Question 1

Is this a Kafka or NATS replacement, or do I still need a broker?

Accepted Answer

You still supply the transport. The crate defines Source, Ack and Sink traits and an in-process channel, with run for events needing no acknowledgement and run_acked or run_source for those that do. Two buses chain through a link so the upstream publish completes only after the downstream acknowledges. The README says explicitly that the transport is not part of this crate.

Question 2

What actually happens when the judge endpoint is down mid-stream?

Accepted Answer

Transient failures get exponential backoff with a cap, bounded or unbounded attempts and a per-call timeout; a typed breaker opens after consecutive transient failures so later events wait out the cooldown instead of burning their own budget, one success closes it and a failed probe doubles it. Permanent failures and exhausted budgets dead-letter immediately with the event, attempt count and cause. If nobody reads the dead-letter stream, failures are still recorded as Outcome::Unhandled rather than dropped silently.

Question 3

Will I pay once per event or once per subscription?

Accepted Answer

Once per event. A subscription is a plain-language description plus examples plus thresholds, the bus compiles them into a question set, and a single request carries all of them. Wrapping the judge in Cached adds an LRU with a 24 h default TTL keyed by a SHA-256 digest of payload and question set, and because the question set changes when you edit a subscription, an edited subscription cannot read a stale answer.

Question 4

How do I test routing logic without hitting the live API?

Accepted Answer

Judge is a trait, so a test double implements it directly and no network code is involved; JevJudge is just one implementation behind the jev feature, which is on by default. The Sleeper trait is injected too, so tests run on a paused clock instead of real backoff waits, and MemoryLedger covers the audit side. Because routing is pure, you can replay a recorded answer set through decide and assert the verdicts.

Question 5

What does the crate actually deny at compile time?

Accepted Answer

Production code denies missing_docs and unsafe_code, plus clippy's unwrap_used, expect_used, panic, todo, unimplemented, unreachable and indexing_slicing. The type-level lifecycle applies the same idea: phases are types, so calling settle on a retrying event does not compile, and only an open breaker can be probed. Acknowledgement takes the handle by value, so acknowledging twice does not compile either.

Question 6

What is a blueprint, and how do two buses chain?

Accepted Answer

A bus is data plus handles; the data half is a Blueprint you can serialise to store, version and ship, and Bus::from_blueprint rebuilds it with a judge, ledger and sleeper. Two buses chain through a link: the upstream publish completes only after the downstream acknowledges, so at-least-once holds across the chain. Crossing processes means carrying the serialised Delivery over your own transport, which is not part of the crate.

Question 7

How does event merging work?

Accepted Answer

merge::windowed is an async stage placed in front of the bus. A Combiner assigns each event a group key and folds a group into one event, flushing when the window elapses, the group reaches max_parts, or the input ends; a timer from an earlier group with the same key never flushes a later one. The merged event's parts list its constituents and the bus records a Composed ledger row, and a rejected group comes out as a MergeFailure carrying every event it held.

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