Real-time communication using Google App Engine Channel API

Google App Engine

Real-time communication using
Channel API

What Is Google App Engine(GAE)?
• Google App Engine lets you run web applications on
Google's infrastructure.

• With App Engine, there are no servers to maintain: you
just upload your application, and it's ready to serve your
users.

• Google App Engine applications can currently be written
in Java, Python, or Go.

Push and Pull
• Pull:
– AJAX

• Push:
– WebSockets

Channel API(Push)
• Creates a persistent connection between your
application and Google servers

• Allows your application to send messages to JavaScript
clients in real time without the use of polling.

Channel API(Push)
Javascript Client:
– Connecting to the channel once it receives the channel’s unique
token from the server
– Listening on the channel for updates
– Sending update messages to the server
The Server:
– Creating a unique channel for individual JavaScript clients
– Creating and sending a unique token to each JavaScript client
so they may connect and listen to their channel
– Receiving update messages from clients via HTTP requests
– Sending update messages to clients via their channels
– Optionally, managing client connection state.

Channel API(Push)
The Client ID:
– The Client ID is responsible for identifying individual JavaScript
clients on the server.
– A Client ID can be anything that makes sense in the design of
your application.
Tokens:
– Tokens are responsible for allowing the JavaScript Client to
connect and listen to the channel created for it.
– The server creates one token for each client using information
such as the client’s Client ID and expiration time.
– Tokens expire after two hours and should also be treated as
secret.

Channel API(Push)
The Channel
– A channel is a one-way communication path through which the
server sends updates to a specific JavaScript client identified by
its Client ID.
– The server receives updates from clients via HTTP requests,
then sends the messages to relevant clients via their channels.
The Message
– Messages are sent via HTTP requests from one client to the
server.
– Once received, the server passes the message to the
designated client via the correct channel identified by the Client
ID.
– Messages are limited to 32K.

Channel API(Push)
<body>
<script>
channel = new goog.appengine.Channel('{{ token }}');
socket = channel.open();
socket.onopen = onOpened;
socket.onmessage = onMessage;
socket.onerror = onError;
socket.onclose = onClose;
</script>
</body> sendMessage = function(path) {
path += '?g=' + message;
var xhr = new XMLHttpRequest();
xhr.open('POST', path, true);
xhr.send();
};

Demo: http://gdg-bucharest.appspot.com

Real-time communication using Google App Engine Channel API

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