Meerkat

Rack middleware for Server-Sent Events (HTML5 SSE).

Requires an EventMachine backed server, like Thin or Rainbows (with the EventMachine backend only).

Features:

• Subscribe for single events
• Subscribe for multiple events with patterns
• Low memory and CPU usage
• Works with all proxies (unlike WebSockets)
• Allows publishing from server side as well as from the client side (with POST request)

Supported backends:

• In memory, using EventMachine Channels, good for single server usage.
• Redis, using em-hiredis and the Pub/Sub API.
• Postgres, using the Notify/Listen API.
  • When a message is published the topic and json payload is inserted into the 'meerkat_pubsub' table, and then a NOTIFY is issued.
  • Listening clients recivies the notification and reads the message from the table and writes it to the Event Stream of its clients.
  • On the next publish all messages older than 5 seconds are deleted.
  • No polling is ever done.
  • This works with PostgreSQL 8 and higher (tested with 8.3 and 9.1).

Usage

config.ru:

require 'bundler/setup'
require 'meerkat' 
require './app'

#Meerkat.backend = Meerkat::Backend::InMemory.new 
#Meerkat.backend = Meerkat::Backend::Redis.new 'redis://localhost/0'
Meerkat.backend = Meerkat::Backend::PG.new :dbname => 'postgres'
map '/' do
  run App
end
map '/stream' do
  run Meerkat::RackAdapter.new
end

On the client:

var source = new EventSource('/stream/mychannel');
var streamList = document.getElementById('stream');
// Use #onmessage if you only listen to one topic
source.onmessage = function(e) {
  var li = document.createElement('li');
  li.innerHTML = JSON.parse(e.data);
  streamList.appendChild(li);
}
var multiSource = new EventSource('/my/event/*');
// You have to add custom event listerns when you 
// listen on multiple topics
multiSource.addEventListener('/my/event/foo', function(e) {
  // Do something
}, false);
multiSource.addEventListener('/my/event/bar', function(e) {
  // Do something
}, false);

To push things from the client:

Meerkat.publish "/mychannel", {:any => hash}
Meerkat.publish "/mychannel/2", 'any string'
Meerkat.publish "/mychannel/3", any_object

The published objects will be JSON serialized (with Yajl) before sent to the backend. Deserialize it in the client.

From the client:

$.post('/stream/mychannel/2', { json: JSON.stringify(my_object) })

A simple POST request, with a parameter called 'json' containing a JSON string.

Read more about Server-Sent Events and the EventSource API on HTML5Rocks.

Examples

A simple demo can be seen here: http://meerkat-demo.herokuapp.com/

It's deployed on Heroku's Cedar stack. It's using the Redis backend, thanks to Redis To Go's free Nano offering.

License

(MIT license)

Copyright (C) 2011 by Carl Hörberg

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.