debian-mirror-gitlab/doc/development/sidekiq_style_guide.md

Sidekiq Style Guide

This document outlines various guidelines that should be followed when adding or modifying Sidekiq workers.

ApplicationWorker

All workers should include ApplicationWorker instead of Sidekiq::Worker, which adds some convenience methods and automatically sets the queue based on the worker's name.

Dedicated Queues

All workers should use their own queue, which is automatically set based on the worker class name. For a worker named ProcessSomethingWorker, the queue name would be process_something. If you're not sure what queue a worker uses, you can find it using SomeWorker.queue. There is almost never a reason to manually override the queue name using sidekiq_options queue: :some_queue.

After adding a new queue, run bin/rake gitlab:sidekiq:all_queues_yml:generate to regenerate app/workers/all_queues.yml or ee/app/workers/all_queues.yml so that it can be picked up by sidekiq-cluster.

Queue Namespaces

While different workers cannot share a queue, they can share a queue namespace.

Defining a queue namespace for a worker makes it possible to start a Sidekiq process that automatically handles jobs for all workers in that namespace, without needing to explicitly list all their queue names. If, for example, all workers that are managed by sidekiq-cron use the cronjob queue namespace, we can spin up a Sidekiq process specifically for these kinds of scheduled jobs. If a new worker using the cronjob namespace is added later on, the Sidekiq process will automatically pick up jobs for that worker too (after having been restarted), without the need to change any configuration.

A queue namespace can be set using the queue_namespace DSL class method:

class SomeScheduledTaskWorker
  include ApplicationWorker

  queue_namespace :cronjob

  # ...
end

Behind the scenes, this will set SomeScheduledTaskWorker.queue to cronjob:some_scheduled_task. Commonly used namespaces will have their own concern module that can easily be included into the worker class, and that may set other Sidekiq options besides the queue namespace. CronjobQueue, for example, sets the namespace, but also disables retries.

bundle exec sidekiq is namespace-aware, and will automatically listen on all queues in a namespace (technically: all queues prefixed with the namespace name) when a namespace is provided instead of a simple queue name in the --queue (-q) option, or in the :queues: section in config/sidekiq_queues.yml.

Note that adding a worker to an existing namespace should be done with care, as the extra jobs will take resources away from jobs from workers that were already there, if the resources available to the Sidekiq process handling the namespace are not adjusted appropriately.

Idempotent Jobs

It's known that a job can fail for multiple reasons. For example, network outages or bugs. In order to address this, Sidekiq has a built-in retry mechanism that is used by default by most workers within GitLab.

It's expected that a job can run again after a failure without major side-effects for the application or users, which is why Sidekiq encourages jobs to be idempotent and transactional.

As a general rule, a worker can be considered idempotent if:

• It can safely run multiple times with the same arguments.
• Application side-effects are expected to happen only once (or side-effects of a second run are not impactful).

A good example of that would be a cache expiration worker.

Ensuring a worker is idempotent

Make sure the worker tests pass using the following shared example:

include_examples 'an idempotent worker' do
  it 'marks the MR as merged' do
    # Using subject inside this block will process the job multiple times
    subject

    expect(merge_request.state).to eq('merged')
  end
end

Use the perform_multiple method directly instead of job.perform (this helper method is automatically included for workers).

Declaring a worker as idempotent

class IdempotentWorker
  include ApplicationWorker

  # Declares a worker is idempotent and can
  # safely run multiple times.
  idempotent!

  # ...
end

It's encouraged to only have the idempotent! call in the top-most worker class, even if the perform method is defined in another class or module.

NOTE: Note: Note that a cop will fail if the worker class is not marked as idempotent. Consider skipping the cop if you're not confident your job can safely run multiple times.

Job urgency

Jobs can have an urgency attribute set, which can be :high, :low, or :throttled. These have the below targets:

Urgency	Queue Scheduling Target	Execution Latency Requirement
:high	100 milliseconds	p50 of 1 second, p99 of 10 seconds
:low	1 minute	Maximum run time of 1 hour
:throttled	None	Maximum run time of 1 hour

To set a job's urgency, use the urgency class method:

class HighUrgencyWorker
  include ApplicationWorker

  urgency :high

  # ...
end

Latency sensitive jobs

If a large number of background jobs get scheduled at once, queueing of jobs may occur while jobs wait for a worker node to be become available. This is normal and gives the system resilience by allowing it to gracefully handle spikes in traffic. Some jobs, however, are more sensitive to latency than others. Examples of these jobs include:

1. A job which updates a merge request following a push to a branch.
2. A job which invalidates a cache of known branches for a project after a push to the branch.
3. A job which recalculates the groups and projects a user can see after a change in permissions.
4. A job which updates the status of a CI pipeline after a state change to a job in the pipeline.

When these jobs are delayed, the user may perceive the delay as a bug: for example, they may push a branch and then attempt to create a merge request for that branch, but be told in the UI that the branch does not exist. We deem these jobs to be urgency :high.

