Zero Downtime Restarts with PM2

When running a live application cluster with PM2, it’s critical to ensure that code changes can be deployed without causing downtime for users. PM2 provides a powerful feature called zero downtime reload, allowing seamless server updates. Here's how to perform zero downtime restarts effectively.

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The Scenario:

Your server cluster is running, and you need to make a change in your server code without affecting the live users. For example:

• Change the timer sound from "ding" to "beep".

• Reduce the timer delay from 9 seconds to 4 seconds.

Updating the server code and restarting the application traditionally would result in downtime, causing inconvenience to users. Instead, we’ll use PM2’s reload feature to apply changes with zero downtime.

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Steps to Implement Zero Downtime Restarts:

1. Make the Code Changes: Update your server code as required. For example:

    // Old: Ding sound and 9-second delay
    app.get('/timer', (req, res) => {
        setTimeout(() => {
            res.send(`Ding Ding Ding ${process.pid}`);
        }, 9000);
    });
   
    // New: Beep sound and 4-second delay
    app.get('/timer', (req, res) => {
        setTimeout(() => {
            res.send(`Beep Beep Beep ${process.pid}`);
        }, 4000);
    });
   

   Save your changes to the server file (e.g., server.js).

2. Verify Current Code Behavior: Before applying changes, confirm the current server behavior. For example, making a request to the endpoint might still take 9 seconds due to the old code.

3. Avoid Traditional Restart: Using a standard restart (e.g., pm2 restart server) would:

   • Terminate all processes at once.

   • Cause downtime while the server restarts.

4. Perform Zero Downtime Reload: Use the pm2 reload command to restart processes one by one:

    pm2 reload server
   

   • This ensures that at least one process remains online at all times.

   • Users can continue accessing the application without interruption.

5. Monitor the Reload Process:

   • Use the pm2 monit command to observe the reload in real-time.

   • Each process will restart sequentially, and the uptime will reset for individual processes.

6. Verify Updated Behavior: After the reload, confirm that the updated server code is running. For example:

   • Make a request to the server endpoint.

   • Observe the response time (4 seconds) and the new behavior ("Beep Beep Beep" sound).

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Benefits of Zero Downtime Reload:

• Ensures continuous application availability, even during code updates.

• Ideal for time-sensitive or high-traffic applications.

• Enhances user experience by avoiding scheduled or unscheduled downtime notifications.

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Monitoring and Debugging:

• Use pm2 monit for a live dashboard of CPU, memory usage, and process status.

• Logs are stored in the specified log file (e.g., logs.txt) for troubleshooting.

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By leveraging PM2’s reload feature, you can deploy updates confidently without disrupting user access, maintaining a reliable and professional application experience.