There is presently no clean way of telling a caller of `perform_later`
the reason why a job failed to enqueue. When the job is enqueued
successfully, the job object itself is returned, but when the job can
not be enqueued, only `false` is returned. This does not allow callers
to distinguish between classes of failures.
One important class of failures is when the job backend experiences a
network partition when communicating with its underlying datastore. It
is entirely possible for that network partition to recover and as such,
code attempting to enqueue a job may wish to take action to reenqueue
that job after a brief delay. This is distinguished from the class of
failures where due a business rule defined in a callback in the
application, a job fails to enqueue and should not be retried.
This PR changes the following:
- Allows a block to be passed to the `perform_later` method. After the
`enqueue` method is executed, but before the result is returned, the
job will be yielded to the block. This allows the code invoking the
`perform_later` method to inspect the job object, even in failure
scenarios.
- Adds an exception `EnqueueError` which job adapters can raise if they
detect a problem specific to their underlying implementation or
infrastructure during the enqueue process.
- Adds two properties to the job base class: `successfully_enqueued` and
`enqueue_error`. `enqueue_error` will be populated by the `enqueue`
method if it rescues an `EnqueueError` raised by the job backend.
`successfully_enqueued` will be true if the job is not rejected by
callbacks and does not cause the job backend to raise an
`EnqueueError` and will be `false` otherwise.
This will allow developers to do something like the following:
MyJob.perform_later do |job|
unless job.successfully_enqueued?
if job.enqueue_error&.message == "Redis was unavailable"
# invoke some code that will retry the job after a delay
end
end
end
