Lambda SQS Partial Batch Failures: Poison Messages Without Infinite Redrive

// sqsPartial.ts
import type { SQSEvent, SQSBatchResponse, SQSRecord } from "aws-lambda";

export async function handler(event: SQSEvent): Promise<SQSBatchResponse> {
  const failures: { itemIdentifier: string }[] = [];

  await Promise.all(
    event.Records.map(async (rec) => {
      try {
        await processOne(rec);
      } catch (err) {
        console.error(JSON.stringify({
          msg: "record_failed",
          messageId: rec.messageId,
          err: String(err),
        }));
        failures.push({ itemIdentifier: rec.messageId });
      }
    })
  );

  // ✅ Only failing IDs — successes are deleted by Lambda/SQS
  return { batchItemFailures: failures };
}

async function processOne(rec: SQSRecord) {
  const body = JSON.parse(rec.body);
  if (!body.orderId) {
    // Poison: validation error — still report failure so it counts toward redrive,
    // or route to a quarantine queue explicitly if you prefer not to retry.
    throw new Error("poison_missing_orderId");
  }
  await idempotentWrite(body); // must tolerate duplicates
}

fn.addEventSource(new lambdaEventSources.SqsEventSource(queue, {
  batchSize: 10,
  reportBatchItemFailures: true, // ✅ required
  maxConcurrency: 40,
}));

# Queue redrive — poison lands here after N receives
RedrivePolicy:
  deadLetterTargetArn: !GetAtt Dlq.Arn
  maxReceiveCount: 5

// ❌ Anti-pattern: catch-all swallow
try {
  await processOne(rec);
} catch {
  // silent — message deleted, data lost
}