GitLab EventStore

Background

The monolithic GitLab project is becoming larger and more domains are being defined. As a result, these domains are becoming entangled with each others due to temporal coupling.

An emblematic example is the PostReceive worker where a lot happens across multiple domains. If a new behavior reacts to a new commit being pushed, then we add code somewhere in PostReceive or its sub-components (Git::ProcessRefChangesService, for example).

This type of architecture:

• Is a violation of the Single Responsibility Principle.
• Increases the risk of adding code to a codebase you are not familiar with. There may be nuances you don't know about which may introduce bugs or a performance degradation.
• Violates domain boundaries. Inside a specific namespace (for example Git::) we suddenly see classes from other domains chiming in (like Ci:: or MergeRequests::).

What is EventStore?

Gitlab:EventStore is a basic pub-sub system built on top of the existing Sidekiq workers and observability we have today. We use this system to apply an event-driven approach when modeling a domain while keeping coupling to a minimum.

This essentially leaves the existing Sidekiq workers as-is to perform asynchronous work but inverts the dependency.

EventStore example

When a CI pipeline is created we update the head pipeline for any merge request matching the pipeline's ref. The merge request can then display the status of the latest pipeline.

Without the EventStore

We change Ci::CreatePipelineService and add logic (like an if statement) to check if the pipeline is created. Then we schedule a worker to run some side-effects for the MergeRequests:: domain.

This style violates the Open-Closed Principle and unnecessarily add side-effects logic from other domains, increasing coupling:

graph LR
  subgraph ci[CI]
    cp[CreatePipelineService]
  end

  subgraph mr[MergeRequests]
    upw[UpdateHeadPipelineWorker]
  end

  subgraph no[Namespaces::Onboarding]
    pow[PipelinesOnboardedWorker]
  end

  cp -- perform_async --> upw
  cp -- perform_async --> pow

With the EventStore

Ci::CreatePipelineService publishes an event Ci::PipelineCreatedEvent and its responsibility stops here.

The MergeRequests:: domain can subscribe to this event with a worker MergeRequests::UpdateHeadPipelineWorker, so:

• Side-effects are scheduled asynchronously and don't impact the main business transaction that emits the domain event.
• More side-effects can be added without modifying the main business transaction.
• We can clearly see what domains are involved and their ownership.
• We can identify what events occur in the system because they are explicitly declared.

With Gitlab::EventStore there is still coupling between the subscriber (Sidekiq worker) and the schema of the domain event. This level of coupling is much smaller than having the main transaction (Ci::CreatePipelineService) coupled to:

• multiple subscribers.
• multiple ways of invoking subscribers (including conditional invocations).
• multiple ways of passing parameters.

graph LR
  subgraph ci[CI]
    cp[CreatePipelineService]
    cp -- publish --> e[PipelineCreateEvent]
  end

  subgraph mr[MergeRequests]
    upw[UpdateHeadPipelineWorker]
  end

  subgraph no[Namespaces::Onboarding]
    pow[PipelinesOnboardedWorker]
  end

  upw -. subscribe .-> e
  pow -. subscribe .-> e

Each subscriber, being itself a Sidekiq worker, can specify any attributes that are related to the type of work they are responsible for. For example, one subscriber could define urgency: high while another one less critical could set urgency: low.

The EventStore is only an abstraction that allows us to have Dependency Inversion. This helps separating a business transaction from side-effects (often executed in other domains).

When an event is published, the EventStore calls perform_async on each subscribed worker, passing in the event information as arguments. This essentially schedules a Sidekiq job on each subscriber's queue.

This means that nothing else changes with regards to how subscribers work, as they are just Sidekiq workers. For example: if a worker (subscriber) fails to execute a job, the job is put back into Sidekiq to be retried.

EventStore advantages

• Subscribers (Sidekiq workers) can be set to run quicker by changing the worker weight if the side-effect is critical.
• Automatically enforce the fact that side-effects run asynchronously. This makes it safe for other domains to subscribe to events without affecting the performance of the main business transaction.

Define an event

An Event object represents a domain event that occurred in a bounded context. Notify other bounded contexts about something that happened by publishing events, so that they can react to it.

