iteration-how-it-works.md

Iteration: how it works

The main idea behind Iteration is to provide an API to describe jobs in an interruptible manner, in contrast with implementing one massive #perform method that is impossible to interrupt safely.

Exposing the enumerator and the action to apply allows us to keep a cursor and interrupt between iterations. Let's see what this looks like with an ActiveRecord::Relation (and Enumerator).

class NotifyUsers
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(cursor:)
    active_record_records_enumerator(User.all, cursor: cursor)
  end

  def each_iteration(user)
    user.notify_about_something
  end
end

1. build_enumerator is called, which constructs ActiveRecordEnumerator from an ActiveRecord relation (User.all)
2. The first batch of records is loaded:

SELECT "users".* FROM "users" ORDER BY "users"."id" LIMIT 100

3. The job iterates over two records of the relation and then receives SIGTERM (graceful termination signal) caused by a deploy.
4. The signal handler sets a flag that makes job_should_exit? return true.
5. After the last iteration is completed, we will check job_should_exit? which now returns true.
6. The job stops iterating and pushes itself back to the queue, with the latest cursor_position value.
7. Next time when the job is taken from the queue, we'll load records starting from the last primary key that was processed:

SELECT "users".* FROM "users" WHERE "users"."id" > 2 ORDER BY "products"."id" LIMIT 100

Exceptions inside each_iteration

When an unrescued exception happens inside the each_iteration block, the job will stop and re-enqueue itself with the last successful cursor. This means that the iteration that failed will be retried with the same parameters and the cursor will only move if that iteration succeeds. This behaviour may be enough for intermittent errors, such as network connection failures, but if your execution is deterministic and you have an error, subsequent iterations will never run.

In other words, if you are trying to process 100 records but the job consistently fails on the 61st, only the first 60 will be processed and the job will try to process the 61st record until retries are exhausted.

Signals

It's critical to know UNIX signals in order to understand how interruption works. There are two main signals that Sidekiq use: SIGTERM and SIGKILL. SIGTERM is the graceful termination signal which means that the process should exit soon, not immediately. For Iteration, it means that we have time to wait for the last iteration to finish and to push job back to the queue with the last cursor position. SIGTERM is what allows Iteration to work. In contrast, SIGKILL means immediate exit. It doesn't let the worker terminate gracefully, instead it will drop the job and exit as soon as possible.

Most of the deploy strategies (Heroku, Capistrano) send SIGTERM before shutting down a node, then wait for a timeout (usually from 30 seconds to a minute) to send SIGKILL if the process has not terminated yet.

Further reading: Sidekiq signals.

Enumerators

Iteration supports any Enumerator. We expose helpers to build enumerators conveniently (active_record_records_enumerator), but it's up for a developer to implement a custom Enumerator.

Consider this example:

class MyJob
  include Sidekiq::Job
  include SidekiqIteration::Iteration

  def build_enumerator(cursor:)
    Enumerator.new do
      Redis.lpop("mylist") # or: Kafka.poll(timeout: 10.seconds)
    end
  end

  def each_iteration(element_from_redis)
    # ...
  end
end

Further reading: ruby-doc, a great post about Enumerators.