--- stage: none group: unassigned info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments --- # 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`](../administration/operations/extra_sidekiq_processes.md). Additionally, run `bin/rake gitlab:sidekiq:sidekiq_queues_yml:generate` to regenerate `config/sidekiq_queues.yml`. ## 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 also picks up jobs for that worker (after having been restarted), without the need to change any configuration. A queue namespace can be set using the `queue_namespace` DSL class method: ```ruby class SomeScheduledTaskWorker include ApplicationWorker queue_namespace :cronjob # ... end ``` Behind the scenes, this sets `SomeScheduledTaskWorker.queue` to `cronjob:some_scheduled_task`. Commonly used namespaces 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 listens 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 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. ## Versioning Version can be specified on each Sidekiq worker class. This is then sent along when the job is created. ```ruby class FooWorker include ApplicationWorker version 2 def perform(*args) if job_version == 2 foo = args.first['foo'] else foo = args.first end end end ``` Under this schema, any worker is expected to be able to handle any job that was enqueued by an older version of that worker. This means that when changing the arguments a worker takes, you must increment the `version` (or set `version 1` if this is the first time a worker's arguments are changing), but also make sure that the worker is still able to handle jobs that were queued with any earlier version of the arguments. From the worker's `perform` method, you can read `self.job_version` if you want to specifically branch on job version, or you can read the number or type of provided arguments. ## 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](https://github.com/mperham/sidekiq/wiki/Best-Practices#2-make-your-job-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 do not have an effect). A good example of that would be a cache expiration worker. A job scheduled for an idempotent worker is [deduplicated](#deduplication) when an unstarted job with the same arguments is already in the queue. ### Ensuring a worker is idempotent Make sure the worker tests pass using the following shared example: ```ruby 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 ```ruby 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. If the worker class isn't marked as idempotent, a cop fails. Consider skipping the cop if you're not confident your job can safely run multiple times. ### Deduplication When a job for an idempotent worker is enqueued while another unstarted job is already in the queue, GitLab drops the second job. The work is skipped because the same work would be done by the job that was scheduled first; by the time the second job executed, the first job would do nothing. #### Strategies GitLab supports two deduplication strategies: - `until_executing` - `until_executed` More [deduplication strategies have been suggested](https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/195). If you are implementing a worker that could benefit from a different strategy, please comment in the issue. ##### Until Executing This strategy takes a lock when a job is added to the queue, and removes that lock before the job starts. For example, `AuthorizedProjectsWorker` takes a user ID. When the worker runs, it recalculates a user's authorizations. GitLab schedules this job each time an action potentially changes a user's authorizations. If the same user is added to two projects at the same time, the second job can be skipped if the first job hasn't begun, because when the first job runs, it creates the authorizations for both projects. ```ruby module AuthorizedProjectUpdate class UserRefreshOverUserRangeWorker include ApplicationWorker deduplicate :until_executing idempotent! # ... end end ``` ##### Until Executed This strategy takes a lock when a job is added to the queue, and removes that lock after the job finishes. It can be used to prevent jobs from running simultaneously multiple times. ```ruby module Ci class BuildTraceChunkFlushWorker include ApplicationWorker deduplicate :until_executed idempotent! # ... end end ``` #### Scheduling jobs in the future GitLab doesn't skip jobs scheduled in the future, as we assume that the state has changed by the time the job is scheduled to execute. Deduplication of jobs scheduled in the feature is possible for both `until_executed` and `until_executing` strategies. If you do want to deduplicate jobs scheduled in the future, this can be specified on the worker by passing `including_scheduled: true` argument when defining deduplication strategy: ```ruby module AuthorizedProjectUpdate class UserRefreshOverUserRangeWorker include ApplicationWorker deduplicate :until_executing, including_scheduled: true idempotent! # ... end end ``` ## Limited capacity worker It is possible to limit the number of concurrent running jobs for a worker class by using the `LimitedCapacity::Worker` concern. The worker must implement three methods: - `perform_work`: The concern implements the usual `perform` method and calls `perform_work` if there's any available capacity. - `remaining_work_count`: Number