spec: a create-from on the process-step axis, and an opt-in append cardinality - #39
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spec: a create-from on the process-step axis, and an opt-in append cardinality#39delchev wants to merge 1 commit into
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…rdinality Proposal 0021. "On event E, append a derived row" - a log entry per step, a protocol line per transition - is not expressible: postings/posts are idempotent per source, rollups/aggregates never insert, and an event-driven create-from is at-most-once by construction. Two additions to the generates event map close it: the process-step binding the rest of the event axis already uses, and mode: once|append. The proposal keeps the back-reference required in both cardinalities (dedup key under once, provenance under append) and states plainly that append is the absence of a guard rather than a state-aware one - a redelivery appends a duplicate, and a voided target's replacement is not what it expresses.
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Adds
proposals/0021-generates-step-axis-and-append.md. Noversions/file is touched — per the proposal-first process, a version document is written once at a release and folded from accepted proposals.The gap
"On event E, append a derived row" — a log entry per workflow step, a protocol line per status change, an activity record per delivery — has no expression today. Every event-driven construct answers a different question:
postings/postsare idempotent per source,rollups/aggregatesrecompute an existing row and never insert, and an event-drivengeneratescreate-from is at-most-once by construction. What remains is a hand-written listener, or anoutbound→inboundloopback whose only purpose is to defeat the guard.Separately, an event-driven create-from binds only to the source's own lifecycle, while notifications, integrations and departures bind to the lifecycle or a process step — so a follow-up document that belongs to a moment in a flow has no trigger to hang off.
The proposal
Two additions to the
generatesevent:map:onStepReached/onStepCompleted: { process, step }, narrowed by one rule of its own: the process must run on thefrom:entity, since the step event is about the record its process runs on and that record is the one the create-from reads;mode:—once(default, today's behaviour) orappend(one target per delivered event).The back-reference stays required in both cardinalities — the dedup key under
once, the created row's provenance underappend.What it deliberately does not claim
appendis the absence of a guard, not a state-aware one. Delivery is at-least-once, so a redelivery appends a duplicate row, and "the target was voided, make another" is not what this cardinality expresses — that needs a state-aware predicate onmode: once. The proposal says so in the edge rules and again in the specification text, because the two are easy to confuse.Implementation tracking: eclipse-dirigible/dirigible#6800.