Add documentation about recurring tasks in README

Author: rosa

README.md

@@ -2,7 +2,7 @@
 
 Solid Queue is a DB-based queuing backend for [Active Job](https://edgeguides.rubyonrails.org/active_job_basics.html), designed with simplicity and performance in mind.
 
-Besides regular job enqueuing and processing, Solid Queue supports delayed jobs, concurrency controls, pausing queues, numeric priorities per job, priorities by queue order, and bulk enqueuing (`enqueue_all` for Active Job's `perform_all_later`). _Improvements to logging and instrumentation, a better CLI tool, a way to run within an existing process in "async" mode, unique jobs and recurring, cron-like tasks are coming very soon._
+Besides regular job enqueuing and processing, Solid Queue supports delayed jobs, concurrency controls, pausing queues, numeric priorities per job, priorities by queue order, and bulk enqueuing (`enqueue_all` for Active Job's `perform_all_later`). _Improvements to logging and instrumentation, a better CLI tool, a way to run within an existing process in "async" mode, and some way of specifying unique jobs are coming very soon._
 
 Solid Queue can be used with SQL databases such as MySQL, PostgreSQL or SQLite, and it leverages the `FOR UPDATE SKIP LOCKED` clause, if available, to avoid blocking and waiting on locks when polling jobs. It relies on Active Job for retries, discarding, error handling, serialization, or delays, and it's compatible with Ruby on Rails multi-threading.
 
@@ -77,7 +77,7 @@ Besides Rails 7.1, Solid Queue works best with MySQL 8+ or PostgreSQL 9.5+, as t
 
 We have three types of processes in Solid Queue:
 - _Workers_ are in charge of picking jobs ready to run from queues and processing them. They work off the `solid_queue_ready_executions` table.
-- _Dispatchers_ are in charge of selecting jobs scheduled to run in the future that are due and _dispatching_ them, which is simply moving them from the `solid_queue_scheduled_executions` table over to the `solid_queue_ready_executions` table so that workers can pick them up. They also do some maintenance work related to concurrency controls.
+- _Dispatchers_ are in charge of selecting jobs scheduled to run in the future that are due and _dispatching_ them, which is simply moving them from the `solid_queue_scheduled_executions` table over to the `solid_queue_ready_executions` table so that workers can pick them up. They're also in charge of managing recurring tasks, dispatching jobs to process them according to their schedule. On top of that, they do some maintenance work related to concurrency controls.
 - The _supervisor_ forks workers and dispatchers according to the configuration, controls their heartbeats, and sends them signals to stop and start them when needed.
 
 By default, Solid Queue will try to find your configuration under `config/solid_queue.yml`, but you can set a different path using the environment variable `SOLID_QUEUE_CONFIG`. This is what this configuration looks like:
@@ -119,6 +119,8 @@ Everything is optional. If no configuration is provided, Solid Queue will run wi
   Finally, you can combine prefixes with exact names, like `[ staging*, background ]`, and the behaviour with respect to order will be the same as with only exact names.
 - `threads`: this is the max size of the thread pool that each worker will have to run jobs. Each worker will fetch this number of jobs from their queue(s), at most and will post them to the thread pool to be run. By default, this is `3`. Only workers have this setting.
 - `processes`: this is the number of worker processes that will be forked by the supervisor with the settings given. By default, this is `1`, just a single process. This setting is useful if you want to dedicate more than one CPU core to a queue or queues with the same configuration. Only workers have this setting.
+- `concurrency_maintenance`: whether the dispatcher will perform the concurrency maintenance work. This is `true` by default, and it's useful if you don't use any [concurrency controls](#concurrency-controls) and want to disable it or if you run multiple dispatchers and want some of them to just dispatch jobs without doing anything else.
+- `recurring_tasks`: a list of recurring tasks the dispatcher will manage. Read more details about this one in the [Recurring tasks](#recurring-tasks) section.
 
 
 ### Queue order and priorities
@@ -265,3 +267,48 @@ Solid Queue has been inspired by [resque](https://github.com/resque/resque) and
 
 ## License
 The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT).
+
+## Recurring tasks
+Solid Queue supports defining recurring tasks that run at specific times in the future, on a regular basis like cron jobs. These are managed by dispatcher processes and as such, they can be defined in the dispatcher's configuration like this:
+```yml
+  dispatchers:
+    - polling_interval: 1
+      batch_size: 500
+      recurring_tasks:
+        my_periodic_job:
+          class: MyJob
+          args: [ 42, { status: "custom_status" } ]
+          schedule: every second
+```
+`recurring_tasks` is a hash/dictionary, and the key will be the task key internally. Each task needs to have a class, which will be the job class to enqueue, and a schedule. The schedule is parsed using [Fugit](https://github.com/floraison/fugit), so it accepts anything [that Fugit accepts as a cron](https://github.com/floraison/fugit?tab=readme-ov-file#fugitcron). You can also provide arguments to be passed to the job, as a single argument, a hash, or an array of arguments that can also include kwargs as the last element in the array.
+
+The job in the example configuration above will be enqueued every second as:
+```ruby
+MyJob.perform_later(42, status: "custom_status")
+```
+
+Tasks are enqueued at their corresponding times by the dispatcher that owns them, and each task schedules the next one. This is pretty much [inspired by what GoodJob does](https://github.com/bensheldon/good_job/blob/994ecff5323bf0337e10464841128fda100750e6/lib/good_job/cron_manager.rb).
+
+It's possible to run multiple dispatchers with the same `recurring_tasks` configuration. To avoid enqueuing duplicate tasks at the same time, an entry in a new `solid_queue_recurring_executions` table is created in the same transaction as the job is enqueued. This table has a unique index on `task_key` and `run_at`, ensuring only one entry per task per time will be created. This only works if you have `preserve_finished_jobs` set to `true` (the default), and the guarantee applies as long as you keep the jobs around.
+
+Finally, it's possible to configure jobs that aren't handled by Solid Queue. That's it, you can a have a job like this in your app:
+```ruby
+class MyResqueJob < ApplicationJob
+  self.queue_adapter = :resque
+
+  def perform(arg)
+    # ..
+  end
+end
+```
+
+You can still configure this in Solid Queue:
+```yml
+  dispatchers:
+    - recurring_tasks:
+        my_periodic_resque_job:
+          class: MyResqueJob
+          args: 22
+          schedule: "*/5 * * * *"
+```
+and the job will be enqueued via `perform_later` so it'll run in Resque. However, in this case we won't track any `solid_queue_recurring_execution` record for it and there won't be any guarantees that the job is enqueued only once each time.