Debug false-ready and dependency outages

Break the artifact on purpose. The most important failure family is ready=true while index or model pin is incompatible. Reproduce one failure with the smallest possible input, inspect the intermediate values, and fix the boundary or algorithm rather than catching every exception. Retrying deterministic bad input only repeats the same mistake; a retry is justified only for a transient dependency.

False-ready is the nightmare: the service accepts traffic while retrieving from the wrong index. Inject DB down, wrong INDEX_VERSION, and wrong MODEL_ID. Assert /ask is not served until ready is true.

The artifact's user-facing goal is specific: serve /ask only when dependencies and version pins match a tested release unit. Its accepted input is request question, tenant auth context, and pinned model/index/config versions. Those statements are intentionally narrower than “build an AI system.” Narrow scope lets us inspect every input and expected result, and it prevents a toy result from being presented as a production claim. System shape for this chapter: a stateless FastAPI service validates requests, queries a separately managed vector index, calls an answer model through an adapter, and emits citations; liveness checks the process while readiness checks dependencies and exact model/index pins. Keep model calls behind adapters, keep authorization and validation in deterministic code, and carry stable IDs and versions through every response. That separation lets you decide whether a bad result came from input handling, retrieval, inference, validation, or deployment. This page's job is the debugging step: break the artifact on purpose. Setup baseline for the chapter (run once per machine, not secrets in git):

python -m venv .venv && source .venv/bin/activate
pip install fastapi uvicorn pydantic httpx
export MODEL_ID=answer-v3 INDEX_VERSION=mini-v1

If hardware or a hosted provider differs, preserve the interface and expected behavior. Do not present provider syntax as universal—when a vendor adapter is unavoidable, keep it behind a thin boundary and test with a fake first. The deliverable is not “it ran once”; it is a reproducible artifact another developer can inspect, including expected output and one deliberate failure related to ready=true while index or model pin is incompatible. Operationally, write down the owner of this stage, the command you ran, the observed output, and the next page's dependency on that output. If you cannot point to a file, fixture, metric, or config key, the stage is not done. Prefer small, reviewable increments: one contract, one path, one metric, one failure, one gate. When tradeoffs appear—latency versus quality, hit rate versus false hits, local privacy versus cloud quality—record both numbers instead of moving the threshold until the report looks green. The chapter ships only when evidence for smoke golden pass rate 100%, readiness true only when pins match, rollback under five minutes and a rehearsed recovery path exist beside containerized FastAPI RAG service with /healthz, /ready, and canary switch.

Save this as lesson.py and run python3 lesson.py. Prefer the standard library or the pinned packages from the setup block so the example stays reproducible.

def readiness(db_ok, index_pin, model_pin, seen_index, seen_model):
    if not db_ok: return False, "db_down"
    if index_pin!=seen_index: return False, "index_mismatch"
    if model_pin!=seen_model: return False, "model_mismatch"
    return True, "ok"
print(readiness(True,"mini-v2","answer-v3","mini-v1","answer-v3"))

Expected output: (False, index_mismatch). Exact floating-point formatting may vary slightly, but the asserted behavior must not. Read the output as evidence about this stage, not merely proof that the interpreter started.

Reproduce ready=true while index or model pin is incompatible as a fixture. Fix one cause at a time. Do not catch broad exceptions and return an apparently successful answer. At the debugging stage, save the smallest failing fixture beside the expected result. Change one cause at a time and rerun the exact command printed above; that makes the repair reviewable and keeps this chapter's progressive artifact reproducible.

Record latency and failure class for each injected fault. Clear degradation beats a wrong success. For this debugging page, preserve the fixture and result as evidence for the next page. Label observations separately from conclusions: a passing assertion establishes the behavior it names, while broader usefulness requires the chapter's full evaluation set and stated operating limits. Primary metrics for the chapter remain smoke golden pass rate 100%, readiness true only when pins match, rollback under five minutes.