Define new event classes under app/events/<namespace>/ with a name representing something that happened in the past:

class Ci::PipelineCreatedEvent < Gitlab::EventStore::Event
  def schema
    {
      'type' => 'object',
      'required' => ['pipeline_id'],
      'properties' => {
        'pipeline_id' => { 'type' => 'integer' },
        'ref' => { 'type' => 'string' }
      }
    }
  end
end

The schema is validated immediately when we initialize the event object so we can ensure that publishers follow the contract with the subscribers.

We recommend using optional properties as much as possible, which require fewer rollouts for schema changes. However, required properties could be used for unique identifiers of the event's subject. For example:

• pipeline_id can be a required property for a Ci::PipelineCreatedEvent.
• project_id can be a required property for a Projects::ProjectDeletedEvent.

Publish only properties that are needed by the subscribers without tailoring the payload to specific subscribers. The payload should fully represent the event and not contain loosely related properties. For example:

Ci::PipelineCreatedEvent.new(data: {
  pipeline_id: pipeline.id,
  # unless all subscribers need merge request IDs,
  # this is data that can be fetched by the subscriber.
  merge_request_ids: pipeline.all_merge_requests.pluck(:id)
})

Publishing events with more properties provides the subscribers with the data they need in the first place. Otherwise subscribers have to fetch the additional data from the database. However, this can lead to continuous changes to the schema and possibly adding properties that may not represent the single source of truth. It's best to use this technique as a performance optimization. For example: when an event has many subscribers that all fetch the same data again from the database.

Update the schema

Changes to the schema require multiple rollouts. While the new version is being deployed:

• Existing publishers can publish events using the old version.
• Existing subscribers can consume events using the old version.
• Events get persisted in the Sidekiq queue as job arguments, so we could have 2 versions of the schema during deployments.

As changing the schema ultimately impacts the Sidekiq arguments, please refer to our Sidekiq style guide with regards to multiple rollouts.

Add properties

1. Rollout 1:
   • Add new properties as optional (not required).
   • Update the subscriber so it can consume events with and without the new properties.
2. Rollout 2:
   • Change the publisher to provide the new property
3. Rollout 3: (if the property should be required):
   • Change the schema and the subscriber code to always expect it.

Remove properties

1. Rollout 1:
   • If the property is required, make it optional.
   • Update the subscriber so it does not always expect the property.
2. Rollout 2:
   • Remove the property from the event publishing.
   • Remove the code from the subscriber that processes the property.

Other changes

For other changes, like renaming a property, use the same steps:

1. Remove the old property
2. Add the new property

Publish an event

To publish the event from the previous example:

Gitlab::EventStore.publish(
  Ci::PipelineCreatedEvent.new(data: { pipeline_id: pipeline.id })
)

Create a subscriber

A subscriber is a Sidekiq worker that includes the Gitlab::EventStore::Subscriber module. This module takes care of the perform method and provides a better abstraction to handle the event safely via the handle_event method. For example:

module MergeRequests
  class UpdateHeadPipelineWorker
    include Gitlab::EventStore::Subscriber

    def handle_event(event)
      Ci::Pipeline.find_by_id(event.data[:pipeline_id]).try do |pipeline|
        # ...
      end
    end
  end
end

Register the subscriber to the event

To subscribe the worker to a specific event in lib/gitlab/event_store.rb, add a line like this to the Gitlab::EventStore.configure! method:

module Gitlab
  module EventStore
    def self.configure!(store)
      # ...

      store.subscribe ::MergeRequests::UpdateHeadPipelineWorker, to: ::Ci::PipelineCreatedEvent

      # ...
    end
  end
end

A worker that is only defined in the EE codebase can subscribe to an event in the same way by declaring the subscription in ee/lib/ee/gitlab/event_store.rb.

Subscriptions are stored in memory when the Rails app is loaded and they are immediately frozen. It's not possible to modify subscriptions at runtime.

Conditional dispatch of events

A subscription can specify a condition when to accept an event:

store.subscribe ::MergeRequests::UpdateHeadPipelineWorker,
  to: ::Ci::PipelineCreatedEvent,
  if: -> (event) { event.data[:merge_request_id].present? }

This tells the event store to dispatch Ci::PipelineCreatedEvents to the subscriber if the condition is met.

This technique can avoid scheduling Sidekiq jobs if the subscriber is interested in a small subset of events.

WARNING: When using conditional dispatch it must contain only cheap conditions because they are executed synchronously every time the given event is published.

For complex conditions it's best to subscribe to all the events and then handle the logic in the handle_event method of the subscriber worker.