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What is PM2?

The Problem with node app.js​

When you SSH into a server and run your app directly:

node app.js

Three things happen that make this unsuitable for production:

  1. The process dies when you close the SSH session — it's tied to your terminal
  2. A crash kills the app — nothing restarts it automatically
  3. Server reboots kill the app — it doesn't come back after a restart

PM2 solves all three.

How PM2 Works​

PM2 runs as a daemon — a background service that outlives your SSH session. It:

  • Starts your Node.js app as a managed process
  • Monitors it and restarts it if it crashes
  • Can generate a startup script so it relaunches after a server reboot
  • Collects stdout/stderr into log files you can read at any time

Installing PM2​

npm install -g pm2

Verify:

pm2 --version

Core Commands​

Starting an App​

pm2 start app.js                        # start with default name ("app")
pm2 start app.js --name my-api          # give it a meaningful name
pm2 start app.js --watch                # restart on file changes (dev use)
pm2 start npm -- start                  # run `npm start` via PM2

Viewing Status​

pm2 status          # list all managed processes with CPU/memory
pm2 list            # alias for status
pm2 show my-api     # detailed info about a specific process

Example output of pm2 status:

┌─────┬──────────┬─────────────┬─────────┬─────────┬──────────┬────────┐
│ id  │ name     │ status      │ cpu     │ memory  │ uptime   │ restarts│
├─────┼──────────┼─────────────┼─────────┼─────────┼──────────┼────────┤
│ 0   │ my-api   │ online      │ 0%      │ 45.3mb  │ 2h       │ 0      │
└─────┴──────────┴─────────────┴─────────┴─────────┴──────────┴────────┘

Logs​

pm2 logs              # stream logs from all processes
pm2 logs my-api       # logs for a specific process
pm2 logs --lines 200  # show last 200 lines
pm2 flush             # clear all log files

Restarting and Stopping​

pm2 restart my-api    # restart (brief downtime)
pm2 reload my-api     # zero-downtime reload (graceful restart)
pm2 stop my-api       # stop without removing from PM2's list
pm2 delete my-api     # stop and remove from PM2's list

Cluster Mode​

Node.js runs on a single thread. On a server with 8 CPU cores, you're only using one. PM2's cluster mode forks your app across all available cores, multiplying throughput:

pm2 start app.js --name my-api -i max   # -i max = one instance per CPU core
pm2 start app.js -i 4                   # exactly 4 instances

PM2 automatically load-balances incoming connections across all instances. This is effectively free horizontal scaling without any code changes.

Surviving Server Reboots​

By default, PM2's process list is lost on reboot. Fix this with:

pm2 startup       # generates and prints a command to run
# → copy and run the printed command (it'll look like: sudo env PATH=... pm2 startup systemd -u ubuntu --hp /home/ubuntu)

pm2 save          # saves the current process list to disk

After this, PM2 will restart all your apps automatically whenever the server boots.

Real-Time Monitoring​

pm2 monit

Opens a terminal dashboard showing CPU usage, memory, logs, and event loop delay for all processes in real time.