Update docs

doc/development/sidekiq_style_guide.md

@@ -9,25 +9,54 @@ 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.
 
-## Default Queue
+## Dedicated Queues
 
-Use of the "default" queue is not allowed. Every worker should use a queue that
-matches the worker's purpose the closest. For example, workers that are to be
-executed periodically should use the "cronjob" queue.
+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`.
 
-A list of all available queues can be found in `config/sidekiq_queues.yml`.
+## Queue Namespaces
 
-## Dedicated Queues
+While different workers cannot share a queue, they can share a queue namespace.
 
-Most 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 a worker's queue name is,
-you can find it using `SomeWorker.queue`.
+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:
+
+```ruby
+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`.
 
-In some cases multiple workers do use the same queue. For example, the various
-workers for updating CI pipelines all use the `pipeline` queue. Adding workers
-to existing queues should be done with care, as adding more workers can lead to
-slow jobs blocking work (even for different jobs) on the shared queue.
+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.
 
 ## Tests
 
@@ -36,7 +65,7 @@ tests should be placed in `spec/workers`.
 
 ## Removing or renaming queues
 
-Try to avoid renaming or removing queues in minor and patch releases.
+Try to avoid renaming or 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 doing rename or remove queue in major release only.