Decoupling an Application with

Symfony Messenger

ConFoo, Montreal, Canada - February 23th, 2024

© David Buchmann

David Buchmann - david@liip.ch

PHP Engineer, Liip SA, Switzerland

We built a data kraken

Product API
massive central index of product data

• Constantly evolving since 2013
• Collecting data from ~40 source systems
  Legacy and future sources, always in the process of migrating to new services
• Cache all data locally in MySQL for quick access
• Aggregate data in Elasticsearch

First approach

• Cronjob looping over all changed products
• Incremental imports: last-modified
• Record command runs and use the timestamp

• Does not scale
• Error during processing: Start over
• Update products many times for different things

Encapsulate Logic

Extract the logic out of commands and controllers.

=> Refactored into Importer classes that can handle a single item at a time.

Decouple with message queue

• Importer can be called from command line but also from message processor
• Importing triggers events on data changes
• Event listeners dispatch further messages as needed for next steps
• Perfect if source system can send messages
• Otherwise cronjob to only pull data and store it, then dispatch messages

Reaping the benefits

• Parallel processing of data
• Dynamically scale the number of workers
• Failure only affects single items, does not block
  • easy retry
  • abort after several failed attempts

Messages within the application

• Also split up heavy tasks in the application
• Multiple message-driven steps

• Easy to trigger the next step
• Using Symfony event system to decouple
• Event listeners can dispatch messages: e.g. we have 2 sources of product data, decide based on category which is relevant

e.g. Compile data into Elasticsearch

• SAP data must be processed as fast as possible, data delivery process is blocking in SAP
• Dispatch message with data as payload
• First consumer stores data locally
• And dispatches an event, which makes another listener queue this product for reindexing
• Different priorities to process urgent changes fast (e.g. prices)

Flexible on the outside,
consistent on the inside

• Some source systems send messages on changes
• Other systems are polled for changes
• Daily import of CSV files with full data
• Temporal data with time limited validity
• We process everything as message

Any questions so far?

Demo Time!

github.com/dbu/messenger-demo

https://md.davidbu.ch

Neuromancer, William Gibson

Symfony Messenger

Event	Message
Blocking	Asynchronous
Can alter behaviour	No influence
Deterministic order	Undefined order (of consumers)
Not scalable	Scalable

Message System Components

• Message (payload application specific)
• Sender (Publisher, Producer)
• Receiver (Subscriber, Consumer)
• Channels (Consume only relevant messages)

Message System Advantages

• Decoupled (mix systems or languages)
• Scalable (add more producers or consumers)
• Buffering/load management (Async)
• Resilient (buffered when receivers are down)
• Reliable (acknowledgement and retry)
• Order (sequential if only 1 instance)
• Extensions (e.g. message priorities)

Why Symfony Messenger?

• High level functionality
• Abstract from specific message transport
• Nice and modern architecture

The flow of Symfony Messenger

https://symfony.com/doc/current/components/messenger.html#concepts

Send Message

Receiving Message

Message

• This is your class. It is serialized by the messenger.
• Small (Entity ID rather than the full entity data)
• Specific

class UpdatePromotion
{
    public function __construct(
        public string $id
    ) {}
}

Sending a message

private MessageBusInterface $bus;
...
$this->bus->dispatch(new UpdatePromotion($id));
            

Stamps (Metadata)

$stamps[] = new DelayStamp(20);
$this->bus->dispatch(
    new UpdatePromotion($id),
    $stamps
);

Routing

• What queue for which message?
• Control which workers to scale
• Better monitoring

framework:
  messenger:
    transports:
      neuromancer:
        dsn: '%env(MESSENGER_TRANSPORT_DSN)%&queue_name=neuromancer'

  routing:
    App\Message\NeuromancerMessage: neuromancer
            

Retry

• In a large system, things will go wrong
  • Software crashes, maschines reboot
  • Network issues
  • Service or system not available
  • Actual logic bugs
• Message goes back to queue
• Exponential backoff
• Failure transport

Keep workers running

• Long-running PHP processes
• Careful about memory leaks.
• supervisord or similar to restart
• Rejuvenation: Restart after number of messsages, time or when memory consumption exceeds threshold
• Restart workers on deployment

Autoscaler

• Own service to start and stop workers
• Application registers its workers with autoscaler
• Autoscaler monitors queue sizes
• Spawns workers up to the limit when queue grows

What is going on?

• Copy all messages to an archive queue with a TTL
• Import state log table, track all steps

Message Transport

• Without a configured transport, messages are processed immediately
• Doctrine (using a database table)
• AMQP with ext-amqp (rabbitmq, kafka)
• Redis with ext-redis
• Amazon SQS
• Plug your own, e.g. to call a custom API

Did i hear «microservices»?

Une Bataille, ~1750, François-Joseph Casanova

Messaging != microservices

• Our application has ~30 types of message and their processors
• Messages and processors all in the same application
• I would not want to maintain 30 tiny Symfony applications just for one single worker each
• Some separate applications that have nothing directly to do with the main data api

Thank you!

github.com/dbu/messenger-demo/

@dbu@phpc.social

David Buchmann, Liip SA