Cold start optimizer

A cold start is not one number — it is runtime boot, dependency/module load, framework init, and first-connection setup stacked on top of each other, and you are usually optimizing a guess about which one dominates. This skill makes it measured: split the init into phases, find the phase that actually costs you, and attack that — shrink the artifact and lazy-load the heavy deps off the first-request path, hoist one-time work to module scope so warm invocations reuse it, right-size memory (more CPU often means a faster and cheaper cold start), and reuse connections across invocations instead of opening a fresh one every cold start. Provisioned concurrency / keep-warm is the last resort for genuinely latency-critical paths, not the first reflex — because it bills you to mask a slow init rather than fixing it.

When to use this skill

• Serverless p99 (or p999) spikes on the first request after a quiet period, while warm requests are fast.
• A function intermittently times out during init — before your handler code even runs.
• Scale-to-zero or aggressive autoscaling is hurting user-facing latency on a path that can't tolerate a 2–5s tail.
• You've been told to "just turn on provisioned concurrency" and want to know whether the init is fixable first (and cheaper).
• A deploy bloated the artifact (new dependency, bundling change) and cold starts regressed.

Instructions

1. Measure the cold start and split it into phases — don't optimize a guess. Force a cold start (deploy a new version, or wait out the platform's idle timeout) and capture the init timeline, not just the total. Most platforms expose it: AWS Lambda INIT_START/REPORT log lines (Init Duration is the pre-handler cost) plus X-Ray init subsegments; GCP/Cloud Run startup probe + request logs; Vercel function logs. Instrument the four phases yourself with timestamps at module load:

   • runtime boot — the platform spinning up the sandbox/container and language runtime (you can't change this much, but you must know its share).
   • dependency/module load — require/import of your code and its tree, top-to-bottom.
   • framework init — ORM bootstrap, DI container, route table build, config parse, schema/codegen load.
   • first-connection setup — DB handshakes, TLS, secret-manager fetches, warm-up calls. Attribute a millisecond cost to each. You optimize the dominant phase; everything else is noise until that one shrinks.

2. Shrink the deployment artifact and lazy-load heavy deps off the first-request path. A giant bundle inflates both runtime boot (more to unpack) and module load (more to parse). Tree-shake and bundle (esbuild/@vercel/nft/webpack) so you ship the function's actual closure, not the whole node_modules; exclude the AWS SDK / platform SDK that the runtime already provides; strip source maps and dev deps from the package. Then find the imports that aren't needed for the first request — a PDF renderer, an image library, an analytics client, a markdown engine — and move them behind a lazy await import() / deferred require inside the code path that needs them, so they never touch init. Grep the entry module for top-level imports of known-heavy packages and ask of each: does request #1 use this?

3. Hoist one-time work to module scope so warm invocations reuse it — but don't connect eagerly. Config parsing, client construction, schema compilation, and validator building should run once at module load and be captured in module-scope variables, so the platform's instance reuse amortizes them across every warm invocation on that instance. The sharp distinction: construct clients at module scope, but connect lazily. Build the DB pool / HTTP client object at module load (cheap, no I/O); open the actual connection on first use inside the handler, and reuse it across subsequent invocations on the same warm instance. Eager top-level await pool.connect() adds connection latency to every cold start and turns a traffic burst into a connection storm.

4. Reuse connections across invocations via instance reuse — never open a fresh connection per cold start. Store the connection/pool in a module-scope (or globalThis) variable so a warm instance hands it back instead of reconnecting. Size the per-instance pool to 1–2 connections, not 20: each concurrent serverless instance gets its own pool, so a large per-instance pool times the instance count will blow past the database's max_connections under burst. For Postgres at high concurrency, point functions at a transaction-mode pooler (PgBouncer/RDS Proxy/Supabase pooler) rather than the database directly. Set a connection idle timeout shorter than the platform's instance-freeze window so dead connections don't accumulate.

5. Right-size memory — on many platforms it buys CPU, so more memory = faster AND cheaper cold start. On Lambda (and similar) CPU and network scale linearly with the memory setting, and a cold start is CPU-bound (parsing, JIT, framework init). Bumping 128MB → 512MB–1GB can cut the cold start by enough that the higher per-ms price × shorter duration is lower total cost — the classic counter-intuitive win. Sweep a few memory settings against the same forced-cold-start workload and pick the point on the cost-vs-latency curve, don't assume the smallest tier is cheapest.

6. Use provisioned concurrency / keep-warm only for genuinely latency-critical paths — after init is already fast. If a path truly can't tolerate any cold tail (checkout, auth, a synchronous user-facing API), provision N warm instances to cover baseline concurrency. But apply it last, sized to real concurrency (not a round number), and only once steps 1–5 have made the init itself fast — because provisioning a 4-second init just means you pay 24/7 to keep a slow thing warm, and any burst beyond your provisioned count still pays the full cold start.

[!WARNING] Opening a fresh DB connection on every cold start — instead of reusing one across warm invocations — is the classic serverless outage. Under a traffic spike, every new instance opens its own connections simultaneously, the database hits max_connections, and every request (warm ones included) starts failing. Construct the client at module scope, connect lazily, reuse across invocations, and cap the per-instance pool low. Use a transaction-mode pooler when instance count can exceed the DB's connection limit.

[!CAUTION] Keep-warm and provisioned concurrency mask a slow init; they don't fix it — and they bill you continuously for the masking. If you reach for them before measuring, you'll pay 24/7 to hide a 3s init that two hours of lazy-loading would have cut to 400ms, and you'll still eat the full cold start on every burst beyond your provisioned count. Fix the init first; provision only the residual.

Output

1. Cold-start breakdown by phase — the measured init timeline showing where the milliseconds actually go, so the dominant cost is obvious before any change:

Cold start breakdown — POST /api/checkout (Lambda, 256MB, node20)
Total cold init: 2,840 ms   (warm: 38 ms)

  runtime boot ................   210 ms   7%   (platform; fixed)
  dependency/module load ......  1,520 ms  54%  <- DOMINANT
      stripe sdk (eager) .........  340 ms
      @prisma/client (eager) .....  610 ms
      pdfkit (eager, unused @ req#1) 470 ms
  framework init ..............    180 ms   6%   prisma engine bootstrap
  first-connection setup ......    930 ms  33%  top-level await pool.connect()

2. Targeted fixes — ordered by the phase that dominates, each with the specific change and why it lands:

1. Lazy-load pdfkit behind await import() in the receipt path .. -470 ms  [HIGH]
   Not used by request #1; only the async receipt job needs it.
2. Move pool.connect() out of top-level await; connect on first
   handler use, reuse across invocations; pool max 2 ................ -930 ms cold,
   + eliminates connection-storm risk under burst .................. [HIGH]
3. Bump memory 256MB -> 1024MB (CPU scales) ................... -640 ms  [HIGH]
   Faster parse + prisma init; est. total cost -18% (shorter ms).
4. Bundle with esbuild, exclude aws-sdk (runtime-provided),
   strip source maps ................................................ -210 ms  [MED]
5. Provisioned concurrency = 3 on /checkout ONLY, after the above ... covers
   baseline concurrency; residual bursts now cost ~600ms not 2,840.  [LAST]

3. Measured before/after — the re-measured cold start after applying the fixes, proving the dominant phase actually shrank (and noting cost impact, since memory and provisioning change the bill):

Cold init: 2,840 ms -> 620 ms  (-78%)   p99 first-request: 3.1s -> 0.7s
Monthly cost: roughly flat (higher memory offset by shorter duration;
provisioned-concurrency on /checkout adds ~$X for 3 warm instances).
Re-measure after a real burst, not a single forced cold start.