node-leader is a micro library that implements the standard ZooKeeper leader election algorithm.
While functional, this is a little bit work-in-progress, as not all ZooKeeper events/errors are handled, nor is error propagation to the client very "nice".
var assert = require('assert');
var elect = require('leader').elect;
var log4js = require('log4js');
var opts = {
zookeeper: 'localhost:2181',
log4js: log4js,
znode: '/my/service/election'
};
elect(opts, function(err, node) {
assert.ifError(err);
if (node.leader) {
console.log('Hooray! I am the leader!');
} else {
node.on('leader', function() {
console.log('I got elected as the leader!');
});
}
});
$ npm install leader
As mentioned above, this is a verbatim implementation of the ZooKeeper "recipe" for doing leader election.
If that's hard to parse, it looks like this in ZooKeeper. All nodes in your
system use some path as the place to watch. Everybody registers with a
ZooKeeper-generated sequential key, and the node with the lowest key wins.
Everyone that's not a leader registers to watch the next node in the
ecosystem, and when that node gets deleted (which happens if the node
disconnects), then $self looks to see if they're the leader, and if not, sets
up a watch on the next node. As an example, let's assume /foo is the place
in ZooKeeper we're talking about:
NodeA -> /foo/01
^
|
Watch (01)
|
|
NodeB -> /foo/02
^
|
Watch (02)
|
|
NodeC -> /foo/03
...
Now, let's say NodeB crashes. NodeC will get a notification from ZooKeeper,
and go look for children of /foo. Upon seeing that 03 is not the lowest,
NodeC will not be the leader, but instead will register to watch NodeA:
NodeA -> /foo/01
^
|
Watch (01)
|
|
NodeC -> /foo/03
...
Now, if some time later, NodeB rejoins (because you know, someone in meatspace
got paged and brought it back up), it would likely get 04 and be watching
NodeC. Lastly, if NodeA crashed, NodeC would get the leader event, and
you'd go do whatever it is you need to do to take over that responsibility.
The MIT License (MIT) Copyright (c) 2012 Mark Cavage
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