Implement one traceable happy path

Code the narrow path that serves release engineer flipping 100% router cut after green smoke: accept → authorize/normalize → call dependency → validate → record. Keep stages named so a trace can show which boundary passed. Success must emit evidence useful to cutover_seconds and postcut_error_rate, not only a 200 with prose. Predict the observable for router → green|blue /v1/rag/answer before running: cutover < 30s; rollback restores green with zero dual-write divergence.

Drive the path with recording fakes or local stubs. Assert call order and arguments. Idempotency keys or stable ids should keep retries from duplicating costly work where the product requires it. Product under test remains RAG API with complete green/v1 and blue/v2 stacks behind one router — resist adding unrelated features mid-path.

For Blue-green deploy lab, paste the CLI/HTTP transcript beside your prediction for router → green|blue /v1/rag/answer. If the oracle is unmet (cutover < 30s; rollback restores green with zero dual-write divergence), stop and debug this page; do not compensate with prompt folktales. Re-run once after a clean process start to catch hidden global state that would invalidate BG-SCHEMA-ONESIDE-11.

Performance sketch: measure local p95 for the fake-backed path so later regressions are obvious. Keep concurrency modest until failure-handling proves limits. Log a single structured event per success with request id, revision, and the evidence field behind cutover_seconds and postcut_error_rate. Avoid hidden global caches in the happy path unless the lab is about caching — and even then key by tenant. If the path calls a model, pin model id in config and echo it in the response for auditability. Remember release engineer flipping 100% router cut after green smoke experiences wall-clock time, not your debugger’s single-step comfort.

Prefer explicit function names over a single god-object handleRequest. Thread a correlation id from ingress to the last log line. When streaming, define what partial failure means before coding. Snapshot one successful response body in fixtures after redaction. If the path writes to a queue, assert message attributes in the fake. Stop adding retries on this page; that is the next concern. In this chapter the product is RAG API with complete green/v1 and blue/v2 stacks behind one router, the human stakeholder is release engineer flipping 100% router cut after green smoke, and the incident id you design against is BG-SCHEMA-ONESIDE-11. Re-state the oracle in your notes — cutover < 30s; rollback restores green with zero dual-write divergence — and keep the invariant visible: blue must pass smoke + /readyz before cut; old green kept warm for 30m rollback. Track cutover_seconds and postcut_error_rate as the scoreboard. Surface under change control: router → green|blue /v1/rag/answer. If you only have forty minutes, finish the fixture for schema migration applied only on blue — green rollback serves 500s before polishing UI. Promotion language stays ternary: promote, hold, or roll back based on evidence, not hope.