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Symfony Messenger: 6 months already and more to come

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SymfonyCon 2018 Presentation by Samuel Roze The Messenger component brings asynchronous processing to Symfony: using message bus(es) you are able to decouple your application and route some (or all) of these "messages" to transports such as the built-in AMQP transport. It’s been more than 6 months since we’ve merged the new Messenger component in Symfony. We will explore the different use cases it has been used for so far and how it will continue to involve in order to facilitate even more.

Hello, good morning. So, roughly six months ago we released the Symfony 4.1. And that was the first time that we actually introduced the Symfony Messenger Component. So today what I want to talk about is obviously this component. I want to quickly introduce it and that's going to be the first part of the talk. And then sort of like explain what's, what has changed in the, during the transition from the 4.1 and the version 4.2, that was actually released a few days ago I think.

Hey Samuel!

Before that, I'll introduce myself very quickly. I'm Samuel Rozé, I'm part of the Symfony core team, the ApiPlatform as well and I've created a bunch of open source things like ContinuousPipe and Tolerance. I don't have any time to present them but you should check these out, it's quite interesting.

Very quickly, the startup I'm working with, we are on a mission in trying to help the old people, the elderly to stay healthier and longer at home instead of going to retirement houses. It's, to me a very interesting project. You should check the website, it's birdie.care. Obviously as most of the technology companies we are actually hiring. So if you're interested, talk to me.

Symfony Messenger? The 5 Concepts

So Symfony Messenger. So what it is? There are five main concepts within the actual, within Symfony Messenger. The first one is messages. You could have guessed it. So it's your PHP object. It's a PHP class that you will create that you will dispatch somewhere and you will dispatch this message to a message bus. Then once the message has been dispatched to the bus, the bus's responsibility is to actually call some things that are going to contain the business logic, your business logic, and these things are called message handlers. It's a bunch of classes, no specific requirement, it's, um, what you want to execute when you receive, when the code receive the message basically. You can have zero to N handlers for a given message.

Just these three points are really super valuable. And if you're into, um, sort of design patterns or sort of like interesting things like CQRS if you've heard about it, the idea is that we can actually structure an entire application around the bus. And we can actually structure an entire application around a bus or multiple buses. But the point is that we can actually do really, really, um, decouple who is dispatching a message from who is actually handling the message. I won't go that much into the details. The important takeaway here is that just these 3 concepts, without talking about asynchronous, without talking about queue all of that, is already extremely valuable and you should actually try to play with them.

Turns out, that also we didn't have a solution for asynchronous things in Symfony. So one of the other concept in the Symfony Messenger component is the notion of transport. So we can actually say, well, instead of actually directly calling the message handler or your message handler, well, the bus can actually switch to something else. We can actually route the message to actual, to a transport. And the transport is really something a bit abstract, but something is responsible of talking to your, to a third party. The most famous one is RabbitMQ being a third party, so queuing mechanism. But technically it can be actually anything, can be Gearman if you want to do some distributed processing for your messages, can be an API, there is no real limit. And the last missing dot here is that if you actually dispatch the message and you route it to a transport, it's in the queue, but nothing will happen. So the fifth sort of concept is this notion of worker. So it's just something that is going to say, or ask the transport: "hey, tell me when I receive a message" And when a message arrives, it will actually call your message handler.

Messenger in Diagram

The same thing as a sort of a graphical representation. You've got a message, you dispatch a message to a message bus, that is handled by one or zero or N handlers.

If you really want to, you can route this message to a subset of transports: to one or multiple transports, instead of directly calling the handler. These transports are responsible for talking to these guys, and we've got the worker that is responsible for receiving the message from the transport and dispatching them back to the message bus. Now the important thing here is that actually, if you actually route the message to a transport within your HTTP server - or your like main PHP thread - um, you actually dispatch the message and your handler won't be called. It will be super fast, you will hand it to the queue. And you need to run this worker somewhere else. So you won't be on your web server, might be somewhere else on another machine, but the code that your handler contains, or your message handler contains, is going to be executed outside of the PHP context or the web context. That's the main, that's the main concept.

Using Messenger!

