Bunyan File Logging With Cluster

index.js

'use strict';

var cluster = require('cluster');
var bunyan = require('bunyan');

if(cluster.isMaster) {
  console.log('Master starting up! PID: ' + process.pid);

  // Fork 20 workers.
  for (var i = 0; i < 20; i = i + 1) {
    cluster.fork();
  }

  // Listen for SIGUSR2 signal sent to master process.
  process.on('SIGUSR2', function () {
    console.log('Master process received SIGUSR2. Alerting workers to reopen logs.');
    for (var id in cluster.workers) {
      cluster.workers[id].send('reopen_log_file');
    }
  });
} else {
  console.log('Worker ' + cluster.worker.id + ' starting up!');

  // Make a file logger.
  var logger = bunyan.createLogger({
    name: 'test-log',
    streams: [
      {
        path: '/var/log/bunyan-cluster/test.log'
      }
    ]
  });

  // In worker processes listen for messages sent from the master process.
  process.on('message', function (msg) {
    if (msg === 'reopen_log_file') {
      console.log('Worker ' + cluster.worker.id + ' is reopening connection to log file.');
      logger.reopenFileStreams();
    }
  });

  // Write to log every 5 seconds.
  setInterval(
    function () {
      logger.info('Worker ' + cluster.worker.id + ' reporting in!');
    }, 
    5000
  );
}

Just an example sketch of how to log to a file using bunyan and cluster.

This script starts 20 workers logging to /var/log/bunyan-cluster/test.log. If test.log is moved somewhere else, say to test.log.0...

mv test.log test.log.0

...the script will keep chugging along but the workers are now writing to nowhere 😱 . We need to tell bunyan to refresh its file connection. We can do this by sending the master process a SIGUSR2 signal, which will in turn tell each thread to reopen its log file.

You can send the signal like this:

kill -31 85434

31 being the code for SIGUSR2 on my mac (which I looked up using kill -l) and 85434 being the pid of the master process (which is output to the console on startup).

After the signal is sent we can see that a new test.log is created and all the workers are writing to it.

If you wanted to delegate log rotation to logrotate you could set it up by making a /etc/logrotate.d/bunyan-cluster with the following content:

/var/log/bunyan-cluster/test.log {
    hourly
    rotate 100
    postrotate
        kill -31 85434
    endscript
}

This will rotate the log file every hour, keeping the last 100 files and sending the refresh connection signal immediately after every rotation.