node-zkplus

zkplus is the API you wish ZooKeeper had for Node.js. The zkplus API resembles Node's fs module quit a bit, with the caveat that data is always assumed to be JSON. That seems sensible and universal for most uses of ZK, and indeed makes this API quite a bit nicer. If you're doing something crazy like storing images and videos in ZooKeeper, you're doing it wrong, so move along.

At a high-level, this API provides facilities for creating "directories", "files", and setting "watches" (ZK only provides the latter as an actual primitive; everything else is approximated here).

Installation

npm install zkplus

Usage

var assert = require('assert');
var zkplus = require('zkplus');

var client = zkplus.createClient({
    connectTimeout: 4000,
    servers: [{
        host: '127.0.0.1',
        port: 2181
    }]
});

client.connect(function (err) {
    assert.ifError(err);
    client.mkdirp('/foo/bar', function (err) {
        assert.ifError(err);
        client.rmr('/foo', function (err) {
            assert.ifError(err);
            client.close();
        });
    });
});

API

zkplus.createClient(options)

Creating a client is straightforward, as you simply invoke the createClient API, which takes an options object with the options below. Note that the servers parameter can be omitted if you only want to talk to a single ZooKeeper node; in that case, you can just use host as a top-level argument (useful for development).

var zkplus = require('zkplus');

var client = zkplus.createClient({
    host: 'localhost'
});

The returned client object will be an instance of a ZKClient:

Class: zkplus.ZKClient

This is an EventEmitter with the following events and methods.

Event: 'close'

function onClose() { }

Emitted when the client has been disconnected from the ZooKeeper server.

Event: 'connect'

function onConnect() { }

Emitted when the client has connected (or reconnected) to the ZooKeeper server.

Event: 'error'

function onError(err) { }

If the client driver has an unexpected error, it is sent here.

ZKClient.connect(callback)

Explicitly connects to the ZooKeeper server(s) passed in at instantiation time. The only argument to the callback is an optional Error.

ZKClient.close([callback])

Shuts down the connection to the ZooKeeper server(s). Emits the close event when done. The only argument to the callback is an optional Error.

ZKClient.create(path, object, [options], callback)

Creates a znode in ZooKeeper. The object parameter must be a JSON object that will be saved as the raw data, and options may include flags. Flags allow you to specify the useful semantics that Zookeeper offers, namely sequences and ephemeral nodes (sequence and ephemeral, respectively).

Additionally, zkplus has additional functionality that enables you to create an ephemeral node that is automatically recreated across connection drops. Recall that Zookeeper ephemeral nodes are deleted when a session closes (as per documentation); emperically the way connections/sessions work, it pretty much means you need to assume that the ephemeral node is dropped on connection loss. Thus, zkplus has an ability to automatically recreate any ephemeral nodes on reconnect. A flag of ephemeral_plus will enable this behavior.

callback is of the form function (err, path), where path is the newly created node (which you would need on using a sequence).

var data = {
    foo: 'bar'
};
var opts = {
    flags: ['sequence', 'ephemeral_plus']
};
client.create('/foo/bar', opts, function (err, path) {
    assert.ifError(err);
    console.log(path); // => /foo/bar/00000000
});

ZKClient.get(path, callback)

Returns the data associated with a znode, as a JS Object (remember, zkplus assumes all data is JSON). Callback is of the form function (err, object)

client.get('/foo/bar/00000000', function (err, obj) {
    assert.ifError(err);
    console.log('%j', obj); // => { "hostname": "your_host_here" }
});

ZKClient.getState()

Returns the state of the underlying ZooKeeper driver. Possible states are:

• connected
• disconnected
• expired
• unknown

ZKClient.mkdirp(path, callback)

Does what you think it does. Recursively creates all znodes specified if they don't exist. Note this API is idempotent, as it will not error if the path already exists. Callback is of the form function (err).

client.mkdirp('/foo/bar/baz', function (err) {
    assert.ifError(err);
});

ZKClient.put(path, object, [options], callback)

Overwrites path with object. Callback is of the form function (err).

client.put('/foo/bar/hello', {value: 'world'}, function (err) {
    assert.ifError(err);
});

ZKClient.readdir(path, callback)

Lists all nodes under a given path, and returns you the keys as relative paths only. The keys returned will be sorted in ascending order. callback is of the form function (err, nodes).

client.readdir('/foo/bar', function (err, nodes) {
    assert.ifError(err);
    console.log(nodes.join()); // => ['00000000', 'baz']
});

ZKClient.rmr(path, callback)

Recursively deletes everything under a given path. I.e., what you'd think rm -r would be. callback is of the form function (err).

client.rmr('/foo/bar', function (err) {
    assert.ifError(err);
});

ZKClient.stat(path, callback)

Returns a ZK stat object for a given path. ZK stats look like:

{
    czxid,           // created zxid (long)
    mzxid,           // last modified zxid (long)
    ctime,           // created (Date)
    mtime,           // last modified (Date)
    version,         // version (int)
    cversion,        // child version (int)
    aversion,        // acl version (int)
    ephemeralOwner,  // owner session id if ephemeral, 0 otw (string)
    dataLength,      //length of the data in the node (int)
    numChildren,     //number of children of this node (int)
    pzxid            // last modified children (long)
}

Reference the ZooKeeper documentation for more info.

client.stat('/foo', function (err, stats) {
    assert.ifError(err);
    console.log('%j', stats);  // => stuff like above :)
});

ZKClient.unlink(path, [options], callback)

Removes a znode from ZooKeeper.

client.unlink('/foo', function (err) {
    assert.ifError(err);
});

client.unlink('/foo', {version: 0}, function (err) { ... });

ZKlient.watch(path, [options], callback)

The watch API makes usable the atrociousness that are ZooKeeper notifications (although, as unusable as they are, they're one of its most useful features). Using this API, you are able to set watches any time the content of a single node changes, or any time children are changed underneath that node. Unlink the raw ZooKeeper API, this will also automatically "rewatch" for you, such that future changes are still fired through the same listener.

The defaults for this API are to listen only for data changes, and not to return you the initial data (i.e., assume you already know what you've got, and just want to get notifications about it). The options parameter drives the other behavior, and specifically allows you to set two flags currently: method and initialData. method defaults to data, and the semantics are such that only content changes to the znode you've passed in via path will be listened for. If you set method to list, then the semantics of the watch are to notify you when any children change (add/del) under path. You cannot listen for both simultaneously; if you want both, you'll need to set two watches. On updates, the returned stream will fire data events.

Additionally, the semantics are not to perform a get, but to only notify you on updates. Setting initialData to true will make the watch fire once "up front".

callback is of the form function (err, listener).

client.watch('/foo', function (err, listener) {
     assert.ifError(err);
     listener.on('error', function (err) {
        console.error(err.stack);
        process.exit(1);
     });

     listener.on('data', function (obj) {
        console.log('%j', obj); // => updated record
     });

     listener.on('end', function () {
        // `end` has been called, and watch will no loner fire
     });
});

client.watch('/foo', { method: 'list' }, function (err, listener) {
     assert.ifError(err);
     listener.on('error', function (err) {
         console.error(err.stack);
         process.exit(1);
     });

     listener.on(data, function (children) {
         console.log('%j', children); // => ['00000000', 'bar', ...]
     });
});

Tests

To launch tests you'll need a running zookeeper instance.

export ZK_HOST=$your_zk_ip_here
cd node-zkplus
make prepush

License

The MIT License (MIT) Copyright (c) 2012 Mark Cavage

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Bugs

See https://github.com/mcavage/node-zkplus/issues.