So now I'm going to show you exactly how from a user perspective or developer perspective, how can we use it.

You all have seen, I guess what a Symfony controller looks like. Now it's fairly recent, but it's a few versions already, we can inject services directly in the action arguments. So in this example here, I'm going to inject a MessageBusInterface that is already configured and wired for you if you have the component installed on your Symfony framework application. What you can do, as I mentioned, you can actually dispatch using, calling dispatch method, on the bus, you can dispatch your message. There is no requirement. I mentioned that already, but there is no requirement: that's how your message looks like. It's your own php class. So whatever you want to do. If you want to actually send a notification, you can call it a SendNotificationMessage. If you want to, I don't know, buy pizza, it can be BuyPizzaMessage. You can put the properties you want, you can put the method, the getters, the setters you want it is completely your own code. There's nothing... to compare with the event-dispatcher for example, where the event needs to extend the Event class, well, we don't need that anymore. At least in this component.

Once you've dispatched your message, guess what? You need a message handler. Same idea: there is no specific requirement on the message handler. The only thing is it needs to be a PHP callable. So typically it can be an anonymous function. In our beautiful world of classes and all of that, it is a class that has a method, a magic method named __invoke(). If you type-hint your message, type-hint your message, as the first argument of the __invoke method, then Symfony will automatically understand that it is this message that you want to handle. And when you dispatch the message this handler is going to be called.

There is a slight tweak here. It's just that in order for Symfony to automatically configure your handler, and automatically add the tag required to actually discover your handlers, you can use this marker interface, and this marker interface is called MessageHandlerInterface and contains nothing. You don't have any requirements, but just to say to Symfony: "Hey, can you wire that handler into the message bus?" Okay, cool.

Now, if you run that code, your handler, whatever your domain logic is will be run synchronously. So that's the simplest way of registering a message handler. You've got another way which is very similar to the... what we've done with event dispatcher component. We've got a MessageSubscriberInterface. What you can say is that, you can say, well, this handler or this class will actually handle this message and this message and this message. You can handle multiple messages and you can also give it a very interesting configuration. So you can say that, so the syntax it's a bit different, it's using yield instead of returning an array. There is a technical reason to that. But the main thing here is the return value or the value of your yield can be a set of options. So you can actually say instead of the default __invoke method, you can say, well, let's call the doSomething() method. You can also configure the bus. So if you've got multiple buses, for most of, you are using an event bus or command bus or this kind of stuff, you can actually say, well, this subscriber or this handler will actually just listen for this message on this specific bus. You've got a notion of priority as well, etc.

An interesting point of using the yield instead of returning just an array is that we can yield multiple time the exact same key. So we can have multiple configuration for the same message, so we can actually, we can have the handler be called twice if we use the same message key but different priorities as the value. It's very interesting. Anyway, I'm not sure you're going to use that all the time, but that's another way to register the message subscribers.

Now, very important, by default, every single message is handled synchronously. And I think it's a pattern that starts to be, we can start to see if you've watched or listened to the Symfony Mailer, announcement from Fabien a few conferences ago, actually the idea would be that the more and more we ship the more, maybe, you would just be able to dispatch a message into the bus. And that would be a sort of a nice way to provide extension points from library perspective: here's all the messages you can dispatch and that is going to do the job.

Async & Transports

Now if you actually have a message handler that is a bit too slow - and that is the main use case. If it is too slow, what you can say is that either you wait. Or you can, well you can configure this message bus to say, I'm going to route this message somewhere else. And to this message you're going to route it to a transport. So how are you going to do it?

Well, first thing you're going to create a transport. So transport is something that can be created from a DSN or URL here. If you.... Actually, who here is actually is using Doctrine? I was expecting more people. So half of the room actually admit they use Doctrine. So the idea is simple: you've got a database URL in Doctrine, so you can configure whether it's MySQL or Postgres database. Here is the same example. The transport: you give it a name, you give it the name rabbit, it's completely up to you give it the name you want, and the main configuration is the DSN. When you create the transport, you run the same code it's going to be as slow as before. Because creating a transport doesn't do anything. What does something is when you route the message... explicitly routing a message to a specific transport. So here you say your App\Message\YourMessage is going to be routed to the rabbit transport. Here I just put the actual class, but it can be a parent class or an interface, that works as well, so we can route a specific set of messages. And if you rerun the application, wow! Now super fast.

