$ npm install rabbit.js
This library provides a simple, socket-oriented API* for messaging in node.js, using RabbitMQ as a backend.
var context = require('rabbit.js').createContext();
context.on('ready', function() {
var pub = context.socket('PUB'), sub = context.socket('SUB');
sub.pipe(process.stdout);
sub.connect('events', function() {
pub.connect('events', function() {
pub.write(JSON.stringify({welcome: 'rabbit.js'}), 'utf8');
});
});
});
*Yes, rather like ZeroMQ. See below.
This library is suitable for co-ordinating peers (e.g., Node.JS programs), acting as a gateway to other kinds of network (e.g., relaying to browsers via SockJS), or simply as a really easy way to use RabbitMQ.
The entry point is createContext, which gives you a factory for
sockets. You supply it the URL to your RabbitMQ server:
var context = require('rabbit.js').createContext('amqp://localhost');
To start sending or receiving messages you need to acquire a socket:
var pub = context.socket('PUB');
var sub = context.socket('SUB');
and connect it to something:
pub.connect('alerts');
sub.connect('alerts');
Sockets act like
Streams; in
particular you will get 'data' events from those that are readable,
and you can write() to those that are writable. If you're expecting
data that is encoded strings, you can setEncoding() to get strings
instead of buffers as data events.
sub.setEncoding('utf8');
sub.on('data', function(note) { console.log("Alarum! " + note); });
pub.write("Emergency. There's an emergency going on", 'utf8');
You can also use pipe to forward messages to or from another stream, making relaying simple:
sub.pipe(process.stdout);
Lastly, note that a socket may be connected more than once, by calling
socket.connect(x) with different xs. What this entails depends on
the socket type (see below), but messages to and from different
connect()ions are not distinguished. For example
var sub2 = context.socket('SUB');
sub2.connect('system');
sub2.connect('notifications');
Here, the socket sub2 will receive all messages published to
'system' and all those published to 'notifications' as well, but
it is not possible to discriminate between the sources.
The socket types, passed as an argument to Context#socket, determine
whether the socket is readable and writable, and what happens to
messages written to it. Socket types (but not necessarily sockets
themselves) should be used in the pairs described below.
To make the descriptions a bit easier, we'll say if
connect(x) is called on a socket for some x, the socket has a
connection to x.
PUBlish/SUBscribe: every SUB socket connected to gets each message sent by a PUB socket connected to ; a PUB socket sends every message to each of its connections. SUB sockets are readable only, and PUB sockets are writable only.
PUSH/PULL: a PUSH socket will send each message to a single connection, using round-robin. A PULL socket will receive a share of the messages sent to each to which it is connected, determined by round-robin at . PUSH sockets are writable only, and PULL sockets are readable only.
REQuest/REPly: a REQ socket sends each message to one of its connections, and receives replies in turn; a REP socket receives a share of the messages sent to each to which it is connected, and must send a reply in turn. REQ and REP sockets are both readable and writable.
A few modules have a socket-server-like abstraction; canonically, the
net module, but also for example SockJS and Socket.IO. These can be
adapted using something similar to the following.
var context = new require('rabbit.js').createContext('amqp://localhost');
var inServer = net.createServer(function(connection) {
var s = context.socket('PUB');
s.connect('incoming');
connection.pipe(s);
});
inServer.listen(5000);
This is a simplistic example; a bare TCP socket won't in general emit data in chunks that are meaningful to applications, even if they are written that way at the far end. A library such as spb can be used encode and decode message streams in byte streams if needed.
Each subdirectory of example has code demonstrating using
rabbit.js with other modules. Install the prerequisites for rabbit.js
first:
rabbit.js$ npm install
Now each example can be run with, e.g.,
rabbit.js$ cd example/sockjs
sockjs$ npm install && npm start
All of the examples assume there is a RabbitMQ server
running locally. The SockJS and
Socket.IO examples both start a website which you can visit at
http://localhost:8080.
rabbit.js was inspired by the RabbitMQ/ZeroMQ adapter I developed with Martin Sústrik. The rationale for using RabbitMQ in a ZeroMQ-based network is largely transferable to rabbit.js:
- RabbitMQ introduces a degree of monitoring and transparency, especially if one uses the web management app;
- RabbitMQ can bridge to other protocols (notably AMQP and STOMP);
- RabbitMQ provides reliable, persistent queues if desired
with some additional benefits:
- since rabbit.js sockets implement the
Streaminterface, one can easily pipe messages around - using RabbitMQ as a backend obviates some configuration management -- just supply all instances the broker URL and you're good to go.
- there's room in the API for more involved routing and other behaviour since AMQP is, well, more complicated let's say.
Here are some notable differences and similarities to ZeroMQ in rabbit.js's API and semantics.
To start, there's no distinction in rabbit.js between clients and
servers (connect and bind in ZeroMQ, following the BSD socket
API), since RabbitMQ is effectively acting as a relaying server for
everyone to connect to. Relatedly, the argument supplied to
connect() is abstract, in the sense that it's just a name rather
than a transport-layer address.
Request and Reply sockets have very similar semantics to those in ZeroMQ. Requesting sockets must take care not to issue more than request at a time, or to label requests (and rely on repliers preserving the label in replies) such that the answers can be correlated with the requests. Actually this is much the same as ZeroMQ; it follows from the possibility of replies coming back out of order due to round-robining. Repliers must respond to requests in the order that they come in, and respond exactly once to each request.
There are no DEALER or ROUTER sockets (a.k.a., XREQ and XREQ) in rabbit.js. In ZeroMQ these are implemented by prefixing messages with a reverse path, which then requires encoding and thereby complication when relaying to other streams or protocols. Instead, rabbit.js notes the reverse path as messages are relayed to a REP socket, and reapplies it when the response appears (giving rise to the ordering requirement on repliers).
rabbit.js makes some simplifying assumptions that must be kept in mind when integrating with other protocols that RabbitMQ supports.
PUB and SUB sockets declare non-durable fanout exchanges named for the
argument given to connect. To send to SUB sockets or receive from
PUB sockets, publish or bind (or subscribe in the case of STOMP) to
the exchange with the same name.
PUSH, PULL, REQ and REP sockets use durable, non-exclusive queues
named for the argument given to connect. If you are replying, be
sure to follow the convention of sending the response to the queue
given in the 'replyTo' property of the request message.
I removed it. It wasn't entirely wrong, but it did have two failings: firstly, it exposed the socket type and the address to the (end) client, while the client ought not to need know about them; secondly, and more fatally, it required a dedicated client connection per socket, which is a problem for e.g., browsers.
The new API avoids these problems by not requiring any particular behaviour from a client connection -- that is totally up to you (if you even use any client connections). If you need to multiplex on a client connection you can do that by, say, prefixing each message with a channel name; or, as the Socket.IO example does, combine two simplex sockets onto a duplex client connection.