Trigger events after pipeline-state has processed an update

[flosell]

Problem

Extensions like LambdaUI want to send updates about the build state (in this case via Websocket) to their clients whenever something changes.

At the moment, they can listen e.g. for :step-result-updated but (because event-bus listeners don't have an order) can't be sure that this event has already been processed by the pipeline-state component, i.e. that they'll actually find the new state when asking the pipeline-state component.

Example: I want to assemble a build history after I received an event that indicates that a pipeline-run is completed. When I receive this event, I need to ask the pipeline-state component for the current state to assemble history information. But I can't be sure that I actually get the new history because it might not have processed the same event I received yet.

Potential Solutions

Extensions needing this information track their own state based on existing events

Pro:
* No additional APIs needed from lambdacd-core
* Their state can be custom-tailored for their use-case

Con:
* Additional effort for extension-developers
* Hard to get right, especially when data has to be loaded from persistence and not from events that occurred during the runtime of the current process

Introduce events that are triggered once the persistence component processed the event

Pro:

• Minimal effort for extension-developers

Con:

• Additional API
• Potentially confusing to developers (:step-result-updated or :step-result-update-persisted???)

Context

Related issues

• Redesign handling of pipeline-state/history Redesign handling of pipeline-state/history #131
• Persistence: Decouple step-updates from flushing to disk to improve performance/resource usage Persistence: Decouple step-updates from flushing to disk to improve performance/resource usage #137: Do we send our events when they are flushed to disk or before?

[flosell]

CC @sroidl

[flosell]

Done and documented in wiki