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.

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.

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);
});

1. 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.

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

pm2 reload server