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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.
Versioning
Version can be specified on each Sidekiq worker class. This is then sent along when the job is created.
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.
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 will automatically be deduplicated 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:
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.
If the worker class is not marked as idempotent, a cop will fail. 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. 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.
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.
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 will have 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:
module AuthorizedProjectUpdate
class UserRefreshOverUserRangeWorker
include ApplicationWorker
deduplicate :until_executing, including_scheduled: true
idempotent!
# ...
end
end
Troubleshooting
If the automatic deduplication were to cause issues in certain
queues. This can be temporarily disabled by enabling a feature flag
named disable_<queue name>_deduplication
. For example to disable
deduplication for the AuthorizedProjectsWorker
, we would enable the
feature flag disable_authorized_projects_deduplication
.
From ChatOps:
/chatops run feature set disable_authorized_projects_deduplication true
From the rails console:
Feature.enable!(:disable_authorized_projects_deduplication)
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 usualperform
method and callsperform_work
if there is any capacity available.remaining_work_count
- number of jobs that will have work to perform.max_running_jobs
- maximum number of jobs allowed to run concurrently.
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
will be passed along to the perform_work
method.
class ScheduleMyDummyCronWorker
include ApplicationWorker
include CronjobQueue
def perform(*args)
MyDummyWorker.perform_with_capacity(*args)
end
end
How many jobs are running?
It will be running max_running_jobs
at almost all times.
The cron worker will check the remaining capacity on each execution and it
will schedule at most max_running_jobs
jobs. Those jobs on completion will
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 that will be performed 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 will be running 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:
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:
- A job which updates a merge request following a push to a branch.
- A job which invalidates a cache of known branches for a project after a push to the branch.
- A job which recalculates the groups and projects a user can see after a change in permissions.
- 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:
- The median job execution time should be less than 1 second.
- 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, 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 will always be a reduction in 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 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 has Total Execution Time and Throughput (RPS). The Shard Utilization panel will show 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:
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 will be dependent on external services in order to complete successfully. Some examples include:
- Jobs which call web-hooks configured by a user.
- 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:
- 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.
- 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
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, 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 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 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.
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
andcpu_s
fields. duration
refers to the total job execution duration, in secondscpu_s
is derived from theProcess::CLOCK_THREAD_CPUTIME_ID
counter, and is a measure of time spent by the job on-CPU.- Divide
cpu_s
byduration
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.
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
- Introduced 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
.
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, leave a code comment pointing to which context will be used 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:
-
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
-
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.
Arguments logging
As of GitLab 13.6, Sidekiq job arguments will be logged by default, unless SIDEKIQ_LOG_ARGUMENTS
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:
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:
- 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.
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 will not require multiple releases if an existing worker already utilizes a parameter hash.
-
Use a parameter hash in the worker to allow future flexibility.
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:
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