Support `one-offs` for firing events only once

[embano1]

Problem

As a user I want to fire (schedule) an action (alarm/script) or event once at a particular time. This can be done by emitting an event "once" (ignoring distr. sys challenges for a second) with customizable data at a particular point in time.

cc/ @lionelvillard

Links:

• https://twitter.com/embano1/status/1511613644777439234?s=20&t=XN6gjYCX3lXsFWLAaKp-Hg
• https://knative.slack.com/archives/C9JP909F0/p1649248557443529?thread_ts=1649234703.449399&cid=C9JP909F0

Persona: Developers, DevOps, operators

Exit Criteria A Knative resource which lets a user fire an event once at a particular time.

• Specify data for event which should be fired
• Specify start (optionally end) timeframe for which the event should be valid (TTL?)

Time Estimate (optional): 1 week.

Additional context (optional)

Evaluated but not meeting exit criteria:

• Knative PingSource: follows cron pattern, i.e. recurring schedule and not "only once"
• Kubernetes Job with suspension which requires a custom controller (logic) and has not simple UX in my opinion (also is not event-oriented)

[github-actions[bot]]

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[pierDipi]

@lionelvillard what do you think?

[lionelvillard]

/remove-lifecycle stale

[lionelvillard]

this is a very useful feature.

[daisy-ycguo]

@lionelvillard I made a simple analysis and came up with a few questions. Would you like to share your opinion?

If we want to implement such an event with a dedicated adapter e.g. "onceadapter" in knative/eventing, there will be a lot duplicated codes similar as pingsource adapter. If we don't want duplicated code, we can just leverage "pingsource" adapter and extend it to support "one-off" event. We introduce a new Runner "oneOffJobRunner" similar as "cronJobsRunner". Then we will use the same event type "PingSource" to support the one-off event.

Another choice is to introduce a new event type in knative-sandbox with a separated repo. Then it will be a new event type and will include the whole structure of a new event source.

Which one do you prefer?

[lionelvillard]

@daisy-ycguo I'm also thinking of extending the current PingSource to support firing events only once. We need to:

• extend the PingSource API with a new date field (similar to OpenWhisk Alarms)
• introduce a new Runner.

So +1 on your proposal!

[daisy-ycguo]

/assign

[pierDipi]

Should we also think about some form of garbage collection for one-off pings?

It seems like the right time to discuss this since it could be the default behavior of a one-off ping source.

[daisy-ycguo]

Sorry I dont have time to work on it recently. /unassign

[liuchangyan]

/assign