Lambda Telemetry API Extensions: Custom Samplers Under Five Milliseconds

// extension/index.ts — illustrative Telemetry API client
const LISTENER = `http://${process.env.AWS_LAMBDA_RUNTIME_API}/2022-07-01/telemetry`;

await fetch(LISTENER, {
 method: "PUT",
 body: JSON.stringify({
 schemaVersion: "2022-11-29",
 types: ["platform", "function"],
 buffering: { timeoutMs: 100, maxBytes: 256_000, maxItems: 200 },
 destination: { protocol: "HTTP", URI: "http://sandbox:4321" },
 }),
});

const queue: unknown[] = [];
const MAX_QUEUE = 500;

Deno.serve({ port: 4321 }, async (req) => {
 const batch = await req.json();
 for (const e of batch) {
 if (queue.length >= MAX_QUEUE) break; // DROP — never block
 if (shouldSample(e)) queue.push(e);
 }
 return new Response("ok");
});

function shouldSample(e: any): boolean {
 if (e.type === "platform.runtimeDone" && e.record?.status === "error") return true;
 if (e.record?.tracing?.sampled) return true;
 return Math.random() < 0.05; // 5% baseline
}

// Measure extension overhead with platform reports + custom metric
// Target: p99(extension_processing_ms) < 5 for latency-sensitive functions

export function assertBudget(started: number) {
 const dt = performance.now() - started;
 if (dt > 5) {
 // metric only — do not throw into the runtime
 process.stdout.write(JSON.stringify({ _aws: { Timestamp: Date.now(), CloudWatchMetrics: [{
 Namespace: "LambdaExt", MetricData: [{ MetricName: "SampleOverBudget", Value: 1, Unit: "Count" }],
 }]}}) + "
");
 }
}