of jobs that have work to perform. - `max_running_jobs`: Maximum number of jobs allowed to run concurrently. ```ruby class MyDummyWorker include ApplicationWorker include LimitedCapacity::Worker def perform_work(*args) end def remaining_work_count(*args) 5 end def max_running_jobs 25 end end ``` Additional to the regular worker, a cron worker must be defined as well to backfill the queue with jobs. the arguments passed to `perform_with_capacity` are passed to the `perform_work` method. ```ruby class ScheduleMyDummyCronWorker include ApplicationWorker include CronjobQueue def perform(*args) MyDummyWorker.perform_with_capacity(*args) end end ``` ### How many jobs are running? It runs `max_running_jobs` at almost all times. The cron worker checks the remaining capacity on each execution and it schedules at most `max_running_jobs` jobs. Those jobs on completion re-enqueue themselves immediately, but not on failure. The cron worker is in charge of replacing those failed jobs. ### Handling errors and idempotence This concern disables Sidekiq retries, logs the errors, and sends the job to the dead queue. This is done to have only one source that produces jobs and because the retry would occupy a slot with a job to perform in the distant future. We let the cron worker enqueue new jobs, this could be seen as our retry and back off mechanism because the job might fail again if executed immediately. This means that for every failed job, we run at a lower capacity until the cron worker fills the capacity again. If it is important for the worker not to get a backlog, exceptions must be handled in `#perform_work` and the job should not raise. The jobs are deduplicated using the `:none` strategy, but the worker is not marked as `idempotent!`. ### Metrics This concern exposes three Prometheus metrics of gauge type with the worker class name as label: - `limited_capacity_worker_running_jobs` - `limited_capacity_worker_max_running_jobs` - `limited_capacity_worker_remaining_work_count` ## 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` | 10 seconds | p50 of 1 second, p99 of 10 seconds | | `:low` | 1 minute | Maximum run time of 5 minutes | | `:throttled` | None | Maximum run time of 5 minutes | To set a job's urgency, use the `urgency` class method: ```ruby 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. 1. A job which invalidates a cache of known branches for a project after a push to the branch. 1. A job which recalculates the groups and projects a user can see after a change in permissions. 1. 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. 1. 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). ### Changing a queue's urgency On GitLab.com, we run Sidekiq in several [shards](https://dashboards.gitlab.net/d/sidekiq-shard-detail/sidekiq-shard-detail), each of which represents a particular type of workload. When changing a queue's urgency, or adding a new queue, we need to take into account the expected workload on the new shard. Note that, if we're changing an existing queue, there is also an effect on the old shard, but that always reduces work. To do this, we want to calculate the expected increase in total execution time and RPS (throughput) for the new shard. We can get these values from: - The [Queue Detail dashboard](https://dashboards.gitlab.net/d/sidekiq-queue-detail/sidekiq-queue-detail) has values for the queue itself. For a new queue, we can look for queues that have similar patterns or are scheduled in similar circumstances. - The [Shard Detail dashboard](https://dashboards.gitlab.net/d/sidekiq-shard-detail/sidekiq-shard-detail) has Total Execution Time and Throughput (RPS). The Shard Utilization panel displays if there is currently any excess capacity for this shard. We can then calculate the RPS * average runtime (estimated for new jobs) for the queue we're changing to see what the relative increase in RPS and execution time we expect for the new shard: ```ruby new_queue_consumption = queue_rps * queue_duration_avg shard_consumption = shard_rps * shard_duration_avg (new_queue_consumption / shard_consumption) * 100 ``` If we expect an increase of **less than 5%**, then no further action is needed. Otherwise, please ping `@gitlab-org/scalability` on the merge request and ask for a review. ## Jobs with External Dependencies Most background jobs in the GitLab application communicate with other GitLab services. For example, PostgreSQL, Redis, Gitaly, and Object Storage. These are considered to be "internal" dependencies for a job. However, some jobs are dependent on external services in order to complete successfully. Some examples include: 1. Jobs which call web-hooks configured by a user. 1. 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. 