Workers: Handling the Async Messages

The only problem is... Nothing happened, obviously. Because this message has been sent to a queue, in this example RabbitMQ queue, but we didn't consume it, so we didn't run the actual handlers. And that's the fifth point, which is just the worker. The worker look like a... just a Symfony command. So you run bin/console messenger:consume-messages into another machine, on another machine, and this will listen for messages and consume them. That's a long-running process. It's got a bunch of options if you want it to be... to auto kill after like X time or X hours, etc. But that's the main thing you need to do. And here your handlers are going to be called for every single message. And that's it.

That's the one on one of the Symfony Messenger. That's how it works and that's mostly all we want to know about it.

What Happened During the Last 6 Months?

Now, I mentioned it's six months already, so I'm going to just do very quick rewind of like what happened and what we changed. So, six months ago Fabien tweeted this things like, yay! We merged it! So probably a year ago I started to create a Symfony Message pull request. We actually broke Github because it was too many comments. And finally somehow it got merged. Super cool. We actually renamed it to Messenger. And the ignorant me was saying: "yay, job's done!". Turns out that actually that was just the beginning of like a lot of work.

So since then, so what happened? Very quickly, um, we only had 33 issues. Super cool eh? It worked super well. But we actually had 150 merged pull request in the component. And if you mention the, sort of, not necessarily the pain but the complexity of getting pull requests and the review process and the Symfony code base. That's a lot of work. Out of these pull requests and other kind of side commits, more than a hundred of them were actually for the preparation of 4.1 release. And 91 commits for the release 4.2. Probably out of this 90, 91, is probably 50 of Nicolas because he destroyed the component recently.

But the good thing is it's actually coming from 21 different contributors and thank you very much to them. That is the beauty of the... Thank you. That's really the beauty of Symfony I guess is that everybody's just here to make it better. Super cool.

Improved in 4.2

So as of a few days ago, it's actually already 100,000 plus downloads. So it's actually been getting used. And now we are actually starting to have much more small tweaks and small features on this component. And I'm going to present three different things that have changed or have been improved in the version 4.2, the new one.

Envelopes

The first one is the notion of envelopes. So I didn't talk about it yet and this is the best moment to introduce it. The notion of the envelope is to say, well, you've got your message, we didn't want to put any pressure on your side - do anything special with the messages... although, when we started to go asynchronous, there is a lot of things actually we... that can go wrong and there is a lot of information we actually want to carry around.

Well it's the same as the beautiful analogy of the postal message. You wrap that into an envelope and you put a bunch of stamps. That's the exact same thing. We have envelopes within Symfony. It's a PHP object, it contains a message and guess what? It has some stamps. So before there were actually named Items, that's the main thing that we actually renamed to make sure that the analogy makes sense for everybody, but it's really that. So we can mark the envelope with a bunch of information. Built-in within the component, we've got three stamps. We've got the ReceivedStamp, we've got the SentStamp and we've got the HandledStamp. So we can actually know when we get an envelope, we can know whether the message has been received from a transport or actually we just sent it to a transport, or we actually, the handler has been called. That's the main, the really important information.

So how we are going to use it? I'm going to show you the... so that's the message, the interface, the main interface you're using. You're going to use every day - the MessageBusInterface. That actually changed in the 4.2. So the main thing is you can dispatch a message. But actually, if you look at the param, param PHP doc, you can dispatch a message or an Envelope. So you can yourself, on your side, create... wrap your own message into an Envelope and add a bunch of stamps, if you want to. But main thing is it actually returns an Envelope now. Before, it was actually returning the return value of the actual message handler. Um, and that guy, Nicolas did... So basically he said, well: Symfony Messenger sounds very interesting, I'm gonna actually look at it. And he found a bunch of things to fix. And one of them was actually a long, long, long, long, long discussion over multiple weeks: how can we manage this return value of the message bus. Beginning was a mixed return value. That sort of made no sense because it's super unpredictable, when you dispatch a message, you don't know what's going to come back. And that is a real problem. So then, we decided to say, okay, let's return the Envelope. And that kind of makes sense.

