Sidekiq Compatibility across Updates

The arguments for a Sidekiq job are stored in a queue while it is scheduled for execution. During a online update, this could lead to several possible situations:

An older version of the application publishes a job, which is executed by an upgraded Sidekiq node.
A job is queued before an upgrade, but executed after an upgrade.
A job is queued by a node running the newer version of the application, but executed on a node running an older version of the application.

Adding new workers

On GitLab.com, we do not currently have a Sidekiq deployment in the canary stage. This means that a new worker than can be scheduled from an HTTP endpoint may be scheduled from canary but not run on Sidekiq until the full production deployment is complete. This can be several hours later than scheduling the job. For some workers, this will not be a problem. For others - particularly latency-sensitive jobs - this will result in a poor user experience.

This only applies to new worker classes when they are first introduced. As we recommend using feature flags as a general development process, it's best to control the entire change (including scheduling of the new Sidekiq worker) with a feature flag.

Changing the arguments for a worker

Jobs need to be backward and forward compatible between consecutive versions of the application. Adding or removing an argument may cause problems during deployment before all Rails and Sidekiq nodes have the updated code.

Deprecate and remove an argument

Before you remove arguments from the perform_async and perform methods., deprecate them. The following example deprecates and then removes arg2 from the perform_async method:

Provide a default value (usually nil) and use a comment to mark the argument as deprecated in the coming minor release. (Release M)

class ExampleWorker
  # Keep arg2 parameter for backwards compatibility.
  def perform(object_id, arg1, arg2 = nil)
    # ...
  end
end

One minor release later, stop using the argument in perform_async. (Release M+1)
```
ExampleWorker.perform_async(object_id, arg1)
```
At the next major release, remove the value from the worker class. (Next major release)
```
class ExampleWorker
  def perform(object_id, arg1)
    # ...
  end
end
```

Add an argument

There are two options for safely adding new arguments to Sidekiq workers:

Set up a multi-step deployment in which the new argument is first added to the worker.
Use a parameter hash for additional arguments. This is perhaps the most flexible option.

Multi-step deployment

This approach requires multiple releases.

Add the argument to the worker with a default value (Release M).

class ExampleWorker
  def perform(object_id, new_arg = nil)
    # ...
  end
end

Add the new argument to all the invocations of the worker (Release M+1).
```
ExampleWorker.perform_async(object_id, new_arg)
```

Remove the default value (Release M+2).

class ExampleWorker
  def perform(object_id, new_arg)
    # ...
  end
end

Parameter hash

This approach doesn't require multiple releases if an existing worker already uses a parameter hash.

Use a parameter hash in the worker to allow future flexibility.

class ExampleWorker
  def perform(object_id, params = {})
    # ...
  end
end

Removing worker classes

To remove a worker class, follow these steps over two minor releases:

In the first minor release

Remove any code that enqueues the jobs.

For example, if there is a UI component or an API endpoint that a user can interact with that results in the worker instance getting enqueued, make sure those surface areas are either removed or updated in a way that the worker instance is no longer enqueued.

This ensures that instances related to the worker class are no longer being enqueued.
Ensure both the frontend and backend code no longer relies on any of the work that used to be done by the worker.
In the relevant worker classes, replace the contents of the perform method with a no-op, while keeping any arguments in tact.

For example, if you're working with the following ExampleWorker:
```
  class ExampleWorker
    def perform(object_id)
      SomeService.run!(object_id)
    end
  end
```
Implementing the no-op might look like this:
```
  class ExampleWorker
    def perform(object_id); end
  end
```
By implementing this no-op, you can avoid unnecessary cycles once any deprecated jobs that are still enqueued eventually get processed.

In a subsequent, separate minor release

Delete the worker class file and follow the guidance in our Sidekiq queues documentation around running Rake tasks to regenerate/update related files.

Add a migration (not a post-deployment migration) that uses sidekiq_remove_jobs:

class RemoveMyDeprecatedWorkersJobInstances < Gitlab::Database::Migration[2.0]
  DEPRECATED_JOB_CLASSES = %w[
    MyDeprecatedWorkerOne
    MyDeprecatedWorkerTwo
  ]
  # Always use `disable_ddl_transaction!` while using the `sidekiq_remove_jobs` method, as we had multiple production incidents due to `idle-in-transaction` timeout.
  disable_ddl_transaction!
  def up
    sidekiq_remove_jobs(job_klasses: DEPRECATED_JOB_CLASSES)
  end

  def down
    # This migration removes any instances of deprecated workers and cannot be undone.
  end
end

Renaming queues

For the same reasons that removing workers is dangerous, care should be taken when renaming queues.

When renaming queues, use the sidekiq_queue_migrate helper migration method in a post-deployment migration:

class MigrateTheRenamedSidekiqQueue < Gitlab::Database::Migration[2.1]
  restrict_gitlab_migration gitlab_schema: :gitlab_main
  disable_ddl_transaction!

  def up
    sidekiq_queue_migrate 'old_queue_name', to: 'new_queue_name'
  end

  def down
    sidekiq_queue_migrate 'new_queue_name', to: 'old_queue_name'
  end
end

You must rename the queue in a post-deployment migration not in a standard migration. Otherwise, it runs too early, before all the workers that schedule these jobs have stopped running. See also other examples.