ETag overrides and redaction

Details on how cache.etag (see Settings) interacts with @Override'd routes and redacted responses. Read Caching & ETags first for the wire-level behavior; this page covers the configuration edge cases.

Redact in the schema, not in an interceptor

Kavo's KavoResponseInterceptor is method-scoped, so it runs innermost: it sets the ETag before any controller- or app-level interceptor runs. An outer interceptor that strips fields per role would ship a hash of the unredacted representation next to a redacted body. A client's If-Match built from that body would then never match.

Shape the response with a per-operation item schema instead. The engine serializes through it before hashing, so the tag matches what the client actually receives.

What an @Override gets for free

An @Override'd method gets the ETag automatically, but only enforces If-Match if it forwards the preconditions itself. The two halves come from different places (ADR-0027):

• The tag is automatic. An override on a single-item operation can return the typed service's item, for example this.base.patchOne(id, body, { app }), and @Kavo hashes it into the same strong ETag a generated route would serve. There's nothing to opt into.
• The precondition is not automatic. If-Match is evaluated inside the engine, against a canonical read. It only runs if the method passes its preconditions parameter on, either as { preconditions } on the typed service, or by returning service.engine.execute({ …, preconditions }).

Before v0.9, the tag was not automatic either, and the host framework filled in its own weak one instead. If you're on an older version, that's the failure to watch for: reads carried an ETag, If-None-Match answered 304, and the tag changed with the body. Everything worked except the 412 that protects data, so a route could look fully conditional while every guarded write was a silent lost update. Assert the tag's shape (/^"[0-9a-f]{64}"$/) in your tests, not just its presence.

The one limit that survives

The engine compares If-Match against what findOne would serve for that id. An override that serves a reshaped representation hands out a tag the check can never match, so every conditional write on it answers 412. Serve the canonical shape, or set cache: { etag: false } for that operation and own the concurrency control yourself.