Now by doing that, what you can do is when you dispatch the message, you get the Envelope back. And on the Envelope, you can get a, you've got a bunch of methods. And the one that is used here is the last() method. So you can get the last stamp of a specific type. So first, obviously it implies that we can have multiple stamps of the same type.

But the first example is if the last stamp of type SentStamp is not null, well it actually means that we sent the message to the transport. Just right now when we called $bus->dispatch(), it has been sent to a transport. And each stamp contains a bunch of information. The second example is that we actually use the same same thing: we get the last stamp of type HandledStamp. But you actually get the stamp into a $handled variable. And what we can say is that, well, the message has been handled. So a message handler has just been called. And we can, on the stamp, call the getResult() method to get much more information. So we can get the result of the handler. Actually, because multiple handlers can be called, you can have multiple HandledStamp and then you can get all the results. Same idea for if you send it to multiple transport at the same time, you can have multiple SentStamp. So that's how you would use the Envelope within your code base.

That's the advanced track. So the stamps themselves, they are automatically added, they are added by the Symfony Messenger - the few of them that I've mentioned. Now, you can very easily create your own. So you can just create a class, implement the empty StampInterface and create your own thing. So you can carry much more information if you want to have a unique identifier for that message, then you can carry it across your application. That you can do. If you want to carry the user context, you can completely do that outside of the message. You can carry much more information.

And the very important thing is the stamps will survive the serialization. So that's, so if when you start to do asynchronous processing, that's where the real troubles comes in, is the serialization process. So your message, your php class, when it's within the php memory, super cool, we can put whatever we want. Now, if we want it to be consumed by another php binary somewhere else, we need to serialize it. We need to put it in a way so that we can put it somewhere as a string and deserialize it into the other php process. So these stamps are going via the serialization process.

So very quickly, that's the detail of the AMQP transport, the default serialization, or the way your message is actually sent, is using the Symfony Serializer using the JSON format.

Tip

In Symfony 4.3, the default serialization has been changed to use PHP's native serialize() and unserialize() function. It is still possible to serialize to JSON, but the new default serialization is much easier to use for most use-cases.

So that is how a message would look like within the RabbitMQ. This payload is literally just your message. So within the payload of... which is in the message of RabbitMQ is just your thing with no constraints. No Symfony, neither php related things. So what it means that you can very easily use the messenger to also communicate with the other types of systems, other language, other frameworks. Now, to carry a bit more information, we use the headers. So we basically put the type of the header to add the type header, to say, well that is the type of message, so we can deserialize the right class. And also we can... another set of headers for each stamp. That's how we carry this information.

Middleware

Very, very related: the second concept is the concept of middleware. And actually there's a moment where I said that the message bus itself, the class MessageBus, which is the only implementation of the MessageBusInterface, is I think it's 10 lines of useful code. The rest is just comments and stuff like that. The real logic of every single bus actually comes from the middleware. I cannot say middlewares because in english we don't say that, it's just middleware, there is no plural apparently. So that's the middleware, multiple of them. You may have seen this kind of stuff. Um, that's the graph that comes from CakePHP that describes how they use middleware, the middleware mechanism to handle the request and the response, the HTTP request and response. The idea is very simple: every single middleware is in a stack. So there is an ordered list of middleware and each middleware is responsible for calling the next one. And they can do specific actions before or after. The Symfony Messenger bus as the exact same way of working.

So there is our middleware, the Symfony Messenger one, and there are two main important middleware. The two or... the two in the middle. The first one, which is basically because... the one in the middle, the last of the stack, so the last to be called is the HandleMessageMiddleware. So that's the middleware that's responsible of saying: okay, I got this message, I am going to find out which handlers are registered for this message and call them.

There is a middleware above that, which is SendMessageMiddleware. And this middleware is responsible for sending the message to a transport. So if this middleware decide that, well I need to send the message to a transport, it won't call the handle message middleware: it will just return the Envelope. So the entire behavior of a message bus is actually within the middleware.

