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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.
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Ajax and Comet:Implementing the Real-Time Web
Alessandro Alinone
Great Indian Developer Summit
May 2008
Contents
• Push Technology: some history
• Web paradigms: from Ajax to Comet
• Comet applications and examples
• Hands-on code lab
• Lightstreamer architecture
Push Technology
1996
2000
2005
2006
2008
Webcasting, narrowcasting
Ajax
Comet
PointCast and 30+ more
Caplin and custom solutions
LightstreamerPushletsKnowNow
Applet-based push
Applet-free push
Gmail, Meebo, Renkoo, etc.Cometd and other open sourceUpdated Caplin and others
Some milestones
Technologies Key Players
Real-Time Web
Sync & Async Modes
wait...
actions by A through time(requests)
wait...
R1
R2
actions by S through time(responses)
Asynchronous modeof a server (S)
with respect to an actor (A)
R1
R2
Synchronous modeof a server (S)
with respect to an actor (A)
actions by A through time(requests)
actions by S through time(responses)
Page Refresh
wait...
wait...
wait...
refresh 1
browser server
wait...
refresh 2
user
wait...
refresh 3
wait...
Typical issues:✔ Low update frequency;
no real time✔ High bandwidth usage✔ High load on Web server
Ajax Polling
wait...
wait...action 1
browser server
action 2
user
wait...
Typical issues:✔ Low update frequency;
no real time✔ High bandwidth usage (but
potentially lower than model 1)
✔ High load on Web server
Advantages:✔ User interface never
blocked
Comet Long Polling
wait...
wait...action 1
browser server
action 2
user
wait...
Typical issues:✔ Medium update frequency;
near real time✔ Medium bandwidth usage
(HTTP headers present in each round-trip cycle)
✔ High load on Web server
Advantages:✔ User interface never
blocked✔ Zero latency on low-
frequency events
Comet Streaming
action 1
browser server
action 2
user
Typical issues:✔ Blocked by some anti-
virus software mounted on proxy servers
Advantages:✔ High update frequency;
true real time✔ Low bandwidth usage
(very little overhead)✔ Low load on the network
infrastructure
Real-Time Web Paradigms
Application Paradigm Method of sending data with respect to the user’s actions
Method of sending data with respect to
the browser’s actions
Traditional Web Application Page Refresh
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
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!
Comet Examples
• Demos:– www.lightstreamer.com/demo
• Real-world applications:– www.hedgestreet.com– www.igmarkets.com.sg – www.extrabet.com– www.yoonew.com
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…]
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
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.
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.
