## Summary
Lambda warm-invocations of logic functions were spending **~440 ms**
re-parsing and re-evaluating the user bundle on every call. The executor
wrote the user code to a **randomly-named** temp file and `import()`-ed
it, so each warm call resolved to a new URL and Node's ESM cache could
never reuse the previous module record.
This PR makes the executor write to a **content-hash filename**, skip
the write when the file already exists, and stop deleting it. Identical
code now reuses the same module record across warm calls in the same
container, dropping warm-invocation overhead by **~30–40%**.
## What changed
- `executor/index.mjs`: temp filename derived from `sha256(code)`, write
skipped when file exists, no `fs.rm` on cleanup.
- `lambda.driver.ts`: single structured `[lambda-timing]` log per
invocation with `totalMs / buildExecutorMs / getBuiltCodeMs /
payloadBytes / invokeSendMs / reportDurationMs / billedMs /
initDurationMs / coldStart`. Goes through the standard NestJS `Logger`.
No behavioural change for callers: same input → same output, same error
semantics.
### Caveat: module-scope state now persists across warm calls
With a stable filename, the user bundle is evaluated **once per warm
container**. Any module-scoped state or top-level side-effects in user
code are now shared across invocations of the same container, instead of
re-running on every call. This is documented in the executor and is the
intended trade-off — module scope should be treated as a per-container
cache, not as per-call isolation.
## Findings — measured impact
Same logic function (`fetch-prs`, ~12k PRs to page through), same
workspace, same Lambda config (eu-west-3, 512 MB), token cache primed.
### Warm invocations
| Phase | Before fix | After fix | Δ |
| -------------------------------------- | ------------- |
-------------- | ------------ |
| Executor `import(userBundle)` | ~440 ms | **~0 ms** | **-440 ms** |
| Lambda billed duration | ~1.5–1.7 s | **~1.0–1.1 s** | **~30–40%** |
| Server-perceived round-trip | ~1.7–2.0 s | **~1.0–1.2 s** |
**~30–40%** |
### Cold starts
Unchanged — the cache helps subsequent warm calls in the same container,
not the first one. Init Duration stays ~130–170 ms; total cold call
~2.5–3.0 s.
### Stress
Could not reproduce the previously-reported \"every ~10th call times
out\" behaviour after the fix:
- 30 sequential calls: max 1.7 s, median ~1.1 s, 0 timeouts
- 50 concurrent calls: max 9.4 s (clear cold-start cluster), median ~1.5
s, 0 timeouts
Hypothesis: the warm-import overhead was eating into the headroom
against the function timeout under bursty load; removing it pushed
everything well below the limit.
## Observability
One structured log line per invocation, sent through the standard NestJS
logger:
\`\`\`
[lambda-timing] fnId=abc123 totalMs=1187 buildExecutorMs=2
getBuiltCodeMs=3 payloadBytes=1466321 invokeSendMs=1180
reportDurationMs=992 billedMs=1000 initDurationMs=n/a coldStart=false
\`\`\`
\`coldStart=true\` whenever Lambda spun up a fresh container; on warm
calls \`buildExecutorMs\` and \`getBuiltCodeMs\` collapse to
single-digit ms, confirming the cache fix is working.
## Test plan
- [ ] CI green.
- [ ] Deploy to a Lambda-backed env, trigger a logic function several
times in a row.
- [ ] Confirm \`[lambda-timing]\` warm invocations show \`totalMs\`
~30–40% lower than before, and \`coldStart=false\` after the first call
in a container.
- [ ] Push a new version of an app; confirm the next call shows higher
\`buildExecutorMs\` (new hash, new file written) followed by warm calls
again.
- [ ] Smoke test: errors thrown by the user handler are still surfaced
correctly.
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