May 2008 - Alessandro Alinone's talk at Great Indian Developer Summit Video of the talk: http://www.youtube.com/watch?v=FsdWIN3YP7I Ajax has proven a successful means to revamp the Web and facilitate the migration of many applications from "thick" clients to browser-based and zero-install clients. But several applications require real-time data to be effective (for example: financial market data visualization; online auctions; online gaming; messaging systems and social networks). The Comet paradigm, as an extension to the "traditional" Ajax techniques, allows any web page to receive real-time data pushed by a specialized server. This talk will introduce Comet from both a technical and historical perspectives. Several examples will be shown and a simple application, based on the Lightstreamer Comet framework, will be developed from scratch.

1. Ajax and Comet:
Implementing the Real-Time Web
Alessandro Alinone
Great Indian Developer Summit
May 2008

2. Contents
•
•
•
•
•
Push Technology: some history
Web paradigms: from Ajax to Comet
Comet applications and examples
Hands-on code lab
Lightstreamer architecture

3. Push Technology
Some milestones
Technologies
1996
2000
Webcasting, narrowcasting
Applet-based push
2005
Ajax
2006
Comet
2008
Key Players
PointCast and 30+ more
Caplin and custom solutions
Lightstreamer
Pushlets
KnowNow
Gmail, Meebo, Renkoo, etc.
Cometd and other open source
Updated Caplin and others
Applet-free push
Real-Time Web

4. Sync & Async Modes
Synchronous mode
of a server (S)
with respect to an actor (A)
R1
Asynchronous mode
of a server (S)
with respect to an actor (A)
R1
wait...
R2
R2
wait...
actions by A
through time
(requests)
actions by S
through time
(responses)
actions by A
through time
(requests)
actions by S
through time
(responses)

5. Page Refresh
Typical issues:
refresh 1
✔
refresh 2
wait...
wait...
wait...
wait...
refresh 3
user
browser
server
High bandwidth usage
✔
wait...
Low update frequency;
no real time
✔
wait...
High load on Web server

6. Ajax Polling
Typical issues:
✔
wait...
High bandwidth usage (but
potentially lower than
model 1)
✔
wait...
Low update frequency;
no real time
✔
action 1
High load on Web server
Advantages:
action 2
user
✔
wait...
browser
server
User interface never
blocked

7. Comet Long Polling
Typical issues:
✔
wait...
Medium bandwidth usage
(HTTP headers present in
each round-trip cycle)
✔
wait...
Medium update frequency;
near real time
✔
action 1
High load on Web server
Advantages:
wait...
user
browser
✔
server
User interface never
blocked
✔
action 2
Zero latency on lowfrequency events

8. Comet Streaming
Typical issues:
✔
action 1
Blocked by some antivirus software mounted on
proxy servers
Advantages:
✔
✔
user
browser
server
Low bandwidth usage
(very little overhead)
✔
action 2
High update frequency;
true real time
Low load on the network
infrastructure

9. Real-Time Web Paradigms
Method of sending
data with respect to
the user’s actions
Method of sending
data with respect to
the browser’s actions
Synchronous
Synchronous
Classic Ajax Application
 Periodic Polling
Asynchronous
Synchronous
Comet Application (a)
 Long Polling - aka:
 Smart Polling
 Asynchronous Polling
 Ajax Push
 Reverse Ajax
Asynchronous
Partially
Asynchronous
Comet application (b)
 Comet Streaming - aka:
 Comet - Forever Frame
 Streaming Ajax
Asynchronous
Asynchronous
Application Paradigm
Traditional Web Application
 Page Refresh

10. Comet Applications
• Where to use Comet
– Finance: market data distribution (quotes, news,
orders, portfolios)
– Gaming and Betting
– Auctions
– Monitor Consoles
– Sports Results
– Social Networks (chat, messaging, presence)
– Transportation (timetables, traffic, etc.)
– Opinion Polls
– … and many more!

11. Comet Examples
• Demos:
– www.lightstreamer.com/demo
• Real-world applications:
– www.hedgestreet.com
– www.igmarkets.com.sg
– www.extrabet.com
– www.yoonew.com

12. Hands-on Code Lab
• Let’s develop a Comet application using
Lightstreamer: “Hello India” chat
– Spec: Push users’ messages messages to a
page displayed in any browsers
– Server-side code: will use Java
– Client-side code: will use JavaScript
[20-minute Lab…]

13. The Architecture
• Lightstreamer (www.lightstreamer.com) is a
comprehensive Comet framework made up of:
• A Comet Server
• Server-side SDKs for: Java, .NET, plain sockets
• Client-side SDKs for: JavaScript (AJAX), ActionScript (Flex),
Java SE, Java ME (midlets), .NET, plain HTTP clients

14. Advanced Features
• Data Filterability
– The nature of some data enables filterability.
– Lightstreamer’s data filtering always ensures consistency and
completeness.
– Different subscription modes are available, based on the data nature
(Merge, Distinct, Raw, Command --> metapush).
• Bandwidth Control
– A maximum bandwidth can be allocated for each user. Data is
dispatched based on the configured bandwidth.
– Bandwidth control is available in both streaming mode and smart-polling
mode.
• Frequency Control
– Each single subscription can request a maximum update frequency.
– Frequency control is available in both streaming mode and smart-polling
mode.

15. Advanced Features (cont’ed)
• Multi-Stage Filtering
– A Pre-Filter is available to preliminarily sample the data on a
global basis.
• Adaptive Streaming
– Lightstreamer automatically detects Internet congestions and
heuristically throttles the data flow based on the available
bandwidth.
– When the network channel is fully available again, the user will
not receive a burst of old updates but will start seeing fresh data
at once (i.e. data aging is avoided).
– Data is aggregated efficiently within TCP packets, with a direct
control over the number of sent packets (a trade-off between
latency and overhead reduction can be configured).
– Adaptive streaming is particularly useful for streaming sessions
held over mobile networks or any unreliable networks.