Tail Workers Gotchas & Debugging

Critical Pitfalls

1. Not Using ctx.waitUntil()

Problem: Async work doesn't complete or tail Worker times out Cause: Handlers exit immediately; awaiting blocks processing Solution:

// ❌ WRONG - fire and forget
export default {
  async tail(events) {
    fetch(endpoint, { body: JSON.stringify(events) });
  }
};

// ❌ WRONG - blocking await
export default {
  async tail(events, env, ctx) {
    await fetch(endpoint, { body: JSON.stringify(events) });
  }
};

// ✅ CORRECT
export default {
  async tail(events, env, ctx) {
    ctx.waitUntil(
      (async () => {
        await fetch(endpoint, { body: JSON.stringify(events) });
        await processMore();
      })()
    );
  }
};

2. Missing tail() Handler

Problem: Producer deployment fails Cause: Worker in tail_consumers doesn't export tail() handler Solution: Ensure export default { async tail(events, env, ctx) { ... } }

3. Outcome vs HTTP Status

Problem: Filtering by wrong status Cause: outcome is script execution status, not HTTP status

// ❌ WRONG
if (event.outcome === 500) { /* never matches */ }

// ✅ CORRECT
if (event.outcome === 'exception') { /* script threw */ }
if (event.event?.response?.status === 500) { /* HTTP 500 */ }

4. Timestamp Units

Problem: Dates off by 1000x Cause: Timestamps are epoch milliseconds, not seconds

// ❌ WRONG: const date = new Date(event.eventTimestamp * 1000);
// ✅ CORRECT: const date = new Date(event.eventTimestamp);

5. Type Name Mismatch

Problem: Using TailItem type Cause: Old docs used TailItem, SDK uses TraceItem

import type { TraceItem } from '@cloudflare/workers-types';
export default {
  async tail(events: TraceItem[], env, ctx) { /* ... */ }
};

6. Excessive Logging Volume

Problem: Unexpected high costs Cause: Invoked on EVERY producer request Solution: Sample events

export default {
  async tail(events, env, ctx) {
    if (Math.random() > 0.1) return;  // 10% sample
    ctx.waitUntil(sendToEndpoint(events));
  }
};

7. Serialization Issues

Problem: JSON.stringify() fails Cause: log.message is unknown[] with non-serializable values Solution:

const safePayload = events.map(e => ({
  ...e,
  logs: e.logs.map(log => ({
    ...log,
    message: log.message.map(m => {
      try { return JSON.parse(JSON.stringify(m)); }
      catch { return String(m); }
    })
  }))
}));

8. Missing Error Handling

Problem: Tail Worker silently fails Cause: No try/catch Solution:

ctx.waitUntil((async () => {
  try {
    await fetch(env.ENDPOINT, { body: JSON.stringify(events) });
  } catch (error) {
    console.error("Tail error:", error);
    await env.FALLBACK_KV.put(`failed:${Date.now()}`, JSON.stringify(events));
  }
})());

9. Deployment Order

Problem: Producer deployment fails Cause: Tail consumer not deployed yet Solution: Deploy tail consumer FIRST

cd tail-worker && wrangler deploy
cd ../producer && wrangler deploy

10. No Event Retry

Problem: Events lost when handler fails Cause: Failed invocations NOT retried Solution: Implement fallback storage (see #8)

Debugging

View logs: wrangler tail my-tail-worker

Incremental testing:

1. Verify receipt: console.log('Events:', events.length)
2. Inspect structure: console.log(JSON.stringify(events[0], null, 2))
3. Add external call with ctx.waitUntil()

Monitor dashboard: Check invocation count (matches producer?), error rate, CPU time

Testing

Add test endpoint to producer:

export default {
  async fetch(request) {
    if (request.url.includes('/test')) {
      console.log('Test log');
      throw new Error('Test error');
    }
    return new Response('OK');
  }
};

Trigger: curl https://producer.example.workers.dev/test

Common Errors

Performance Notes

• Max 100 events per invocation
• Each consumer receives all events independently
• CPU limits same as regular Workers
• For high volume, use Durable Objects batching