Repository navigation
[Not a bug] When to use ICommand vs IRequest? Docs say differently from the code examples? #1875
Description
Hello all!
This library is excellent!!! Very clean, easy to use and powerful!
I am confused on when to use ICommand vs IRequest
The code and documentation says IRequest is a base class of IEvent and ICommand. That is great. I assume when you want to be more explicit and ensure your Command is only processed by command handlers and not Event Handlers you would use ICommand, and when you want to be more loose about it, you could use IRequest?
Here is the only part in the documentation where it speaks about this: https://paramore.readthedocs.io/en/latest/DispatchingARequest.html#using-the-base-class-when-dispatching-a-message
From the source code:
I am confused, because if I understand this correctly, the docs are telling us to use ICommand or IEvent, rather than IRequest.
However, in all of the code examples in the documentation, IRequest is used. From looking at the code, it appears IRequest is what you would want to use, however the documentation makes me think that I should not use IReqest
Can someone please clarify what is best practice for this? Thanks!
Activity
preardon commentedon Nov 30, 2021
Hi @jeffward01
IEvent and ICommand are the correct ones to use, when you are using an external bus it needs to map to one of these two types to dispatch the messages. I would recommend being explicit and selecting one or the other, I assume at the minute you are only dispatching internally (CommandProcessor.Send) as such if I had to guess most of what you are doing should be ICommand (I personally just use Command)
For further clarification the way I define the difference between a Command and an Event is, a Command is a request to do something (i.e. Create new Order) and an Event is a notification to say something has been done (i.e. NewOrderVersionCreated)
I hope this helps, I am going to tag @iancooper in as he is the expert here and can point out anyvhere that I may have missed something out / add more detail
iancooper commentedon Dec 1, 2021
Command - expects only one handler - an instruction for work
Event - zero or more handlers - an observable fact
Request - base class, a message that I want to handle, could be a command or event
But @preardon is right
jeffward01 commentedon Dec 8, 2021
You both read my mind! sorry for the slow reply. @preardon @iancooper
Exactly, just testing internally, I am thinking for my architecture I plan to use Brigher / Darker for in-process and CAP for out-process.
Question 1
The main reason for this is that CAP has an NATS integration. I know this is the Brigher repo, but am I making a mistake by not leveraging Brighter for out-process items? Do you have any example handlers I can examine so that I can easily implement NATS for Brighter?
Question 2
This is very helpful! So in theroy, is this 'good practice:
API Endpoint POST --> Handler (ICommand) --> The handler then calls a second .Publish method which goes to --> Handler (IEvent) ....I think what I am trying to ask in question 2 is "Is it bad practice to have "Handlers" call other "Handlers", or should this be handled with listening events such as the Mediatr INotification event?
Question 3
I feel improper opening up an issue to discuss these FAQ questions, is there a better place I can go in the future for these questions?
Final Notes
This is very helpful! For example I was thinking that the
IEventclass functioned a little bit likeINotificationfrom Mediatr or Azure Event Grid, which is basically "Hello everyone! We have a new order received event, go take your places and take care of it" and signals the rest of the application to 'do work'.But in Brighter, its a confirmation event, which is much more like DDD. Thanks for clarifying this!! I appreciate it very much!!!
Thank you again for the help!! you guys are great
preardon commentedon Dec 8, 2021
1: one of the best examples to follow for implementing a Messaging Transport for brighter is the RMQ one, there are only a few interfaces that need to be implemented and should be fairly straightforward
2: Firstly for out of proc messaging please use Deposit / DepositPost these use the outbox pattern, also if you want your outbox to participate in the same transaction as your database you will have to use DepositPost. Send and Publish are for in proc works.
The question I would ask here is why is a handler calling another handler, I have many command handler that create events or even other commands i.e.
API post (new Order)-> ICommand (Create new order) -> creates Event (NevOrderVersionEvent>
Or even
API post -> ICommand (start Batch job) -> chunks batch job and create a command for each
The only thing I would say about the batch job route is that we don't yet support bulk but I'm hoping this will be a v9.next feature
3: There is the discussions tab on GitHub but the main thing is that this is somewhere where others can see and benefit from it
iancooper commentedon Dec 8, 2021
We have an issue, #1208 and I was planning on tackling in the 9.x timescale. If you feel up to it, you could have a stab. But if you are happy to help test it, I could probably pick up that issue over the weekend and create an alpha version of NATS for you. You might find it easier to PR fixes for that than write from scratch. It was on my ToDo list anyway
iancooper commentedon Dec 8, 2021
Yes, that it how it tends to be done.
iancooper commentedon Dec 8, 2021
?3: There is the discussions tab on GitHub but the main thing is that this is somewhere where others can see and benefit from it
Indeed, we care about folks being able to find answers, and are happy to answer questions. We can move this issue into discussions, but let's work through the conversation first
jeffward01 commentedon Dec 8, 2021
Thanks guys for being so helpful with all of this, its been super helpful and lightening fast reply times are something that I can aspire to haha.
My replies:
Question 1.)
Big thanks on sharing the #1208 issue link! I found the code here for RMQ Messaging Transport. I'd love to take a stab at it and let you guys know how it goes, despite my questions i'm a pretty competent developer, all my code is on Azure DevOps, which makes my github is rather sparse. I am looking forward to learning more about brighter and the inner workings!
What attracted me to brighter was how clean it was, and how similar it was to Mediatr, but with multiple very important 'power-up' features like out-process messaging queues. When I look at CAP, it appears CAP only does out-of-process messaging while Brighter does in and out of process messaging. (not sure if this is accurate for CAP, but pretty sure it is)
Question 2.)
Thanks, this makes alot of sense, basically just add a 'listener' handler which waits for the
IEventwhich will be called by the first handler. Perfect!^^ Thank you for mentioning these and pointing me in the right direction!
Question 3.)
Big thanks for the clarification, personally I think its best on the Issues area, rather than discussions. I tend to check Issues more often rather than the discussion area, but I know that the discussion area is built for this. Wherever you decide to put it works for me. I will start to check the discussions area.
Last question on the topic, (i'll open a new topic if I have further questions because my original question was 100% answered)
I plan to take a stab at #1208 - I am rather new to Brighter codebase, and I just wanted to ask you guys this question to see if my understanding is correct. I will also try and answer my own question and you can see if im on-target or not.
Question 4 (final)
If CAP already implements NATS, what is the advantage to using Brighter? Why would a developer select the Brigher / Darker package over CAPS / MediatR combo.
Jeff's Answer:
Brighter provides both the familiar style of MediatR while also including out-of-process messaging patterns (including outbox) for out-of-proc communication for things such as Microservices. A developer will choose brighter / darker when looking for a messaging library which can do it all (in / out) without additional bells and whistles required. Instead of being dependent on CAP to do the logging and errors logging (I think this is bad practice as it fragments away from the logging / error infrastructure already in place), with Brighter it makes the developers use their own logging infrastructure
The only real advantages (feature wise) CAP would have over Brighter / Darker at this current point in time (roadmap not included) would be:
Thanks guys for your patience with my on this! I'll give #1208 a go! it'll take me a few days to spin-up on it, I have a few dependencies I have to take care of first on some other projects I am working on. I need NATS integration for my final implementation so I certainly will work on it asap <3
iancooper commentedon Dec 8, 2021
iancooper commentedon Dec 12, 2021
Going to move this to a discusion