The Real-time Web seems to be the buzz of the moment, and there has been quite a debate comparing HTTP, XMPP and related technologies (Comet, Web sockets). Generally HTTP is associated with the Pull model, while XMPP is associated with the Push model. But it’s very well possible to design an architecture that follows the Push model using HTTP.
Let’s see an example to illustrated the point: Nick and Debbie are friends and they have subscribed to each other’s feed to receive updates. Their feeds are hosted on different servers.
In the Pull model, Nick has to poll Debbie’s server every time to check for updates from Debbie, and vice-versa.
In the Push model, the flow goes something like this:
- Debbie publishes a new entry on her server (Push);
- Debbie’s server lets Nick’s server know that Debbie has published a new entry (Push);
- Nick polls his own server to receive updates (Pull).
Notice that the second step is a Push implemented in HTTP. Nick’s server didn’t have to poll Debbie’s server every time to check for updates.