How do they make real time data live on a web page?

Question

How do they do this? I would like to have web pages with data fields that change in real time as a person views the web page. Here is an example.

How do they do this

Answer 1

There are two approaches:

Polling

Client requests data on a regular basis. Uses network and server resources even when there is no data. Data is not quite 'live'. Extremely easy to implement, but not scalable.

Push

Server sends data to the client, so client can simply wait for it to arrive instead of checking regularly. This can be achieved with a socket connection (since you are talking about web pages, this doesn't really apply unless you are using Flash, since support for sockets in the browser in the browser is currently immature) - or by using the technique known as 'comet'.

Neither socket connections nor comet are particularly scalable if the server end is implemented naively.

- To do live data on a large scale (without buying a boat load of hardware) you will need server software that does not use a thread for each client.