And you can create your own. So in order to create your own, that's just a class that implements a MiddlewareInterface. It has just one method. I kind of mentioned it: it takes as an input an Envelope, and as an output another Envelope.

It also takes the stack as - the middleware stack as an argument so that it can call the next one. So an empty middleware that's is completely transparent, will look like that. The main thing is return stack: from the stack and get the next middleware, and call the handle method again. I just call the next middleware. And here, I can actually add some behavior, I can add some things, things that are going to happen before and some things that are going to have to happen after. After what? Or before what? After and before the next middleware. And if you look at the previous slide, it actually means it is going to be running something before or after either the message has been sent or the message has been handled.... your message handler has been called. Yep?

So, I'm gonna show you an example middleware, and I'll walk you through. The main thing here is that line at the bottom, which return is, return $stack->next()->handle(). That's super important: as a middleware, I need to call the next ones. Here, the concept is to say: well, I can create a middleware that's responsible of auditing what are the commands or what are the messages that went through. So the first thing that I'm going to do is to get the message from the envelope. So that's your php class. Get the message so that after, I can display a bunch of messages that contains the class of the message. So it's going to say: well, either I received the message or I started to dispatch a message. To get the class at the end of the line, just get the class of the message. Now there is one thing that if - and you've seen that already with the two other stamps, but it's the same mechanism here - if from the envelope we've got the stamp that says ReceivedStamp, it actually means that we are in the consumer. We are under, bin/console messenger:consume-messages side. So in the logs, I won't put a "started with message", I would just put, like, "received message" because that's the context.

So when you look at your log a few days later or something, the future you will actually thank you very much - that you didn't put the same message and that you don't know when things have happened. We don't know exactly, um, what was the context of the message execution. Because the middleware they were called when we received the message from the transports or, when we actually dispatched it from the web application. Because the messages all the time go from the bus.

Now, there is this notion of stamps and this custom stamp. So here is a simple example where you can create your custom stamp - here I have named it AuditStamp. It will just contain an identifier. So we can actually stick an identifier to this given specific envelope, and you can stick this identifier so that in your log you can correlate the log that have happened on the web server: when you created the message, when you dispatch and send the message and the logs that are happening in the CLI, in the background process, because you're going to have the same identifier for this specific message. Because this stamp, if it doesn't exist - that's the first line - if the AuditStamp does not exist, you add the stamp on the envelope. And because these stamps are like, carried around with the serialization process, you're going to have your AuditStamp when you consume the message. And that's this middleware. So this notion of stamps and envelopes - coupled with the middleware - in my perspective are super, super valuable because we can do such things.

When you create your own middleware, well the only thing you need to do is to register it. So you need to explicitly say this bus is going to have this middleware. This example is saying you can create multiple buses, this is just an example of creating multiple buses. And here one of them that we called command_bus will have this AuditMiddleware that we've created. So every message that we dispatch into this command_bus, will go through our new AuditMiddleware. while the rest, or if the message is going to the event_bus, won't go through this middleware. That's an example a bit more complex because we've got multiple buses, but imagine and replace, just remove the event_bus and that's the normal behavior of just having one bus in your Symfony application.

Now there's just a small clarification: the stack. Because that's the main thing as well that's changed in the MiddlewareInterface. Before we had a sort of magic second parameter. Instead of the stack, there was "next", so we could just call $next() parenthesis and call the next middleware. Well the problem was, by doing that, the next, the function that was behind the next variable was actually within the message bus class. So what it meant is that the stack trace was super, super horrible. And if you're using a profiler or something like that, you will always see the middleware going back to the message bus, going back to the next middleware, etc, etc. That way it's actually much more explicit as well: you get the stack and you get directly the next middleware within your own middleware. So you just go: $nextMiddleware->handle() and that's it. It directly called the next middleware. The stack trace is very good or very simple when you get exceptions. And that's very useful.

So that's the sort of, um, details of what's changed and how, about the middleware.

