
<td><p><strong>no</strong></p></td>
<td><p>yes (<code class="docutils literal notranslate"><span class="pre">runState()</span></code>, <code class="docutils literal notranslate"><span class="pre">isPaused()</span></code>, <code class="docutils literal notranslate"><span class="pre">listenerState()</span></code>,
<code class="docutils literal notranslate"><span class="pre">lastTransition()</span></code> all <code class="docutils literal notranslate"><span class="pre">const</span></code>)</p></td>