Safe code hotswap for zero downtime re-deploys.

An application in production should not need to be stopped in order to utilize new code.

liveswap is a library that helps you update an application’s code without stopping it. It also ships with a command-line tool and it doesn’t impose any special requirements or conventions on your application code.

It works by creating a light-weight master process that runs your application code as worker processes. It then starts listening for instructions.

When you send an upgrade instruction, workers take turns disconnecting their underlying servers. When a server is disconnected, it lets the old connections finish up, but no new connections are accepted. When all of the worker’s connections have been closed, it will be retired and a new one will be created using the new code. For more in depth information read this blog post.

npm install liveswap -g

Using the node.js hello world example, in a filed named index1.js...

var http = require('http')
http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'})
  res.end('Hello World\n')
}).listen(1337, '127.0.0.1')
console.log('Server running at http://127.0.0.1:1337/')

Start the application using liveswap from the command-line...

liveswap -s index1.js

Alternatively, start liveswap programmatically...

var liveswap = require('liveswap')
liveswap('./index1.js')

Now we have an updated version of the code in index2.js...

var http = require('http')
http.createServer(function (req, res) {
  res.writeHead(404, {'Content-Type': 'text/plain'})
  res.end('Not Found\n')
}).listen(1337, '127.0.0.1')
console.log('Server running at http://127.0.0.1:1337/')

Update the current code by sending an update command to the server.

liveswap -u ./index2.js

After sending an update message the worker processes will wait for any current connections to end before restarting with the new code.

Options:
  -p, --port     <port> specify the liveswap server port.                      [default: 3000]
  -a, --address  <address> specify the ip address to run on.                   [default: "127.0.0.1"]
  -f, --forks    <number> specify how many worker processes to spawn           [default: 2]
  -u, --upgrade  [<path>] specify the source code to upgrade to.
  --pre-upgrade  <path> a module to handle pre upgrade logic.
  -k, --kill     kill all forked worker processes and respawn.
  -d, --die      kill all forked worker processes and quit master process.
  -m, --message  <message> send a message to all the forked worker processes.
  -s, --start    <path> start a node process cluster.
  -H, --head     <path> path to HEAD file
  -z             disable zero-downtime, upgrade will kill processes
  -v, --version  print program version and exit

Pre-upgrade allows you to require a module that will be executed before each time the upgrade happens.

liveswap --pre-upgrade ./pull.js --start ./index1.js

The pre-upgrade module should export a single function as its interface. Here's an example of what that module might look like:

function preupgrade(data, callback) {
  console.log('executing pre-upgrade script...');

  var err, value = data.value;
  try {
    // execute pre-upgrade code
  } catch (e) {
    err = e.toString();
  }

  callback(err, value);
}
module.exports = preupgrade;

The main export of this library accepts an options object or a string. If a string is specified, it will be interpreted as the target option.

liveswap({
  target: './index.js',
  port: 9008,
  address: '0.0.0.0',
  forks: 2,
  head: '/tmp/HEAD',
  'pre-upgrade': './pull.js',
  'zero-downtime': false
})