Clustering / Multi-process Logging

If you’re running log4js in an application that uses node’s core cluster then log4js will transparently handle making sure the processes don’t try to log at the same time. All logging is done on the master process, with the worker processes sending their log messages to the master via process.send. This ensures that you don’t get multiple processes trying to write to the same file (or rotate the log files) at the same time.

This can cause problems in some rare circumstances, if you’re experiencing weird logging problems, then use the disableClustering: true option in your log4js configuration to have every process behave as if it were the master process. Be careful if you’re logging to files.

I’m using PM2, but I’m not getting any logs!

To get log4js working with PM2, you’ll need to install the pm2-intercom module.

pm2 install pm2-intercom

Then add the value pm2: true to your log4js configuration. If you’re also using node-config, then you’ll probably have renamed your NODE_APP_INSTANCE environment variable. If so, you’ll also need to add pm2InstanceVar: '<NEW_APP_INSTANCE_ID>' where <NEW_APP_INSTANCE_ID> should be replaced with the new name you gave the instance environment variable.

log4js.configure({
  appenders: { out: { type: "stdout" } },
  categories: { default: { appenders: ["out"], level: "info" } },
  pm2: true,
  pm2InstanceVar: "INSTANCE_ID",
});

I’m using Passenger, but I’m not getting any logs!

Passenger replaces the node.js core cluster module with a non-functional stub, so you won’t see any output using log4js. To fix this, add disableClustering: true to your configuration. Again, be careful if you’re logging to files.

I’m not using clustering/pm2/passenger but I do have multiple processes that I’d like to all log to the same place

Ok, you probably want to look at the tcp-server and tcp appender documentation.