1. Errors in jobs with external dependencies have higher alerting thresholds as there is a likelihood that the cause of the error is external. ```ruby class ExternalDependencyWorker include ApplicationWorker # Declares that this worker depends on # third-party, external services in order # to complete successfully worker_has_external_dependencies! # ... end ``` 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, waiting on network responses from other services such as Redis, PostgreSQL, and Gitaly. Since Sidekiq is a multi-threaded 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 multi-threading - it relies on the [GIL](https://thoughtbot.com/blog/untangling-ruby-threads#the-global-interpreter-lock) 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 leads to contention on the GIL which has the effect 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 has an impact on the latency requirements for the worker. For this reason, `memory` bound, `urgency :high` jobs are not permitted and 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 restriction on high urgency memory-bound workers. ## Declaring a Job as CPU-bound This example shows how to declare a job as being CPU-bound. ```ruby 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`](https://www.rubydoc.info/stdlib/core/Process:clock_gettime) 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 category All Sidekiq workers must define a known [feature category](feature_categorization/index.md#sidekiq-workers). ## Job weights Some jobs have a weight declared. This is only used when running Sidekiq in the default execution mode - using [`sidekiq-cluster`](../administration/operations/extra_sidekiq_processes.md) does not account for weights. As we are [moving towards using `sidekiq-cluster` in Core](https://gitlab.com/gitlab-org/gitlab/-/issues/34396), newly-added workers do not need to have weights specified. They can simply use the default weight, which is 1. ## Worker context > - [Introduced](https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/9) in GitLab 12.8. To have some more information about workers in the logs, we add [metadata to the jobs in the form of an `ApplicationContext`](logging.md#logging-context-metadata-through-rails-or-grape-requests). In most cases, when scheduling a job from a request, this context is already deducted from the request and added to the scheduled job. When a job runs, the context that was active when it was scheduled is 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, leave a code comment pointing to which context to use when disabling the cops. When you do provide objects to the context, make sure that the route for namespaces and projects is pre-loaded. This can be done by using the `.with_route` scope defined on all `Routable`s. ### Cron workers The context is automatically cleared for workers in the Cronjob queue (`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: ```ruby 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: ```ruby 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: ```ruby 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: ```ruby 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. ## Arguments logging As of GitLab 13.6, Sidekiq job arguments are logged by default, unless [`SIDEKIQ_LOG_ARGUMENTS`](../administration/troubleshooting/sidekiq.md#log-arguments-to-sidekiq-jobs) is disabled. By default, the only arguments logged are numeric arguments, because arguments of other types could contain sensitive information. To override this, use `loggable_arguments` inside a worker with the indexes of the arguments to be logged. (Numeric arguments do not need to be specified here.) For example: ```ruby class MyWorker include ApplicationWorker loggable_arguments 1, 3 # object_id will be logged as it's numeric # string_a will be logged due to the loggable_arguments call # string_b will be filtered from logs # string_c will be logged due to the loggable_arguments call def perform(object_id, string_a, string_b, string_c) end 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. 1. A job is queued before an upgrade, but executed after an upgrade. 1. 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 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: 1. Provide a default value (usually `nil`) and use a comment to mark the argument as deprecated in the coming minor release. (Release M) ```ruby class ExampleWorker # Keep arg2 parameter for backwards compatibility. def perform(object_id, arg1, arg2 = nil) # ... end end ``` 1. One minor release later, stop using the argument in `perform_async`. (Release M+1) ```ruby ExampleWorker.perform_async(object_id, arg1) ``` 1. At the next major release, remove the value from the worker class. (Next major release) ```ruby class ExampleWorker def perform(object_id, arg1) # ... end end ``` #### Add an argument There are two options for safely adding new arguments to Sidekiq workers: 1. Set up a [multi-step deployment](#multi-step-deployment) in which the new argument is first added to the worker. 1. Use a [parameter hash](#parameter-hash) for additional arguments. This is perhaps the most flexible option. ##### Multi-step deployment This approach requires multiple releases. 1. Add the argument to the worker with a default value (Release M). ```ruby class ExampleWorker def perform(object_id, new_arg = nil) # ... end end ``` 1. Add the new argument to all the invocations of the worker (Release M+1). ```ruby ExampleWorker.perform_async(object_id, new_arg) ``` 1. Remove the default value (Release M+2). ```ruby 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. 1. Use a parameter hash in the worker to allow future flexibility. ```ruby class ExampleWorker def perform(object_id, params = {}) # ... end end ``` ### 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: ```ruby 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 ```