In this article, we will analyze the causes of Heroku H12 timeout errors, explore debugging techniques, and provide best practices to optimize application performance and request handling.

Understanding H12 Timeout Errors in Heroku

H12 timeout errors occur when a request takes longer than 30 seconds to complete, causing Heroku’s router to terminate the connection. Common causes include:

• Long-running queries or processing tasks blocking request handling.
• Improper worker dyno configuration leading to overloaded web dynos.
• Slow database queries causing request bottlenecks.
• High memory or CPU usage leading to process starvation.
• Network latency or slow external API calls delaying responses.

Common Symptoms

• Requests failing with H12 Request timeout errors in logs.
• Application becoming unresponsive under load.
• Slow database queries affecting response times.
• Memory and CPU usage spikes in Heroku Metrics.
• Intermittent failures even when the app appears healthy.

Diagnosing H12 Timeout Errors

1. Checking Heroku Logs

Inspect logs to identify timeouts:

heroku logs --tail

2. Monitoring Dyno Load

Check dyno performance and resource usage:

heroku ps

3. Analyzing Slow Database Queries

Identify queries taking too long to execute:

SELECT pid, age(clock_timestamp(), query_start), query FROM pg_stat_activity WHERE state != 'idle';

4. Tracking External API Calls

Log API call durations to detect slow dependencies:

console.time("API Request");
fetch("https://external-api.com/data").then(() => console.timeEnd("API Request"));

5. Debugging Long-Running Processes

Ensure background jobs don’t block the main thread:

setTimeout(() => console.log("Processing done"), 10000);

Fixing H12 Timeout Errors in Heroku

Solution 1: Moving Heavy Tasks to Background Workers

Use worker dynos for long-running tasks:

heroku scale worker=1

Solution 2: Optimizing Database Queries

Ensure indexes are used efficiently:

CREATE INDEX idx_user_email ON users(email);

Solution 3: Using Asynchronous Processing

Offload heavy computations to background jobs:

const queue = require("bull");
const jobQueue = new queue("processing");
jobQueue.process(async (job) => {
    await performHeavyComputation(job.data);
});

Solution 4: Increasing Request Timeout for External APIs

Use a timeout handler for slow API requests:

fetch("https://external-api.com/data", { timeout: 5000 })

Solution 5: Scaling Dynos to Handle More Requests

Increase dyno count for better load distribution:

heroku ps:scale web=2

Best Practices for Avoiding H12 Timeouts

• Move long-running tasks to worker dynos.
• Optimize database queries for faster execution.
• Use asynchronous processing for heavy computations.
• Monitor Heroku logs and dyno metrics for performance issues.
• Scale dynos to handle increased request loads.

Conclusion

H12 timeout errors in Heroku can disrupt application performance and user experience. By optimizing request handling, leveraging background workers, and scaling dynos appropriately, developers can prevent timeouts and ensure a responsive application.

FAQ

1. Why does my Heroku app keep hitting H12 timeouts?

Long-running requests, slow database queries, or excessive CPU/memory usage can cause timeouts.

2. How can I diagnose H12 errors?

Check Heroku logs, monitor dyno performance, and analyze slow queries.

3. What is the best way to prevent H12 timeouts?

Move blocking tasks to worker dynos and optimize queries for better performance.

4. Can scaling dynos help with H12 errors?

Yes, increasing dyno count can help distribute load and prevent request congestion.

5. How do I optimize external API calls on Heroku?

Use request timeouts and caching mechanisms to reduce API response delays.