Subjects

Why
What
How
When
Where

About Me

20+ years in software industry
1985-1996 AT&T Bell Labs/Lucent
1997+ Just Objects B.V.
- independent consultant/developer/trainer
- emphasis on OOT/Java/Multimedia/Webtech/AJAX
- OSS projects: KeyWorx, Pushlets, GeoTracing, CowCatcher
- 2004+: locative media and GIS projects
- 2006+: fulltime GeoTracing projects
Neogeographer

Pushlets in a Nutshell

HTTP-based publish/subscribe

publish: GET/POST or method call
subscribe: GET stream, pull and poll modes
topic tree
protocol formats: JS, XML, (JSON)
use any client that speaks HTTP
extensibility: declarative classes e.g. event sources
integration: separate .war or within app .war
impl: Java (J2EE/servlets), JavaScript (AJAX)

Why

IP-port limitations in (corporate) firewalls
instability of applet/Flash solutions
pure HTML solution gives optimal web-page integration
page-refresh often expensive/annoying

Protocol

session: join/leave
- session id (no cookies)
- all requests must include session id
subscription: subscribe/unsubscribe
- subscribe to one or more "subjects" (topics)
publication
- publish events to topic
data: listen
- receive messages (called Events)
- three modes (poll, stream, pull)
- may be combined with join (join-listen) for simple clients

Topic Tree

publish/subscribe to "subjects"
- strings, e.g. /chat/room42
- hierarchical, e.g. /stocks/aex , /stocks/aex/philips
- subscribe to levels below in hierachy
contract between publisher/subscriber
- application-specific

Dataflow

Data Listen Modes

These determine behaviour in C/S interaction

stream: like HTTP video streaming
poll: ordinary polling
pull: like fishing
- events present: return
- events not present: block until timeout or event avail
client indicates mode or server may enforce (config or browser)

Poll Mode

Stream Mode

Pull Mode (1) - Block

Pull Mode (2) - Message Avail

Pull Mode (3) - Timeout

Event Publication

local
- using configured EventSources
- method call through Dispatcher
remote (HTTP)
- using protocol

Web Client Example


<html>
  <head>
   <script type="text/javascript" src="../../lib/js-pushlet-client.js"></script>

   <script type="text/javascript">
     function init() {
       p_join_listen('/stocks/aex');
     }

     function onData(pushletEvent) {
       var stock = pushletEvent.get('name');
       var rate = pushletEvent.get('rate');
     }
    </script>
  </head>
  <body onLoad="init()">
    <script type="text/javascript">p_embed()</script>
       .
       . (HTML elements to present rates)
  </body>
  </html>

Overview

Subjects

About Me

Pushlets in a Nutshell

HTTP-based publish/subscribe

Why

Protocol

Topic Tree

Dataflow

Data Listen Modes

Poll Mode

Stream Mode

Pull Mode (1) - Block

Pull Mode (2) - Message Avail

Pull Mode (3) - Timeout

Event Publication

JavaScript Clients

Web Client Example

Other Clients

Configuration

Extensibility

Integration and Deployment