Symfony Messenger Auto Scaling

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v0.1.3 2020-11-11 20:51 UTC

This package is auto-updated.

Last update: 2021-11-12 00:06:48 UTC


PHP Tests

The Symfony Messenger Auto Scaling package provides the ability to dynamically scale the number of workers for a given set of receivers to respond to dynamic workloads.

It's not uncommon for certain types of workloads to fluctuate throughput for lengthy periods of time that require the number of queue consumers to dynamically scale to meet demand. With this auto scaling package, that is now achievable with symfony's messenger system.


Install with composer at krak/symfony-messenger-auto-scale.

If symfony's composer install doesn't automatically register the bundle, you can do so manually:


return [
  Krak\SymfonyMessengerAutoScale\MessengerAutoScaleBundle::class => ['all' => true],


After the bundle is loaded, you need to configure worker pools which will manage procs for a set of messenger receivers.

  console_path: '%kernel.project_dir%/tests/Feature/Fixtures/console'
      min_procs: 0
      max_procs: 5
      receivers: "sales*"
      heartbeat_interval: 5
      min_procs: 0
      max_procs: 5
      backed_up_alert_threshold: 100
      receivers: "*"
      heartbeat_interval: 10      

Once configured, you can start the consumer with the krak:auto-scale:consume command which will start up and manage the worker pools.

Matching Receivers

Each pool config must have a receivers property which is a simple Glob that will match any of the current transport names setup in the messenger config.

It's important to note, that a receiver can ONLY be apart of one pool. So if two pools have receiver patterns that match the same receiver, then the first defined pool would own that receiver.

Receiver Priority

By default, if a pool matches more than one receiver, the order in which the receivers are defined in the framework messenger configuration will be the order in which they are consumed.

Let's look at an example:

# auto scale config
      receivers: '*' # this will match all receivers defined

# messenger config
      transport1: ''
      transport2: ''
      transport3: ''

Every worker in the pool will first process messages in transport1, then once empty, they will look at transport2, and so on. Essentially, we're making a call to the messenger consume command like: console messenger:consume transport1 transport2 transport3

If you'd like to be a bit more explicit about receiver priority, then you can define the priority option on your transport which will ensure that receivers with the highest priority will get processed before receivers with lower priority. If two receivers have the same priority, then the order in which they are defined will take precedent.

Let's look at an example:

# auto scale config
      receivers: '*' # this will match all receivers defined

# messenger config
        dsn: ''
        options: { priority: -1 }
        dsn: ''
        options: { priority: 1 }
      transport2: '' # default priority is 0

This would have the same effect as the above configuration. Even though the transports are defined in a different order, the priority option ensures they are in the same order as above.

Disabling Must Match All Receivers

By default, the bundle will throw an exception if any receivers are not matched by the pool config. This is to help prevent any unexpected bugs where you the receiver name is for some reason not matched by a pool when you expected it to.

To disable this check, update the must_match_all_receivers config option to false:

  must_match_all_receivers: false

Configuring Heartbeats

By default, each worker pool will log a heartbeat event every 60 seconds. If you want to change the frequency of that, you use the pool heartbeat_interval to define the number of seconds between subsequent heartbeats.


You can access the PoolControl from your own services if you want to build out custom monitoring, or you can just use the krak:auto-scale:pool:* commands that are registered.

Auto Scaling

Auto scaling is managed with the AutoScale interface which is responsible for taking the current state of a worker pool captured in the AutoScaleRequest and returning the expected num workers for that worker pool captured in AutoScaleResponse.

The default auto scale is setup to work off of the current queue size and the configured message rate and then will clip to the min/max procs configured. There also is some logic included to debounce the auto scaling requests to ensure that the system is judicious about when to create new procs and isn't fluctuating too often.

Here is some example config and we'll go over some scenarios:

      max_procs: 5
      message_rate: 100
      scale_up_threshold_seconds: 5
      scale_down_threshold_seconds: 20
      receivers: "catalog"
      min_procs:  5
      message_rate: 10
      scale_up_threshold_seconds: 5
      scale_down_threshold_seconds: 20
      receivers: "sales"
Seconds from Start Catalog Pool Queue Size Catalog Pool Num Workers Sales Pool Queue Size Sales Pool Num Workers Notes
n/a 0 0 0 0 Initial State
0 0 0 0 5 First Run, scaled up to 5 because of min procs
2 1 1 60 5 Scale up to 1 on catalog immediately, but wait until scale up threshold for sales
5 0 1 50 5 Wait to scale down on for catalog, reset counter for sales for scale up because now a scale up isn't needed
6 0 1 60 5 Wait to scale up on sales again, timer started, needs 5 seconds before scale up
11 0 1 60 6 Size of queue maintained over 60 for 5 seconds, so now we can scale up.
22 0 0 60 6 Catalog now goes back to zero after waiting 20 seconds since needing to scale down

Defining your own Auto Scale algorithm

If you want to augment or perform your own auto-scaling algorithm, you can implement the AutoScale interface and then update the Krak\SymfonyMessengerAutoScale\AutoScale to point to your new auto scale service. The default service is defined like:

use Krak\SymfonyMessengerAutoScale\AutoScale;

$autoScale = new AutoScale\MinMaxClipAutoScale(new AutoScale\DebouncingAutoScale(new AutoScale\QueueSizeMessageRateAutoScale()));


The alerting system is designed to be flexible and allow each user define alerts as they see. Alerts are simply just events that get dispatched when a certain metric is reached as determined by the services that implement RaiseAlerts.

To actually trigger the alerts, you need to run the krak:auto-scale:alert command which will check the state of the pools and raise alerts. Put this command on a cron at whatever interval you want alerts monitored at.

Subscribing to Alerts

You simply just can create a basic symfony event listener/subscriber for that event and you should be able to perform any action on those events.


This alert will fire if the there are too many messages for the given queue. To enable this on a pool, you need to define the backed_up_alert_threshold config value.

# ...
      backed_up_alert_threshold: 100

If there are over 100 messages in the sales pool, then the PoolBackedUpAlert will fire on the next check.

Creating Your Own Alerts

To create an alert, you need to subscribe to the RaiseAlerts interface, then register that service, and if you enable auto configuration, it should automatically get tagged with messenger_auto_scale.raise_alerts.

Accessing Supervisor Pool Config from Symfony App

When installing this as a bundle in a symfony app, it can be helpful to provide access to some internal config structures. The library exposes services which can be injected/accessed to provide access to the internal config.

Supervisor Pool Config Array

krak.messenger_auto_scale.supervisor_pool_configs stores list<SupervisorPoolConfig> based off of the auto scale config.

Receiver To Pool Names Array

krak.messenger_auto_scale.receiver_to_pool_mapping stores array<string, string> which maps the messenger reciever ids to the auto scale pool names.


You can run the test suite with: composer test

You'll need to start the redis docker container locally in order for the Feature test suite to pass.

Keep in mind that you will need to have the redis-ext installed on your local php cli, and will need to start up the redis instance in docker via docker-compose.