federation

Introduction

Federation is a federated event emitter

Federation is a messaging network built on top of Axon, that lets any two nodes in the federation send routed messages to one and other.

Goals

• fault tolerance
• zero-configuration setup
• minimal and familiar api

Install

Running a Federation Base Server

All federations require a base server. Base servers are zero-configuration:

$ npm install -g federation
$ federation start
Federation Base Started at axon://10.0.1.1

Joining a Federation

Once a base server is established, modify your modules to join the federation.

$ npm install --save federation

In your server.js file add:

var fed  = require('federation').join('axon://10.0.1.1');
var node = fed.node(options);

The above node object is a citizen of the network, who can send and receive messages to other nodes.

Usage

A federation node operates like an event-emitter.

Send a message:

var launch_code = "12345puppy";
node.emit('president',launch_code);

Receive a message:

node.on('message',function(message){
    launch_the_missiles(message);
});

Address Space

Messages are sent to address that look like paths, both absolute and relative.

• absolute path - /america/whitehouse/president
• relative path - vice-president

Relative paths are resolved to an absolute path based on the address of the sender.

# given the following addresses
/canada/parliament/prime-minister <-- node A
/canada/parliament/house-speaker  <-- node B

# node A can message node B with the address `house-speaker`

Message Handlers

Emitted messages can hint at the type of message using a hash-tag (#). Nodes can choose to handle messages according to their hinted type.

node.emit('planet-express/fry#package',package);

The receiving node is planet-express/fry. Fry can catch that message with:

node.on('#package',function(package){
    put_in_spaceship(package);
});

Hinted messages that are not caught will be emitted as message.

Anonymous Handlers

You can receive replies to your messages by specifying anonymous handlers:

node.emit('planet-express/bender',beer).on('#reply',function(money){
    // thanks for the money bender!
});

A receiver can reply to messages by catching a reference to the sender in the handler:

node.on('message',function(message,sender){
    sender.emit('#reply','THANK YOU');
});

Handling Errors

Multiple anonymous handlers can be registered to the same transaction:

node.emit('bender#kill-all-humans',message)
.on('#reply',function(reply){
  // handle reply
})
.on('#error',function(err){
  // handle error
});

Specification

The specification contains the technical details of the project.

• federation uses the Axon PubEmitter / SubEmitter socket
• default port: 8973

Future Features

The following are features that may be cool in a beta release, but are not planned for the alpha.

• broadcast / multicast addresses
• multiple addresses