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.

You must always add any new queues to app/workers/all_queues.yml or ee/app/workers/all_queues.yml otherwise your worker will not run.

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.

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 latency_sensitive.

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 latency_sensitive worker: consider redesigning the worker, or splitting the work between two different workers, one with latency_sensitive code that executes quickly, and the other with non-latency_sensitive, which has no execution latency requirements (but also has lower scheduling targets).

This can be summed up in the following table:

Latency Sensitivity	Queue Scheduling Target	Execution Latency Requirement
Not latency_sensitive	1 minute	Maximum run time of 1 hour
latency_sensitive	100 milliseconds	p50 of 1 second, p99 of 10 seconds

To mark a worker as being latency_sensitive, use the latency_sensitive_worker! attribute, as shown in this example:

class LatencySensitiveWorker
  include ApplicationWorker

  latency_sensitive_worker!

  # ...
end

Jobs with External Dependencies

Most background jobs in the GitLab application communicate with other GitLab services, eg 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 latency_sensitive 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 latency sensitive 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, latency_sensitive 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.

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

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