Extra effort is made to ensure that these jobs are started within a very short period of time after being scheduled. However, in order to ensure throughput, these jobs also have very strict execution duration requirements:

1. The median job execution time should be less than 1 second.
2. 99% of jobs should complete within 10 seconds.

If a worker cannot meet these expectations, then it cannot be treated as a urgency :high worker: consider redesigning the worker, or splitting the work between two different workers, one with urgency :high code that executes quickly, and the other with urgency :low, which has no execution latency requirements (but also has lower scheduling targets).

Jobs with External Dependencies

Most background jobs in the GitLab application communicate with other GitLab services. For example, Postgres, Redis, Gitaly, and Object Storage. These are considered to be "internal" dependencies for a job.

However, some jobs will be dependent on external services in order to complete successfully. Some examples include:

1. Jobs which call web-hooks configured by a user.
2. Jobs which deploy an application to a k8s cluster configured by a user.

These jobs have "external dependencies". This is important for the operation of the background processing cluster in several ways:

1. Most external dependencies (such as web-hooks) do not provide SLOs, and therefore we cannot guarantee the execution latencies on these jobs. Since we cannot guarantee execution latency, we cannot ensure throughput and therefore, in high-traffic environments, we need to ensure that jobs with external dependencies are separated from high urgency jobs, to ensure throughput on those queues.
2. Errors in jobs with external dependencies have higher alerting thresholds as there is a likelihood that the cause of the error is external.

class ExternalDependencyWorker
  include ApplicationWorker

  # Declares that this worker depends on
  # third-party, external services in order
  # to complete successfully
  worker_has_external_dependencies!

  # ...
end

NOTE: Note: Note that a job cannot be both high urgency and have external dependencies.

CPU-bound and Memory-bound Workers

Workers that are constrained by CPU or memory resource limitations should be annotated with the worker_resource_boundary method.

Most workers tend to spend most of their time blocked, wait on network responses from other services such as Redis, Postgres and Gitaly. Since Sidekiq is a multithreaded environment, these jobs can be scheduled with high concurrency.

Some workers, however, spend large amounts of time on-CPU running logic in Ruby. Ruby MRI does not support true multithreading - it relies on the GIL to greatly simplify application development by only allowing one section of Ruby code in a process to run at a time, no matter how many cores the machine hosting the process has. For IO bound workers, this is not a problem, since most of the threads are blocked in underlying libraries (which are outside of the GIL).

If many threads are attempting to run Ruby code simultaneously, this will lead to contention on the GIL which will have the affect of slowing down all processes.

In high-traffic environments, knowing that a worker is CPU-bound allows us to run it on a different fleet with lower concurrency. This ensures optimal performance.

Likewise, if a worker uses large amounts of memory, we can run these on a bespoke low concurrency, high memory fleet.

Note that memory-bound workers create heavy GC workloads, with pauses of 10-50ms. This will have an impact on the latency requirements for the worker. For this reason, memory bound, urgency :high jobs are not permitted and will fail CI. In general, memory bound workers are discouraged, and alternative approaches to processing the work should be considered.

If a worker needs large amounts of both memory and CPU time, it should be marked as memory-bound, due to the above restrction on high urgency memory-bound workers.

Declaring a Job as CPU-bound

This example shows how to declare a job as being CPU-bound.

class CPUIntensiveWorker
  include ApplicationWorker

  # Declares that this worker will perform a lot of
  # calculations on-CPU.
  worker_resource_boundary :cpu

  # ...
end

Determining whether a worker is CPU-bound

We use the following approach to determine whether a worker is CPU-bound:

• In the Sidekiq structured JSON logs, aggregate the worker duration and cpu_s fields.
• duration refers to the total job execution duration, in seconds
• cpu_s is derived from the Process::CLOCK_THREAD_CPUTIME_ID counter, and is a measure of time spent by the job on-CPU.
• Divide cpu_s by duration to get the percentage time spend on-CPU.
• If this ratio exceeds 33%, the worker is considered CPU-bound and should be annotated as such.
• Note that these values should not be used over small sample sizes, but rather over fairly large aggregates.

Feature Categorization

Each Sidekiq worker, or one of its ancestor classes, must declare a feature_category attribute. This attribute maps each worker to a feature category. This is done for error budgeting, alert routing, and team attribution for Sidekiq workers.

The declaration uses the feature_category class method, as shown below.

class SomeScheduledTaskWorker
  include ApplicationWorker

  # Declares that this worker is part of the
  # `continuous_integration` feature category
  feature_category :continuous_integration

  # ...
end

The list of value values can be found in the file config/feature_categories.yml. This file is, in turn generated from the stages.yml from the GitLab Company Handbook source.

Updating config/feature_categories.yml

Occasionally new features will be added to GitLab stages. When this occurs, you can automatically update config/feature_categories.yml by running scripts/update-feature-categories. This script will fetch and parse stages.yml and generate a new version of the file, which needs to be checked into source control.

