Support for ActiveSupport Notifications about maintenance_tasks

[nvasilevski]

Instead of using callbacks

Perhaps I'm missing a particular use-case but generally Active Support notifications are not a substitution for callbacks. A callback is something that is specific to a single maintenance task and will go away along with the task while notifications subscriptions will most likely stay in the application itself as it will be global for all tasks. Also the execution context is different, callbacks are executed within the task itself which means you can access data available in the task or choose how the task should proceed with the execution. While a notification is a fact of an event with no control on the even itself and the context is limited by the values passed along with the notification.

Having said that, I still think that it might be reasonable to have active support notifications added to maintenance tasks. However, since maintenance_tasks is using job-iteration (https://github.com/Shopify/job-iteration) under the hood - it already has all notifications issued by job-iteration, for example - https://github.com/Shopify/job-iteration/blob/c5c299af110bceff60091c5bfec8e2353b069198/lib/job-iteration/iteration.rb#L140

The naming like build_enumerator.iteration is not the friendliest in scope of the maintenance tasks gem as it requires some internal knowledge of job-iteration while maintenance tasks is trying to provide a higher-level API for job-iteration so maybe mirroring job-iteration notifications in maintenance tasks is reasonable assuming that this improves the UX and removes the need to be aware of job-iteration internals without necessity.

[elfassy]

A callback is something that is specific to a single maintenance task and will go away along with the task

That doesn't seem to be what's hinted at in the readme:
https://github.com/Shopify/maintenance_tasks/blob/main/README.md?plain=1#L378-L391

[nvasilevski]

That doesn't seem to be what's hinted at in the readme

This is just a way to mass-define a callback. It's fair that in this case the callback won't go away with the deletion of the task but execution wise it still stays specific to a given task. So a callback is something that is attached to a task when it comes to an execution and a notification subscriber is something that exists in an application and just listens for events. Ultimately in an application there could be two maintenance tasks frameworks that issue the same notifications on similar events while callbacks will have to be duplicated in both of them even if it's going to be a single meta-programming solution that evals code in parent classes. This is why I don't consider notifications to be a complete substitution to callbacks. We should have both

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