Lambda Log Buffering: Cut CloudWatch Ingestion Without Losing Correlation

// handler.ts
import { Logger } from "@aws-lambda-powertools/logger";
import { injectLambdaContext } from "@aws-lambda-powertools/logger/middleware";
import middy from "@middy/core";

const logger = new Logger({
  serviceName: "checkout",
  logLevel: "INFO",
  // Powertools buffer: hold debug/info until error or explicit flush
});

export const handler = middy(async (event, ctx) => {
  logger.appendKeys({
    correlationId: event.headers?.["x-correlation-id"] ?? ctx.awsRequestId,
    tenantId: event.requestContext?.authorizer?.tenantId,
  });

  logger.info("start", { path: event.rawPath });
  try {
    const result = await checkout(event);
    // ✅ Sample successes: 1% INFO after success
    if (Math.random() < 0.01) logger.info("success_sample", { resultCode: result.code });
    return result;
  } catch (err) {
    // ✅ Errors always flush full buffer + stack
    logger.error("checkout_failed", { err });
    throw err;
  }
}).use(injectLambdaContext(logger, { clearState: true }));

// lib/sample.ts
export function shouldLogSuccess(sampleRate = 0.01) {
  return Math.random() < sampleRate;
}

import { Metrics, MetricUnit } from "@aws-lambda-powertools/metrics";
const metrics = new Metrics({ namespace: "Checkout", serviceName: "checkout" });

metrics.addMetric("CheckoutOk", MetricUnit.Count, 1);
metrics.publishStoredMetrics();