Transports

Now, the last thing I want to talk about quickly are the transports. The first thing, again, and I think it's really important to reiterate that, is that you don't need transports to benefit from the Messenger component. By itself, the structure that it offers you is really interesting for you to use. So we don't need transports. And I think it's been built in a way so that we can have very nice or an easy migration path. As in, we do synchronous as much as possible. The reason is, asynchronous, for a lot of reasons, including the fact that you might have inconsistencies between when you run the message and when the message has been dispatched, with the fact that you may have something that actually needs to return the value from the synchronous background process. We need to have sort of like asynchronous notifications via email or whatever. There is much more complexity.

So you can start synchronous. And then you can configure the transport and then you can write a message to be asynchronous. That is sort of like the great path, as has been planned. So the transport, we have a built-in AMQP transport. So if you've got, the um, the only requirement is the AMQP php extension. If you've got that, you can actually directly use RabbitMQ, and that's it.

Now there is an amazing php library is called Enqueue , which has 1+ transports. So if you're using AWS, SQS transport, if you are using google, you've got Google Pub/Sub, so using more exotic things like Kafka, you can actually use them. Even just a file as well, this kind of stuff. There is an integration with Enqueue there is an adapter so that you can directly create using the EnqueueBundle all these transports and use them directly in the messenger. So you can directly route the messages to an Enqueue transport. So it means that already right now you've got dozens of actually transport you can use.

One thing that we changed, which is very important for you if you want to create transports, is that we simplified the serialization. Before we had the notion of encoder and decoder. Now it's very simple. You've got just one class, that is responsible of serializing and unserializing the messages. You can, so I mentioned by default, using Symfony Serializer, if you want to use protobuf, if you want to use csv, you can easily create a class that implements the SerializerInterface and roll your own.

What Now? What's Next?

So what about now? So now the Symfony Messenger component, it's still experimental in 4.2. So what it means is that maybe in 4.3 we're going to change things as well. So the idea is to say for new components, because we - and especially this one which touches more than the scope of just the php process - there was potentially things we didn't see coming. And maybe there are design or structure in the component that we need to tweak to make sure you actually benefit for everybody. I think it's very unlikely, but it might change from 4.2 to 4.3. It doesn't mean that it is not usable, it just means that if you, when you will upgrade from 4.2 to 4.3, you might have some slight things to change.

It is much better than before. And really I think the massive work of the community has been great in a way that: it's super easier. The error messages are like, make much more sense, and you've got this notion of stamps that actually to me seems very, very simple to grasp.

Now, the documentation deserves a lot more love. It's a super hard problem, especially for components moving super fast like this one. If you feel you can help, that's an amazing contribution to do, and it will help a lot of people. So on that. Thank you very much.

Leave a comment!

  • 2019-03-18 weaverryan

    Hey Scottie Gutman!

    Funny you ask - I'm working a lot right now to get a few of the "transport" things better - I hope we'll have SQS sometime soon in core Symfony, but not in time to help you, unfortunately :(.

    So, you're right that everything will be handled in the resolve:ENQUEUE_DSN. And you're going to need to do some digging to figure out what's going wrong. I'll give you a few pieces of code to put var_dump/die into, to see where config is being lost:

    * https://github.com/php-enqu... - this part is responsible for parsing the DSN

    * https://github.com/php-enqu... - this part is responsible for sending data to SQS. Well, it starts the process - there's nothing specific to SQS here

    * https://github.com/php-enqu... - this is eventually called by the above and sends the message

    Let me know if that helps! This stuff is still too hard with Messenger, and while Enqueue is great, it creates too many layers.

    Cheers!

  • 2019-03-17 Scottie Gutman

    Hello Guys, I was wondering if you have every tried to setup a combination of Symfony 4 messenger, php-enqueue, AWS SQS & SNS. I am using a load balancer across a few systems, and wanted the messages to go out and come back in so they would be distributed. I have setup pieces of this, but I am missing the "special sauce" and wanted your take. I put some more info here: https://stackoverflow.com/q...

  • 2019-03-06 Diego Aguiar

    Hey CharlES

    Thanks for your suggestion. At the moment we don't have plans for it, but we always take in consideration what our users wants to watch :)

    Cheers!

  • 2019-03-06 CharlES

    Waiting for a course on symfony messenger like eating day to day.