Excluding Sidekiq workers from feature categorization

A few Sidekiq workers, that are used across all features, cannot be mapped to a single category. These should be declared as such using the feature_category_not_owned! declaration, as shown below:

class SomeCrossCuttingConcernWorker
  include ApplicationWorker

  # Declares that this worker does not map to a feature category
  feature_category_not_owned!

  # ...
end

Job weights

Some jobs have a weight declared. This is only used when running Sidekiq in the default execution mode - using sidekiq-cluster does not account for weights.

As we are moving towards using sidekiq-cluster in Core, newly-added workers do not need to have weights specified. They can simply use the default weight, which is 1.

Worker context

To have some more information about workers in the logs, we add metadata to the jobs in the form of an ApplicationContext. In most cases, when scheduling a job from a request, this context will already be deducted from the request and added to the scheduled job.

When a job runs, the context that was active when it was scheduled will be restored. This causes the context to be propagated to any job scheduled from within the running job.

All this means that in most cases, to add context to jobs, we don't need to do anything.

There are however some instances when there would be no context present when the job is scheduled, or the context that is present is likely to be incorrect. For these instances we've added rubocop-rules to draw attention and avoid incorrect metadata in our logs.

As with most our cops, there are perfectly valid reasons for disabling them. In this case it could be that the context from the request is correct. Or maybe you've specified a context already in a way that isn't picked up by the cops. In any case, please leave a code-comment pointing to which context will be used when disabling the cops.

When you do provide objects to the context, please make sure that the route for namespaces and projects is preloaded. This can be done using the .with_route scope defined on all Routables.

Cron-Workers

The context is automatically cleared for workers in the cronjob-queue (which include CronjobQueue), even when scheduling them from requests. We do this to avoid incorrect metadata when other jobs are scheduled from the cron-worker.

Cron-Workers themselves run instance wide, so they aren't scoped to users, namespaces, projects, or other resources that should be added to the context.

However, they often schedule other jobs that do require context.

That is why there needs to be an indication of context somewhere in the worker. This can be done by using one of the following methods somewhere within the worker:

1. Wrap the code that schedules jobs in the with_context helper:

  def perform
    deletion_cutoff = Gitlab::CurrentSettings
                        .deletion_adjourned_period.days.ago.to_date
    projects = Project.with_route.with_namespace
                 .aimed_for_deletion(deletion_cutoff)

    projects.find_each(batch_size: 100).with_index do |project, index|
      delay = index * INTERVAL

      with_context(project: project) do
        AdjournedProjectDeletionWorker.perform_in(delay, project.id)
      end
    end
  end

1. Use the a batch scheduling method that provides context:

  def schedule_projects_in_batch(projects)
    ProjectImportScheduleWorker.bulk_perform_async_with_contexts(
      projects,
      arguments_proc: -> (project) { project.id },
      context_proc: -> (project) { { project: project } }
    )
  end

or when scheduling with delays:

  diffs.each_batch(of: BATCH_SIZE) do |diffs, index|
    DeleteDiffFilesWorker
      .bulk_perform_in_with_contexts(index *  5.minutes,
                                     diffs,
                                     arguments_proc: -> (diff) { diff.id },
                                     context_proc: -> (diff) { { project: diff.merge_request.target_project } })
  end

Jobs scheduled in bulk

Often, when scheduling jobs in bulk, these jobs should have a separate context rather than the overarching context.

If that is the case, bulk_perform_async can be replaced by the bulk_perform_async_with_context helper, and instead of bulk_perform_in use bulk_perform_in_with_context.

For example:

    ProjectImportScheduleWorker.bulk_perform_async_with_contexts(
      projects,
      arguments_proc: -> (project) { project.id },
      context_proc: -> (project) { { project: project } }
    )

Each object from the enumerable in the first argument is yielded into 2 blocks:

The arguments_proc which needs to return the list of arguments the job needs to be scheduled with.

The context_proc which needs to return a hash with the context information for the job.

Tests

Each Sidekiq worker must be tested using RSpec, just like any other class. These tests should be placed in spec/workers.

Sidekiq Compatibility across Updates

Keep in mind that 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:

1. An older version of the application publishes a job, which is executed by an upgraded Sidekiq node.
2. A job is queued before an upgrade, but executed after an upgrade.
3. 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.

Changing the arguments for a worker

Jobs need to be backwards- and forwards-compatible between consecutive versions of the application.

This can be done by following this process:

1. Do not remove arguments from the perform function.. Instead, use the following approach
   1. Provide a default value (usually nil) and use a comment to mark the argument as deprecated
   2. Stop using the argument in perform_async.
   3. Ignore the value in the worker class, but do not remove it until the next major release.

Removing workers

Try to avoid removing workers and their queues in minor and patch releases.

During online update instance can have pending jobs and removing the queue can lead to those jobs being stuck forever. If you can't write migration for those Sidekiq jobs, please consider removing the worker in a major release only.

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, as show in this example:

class MigrateTheRenamedSidekiqQueue < ActiveRecord::Migration[5.0]
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  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