Have you ever wondered that when you do online shopping you never loose the items in your cart unless you explicitly delete them. What does an application server do for you in the background to make e-commerce run smooth without even letting the user know that a server failure happened.
The above short paragraph lays the base and even answers the questions on "why we need clustering"? Here is a diagram which pictorially explains what happens to session data when a server failure occurs.
key components here : cookies, session data, oracle weblogic, clustering, primary server and secondary server.
Here is an explanation and chronology of events that happen in the background.
Pre-requisite : The 4 weblogic servers should be in a cluster. Weblogic plugin should be configured to route the requests to WLS.
1. A customer access the website for the first time and his request goes via the weblogic plugin (sitting on apache) and then hits the application server.
2. In a response weblogic renders the application page and also sends back a cookie which has a primary and secondary weblogic server information. Plus, it creates a session for the user on the primary weblogic server and a copy of the session goes on the secondary server.
3. User from the same browser then access the site again and the request lands on apache and the weblogic plugin parses it, the plugin then extracts the weblogic primary and secondary server details from the cookie that comes along with the user's request.
The action starts here.....
4. The primary server which was serving user's request all this while goes down.
5. The next incoming request coming from the same user from the same browser, reaches the weblogic plugin ( apache ) .
6. Plugin knows that the primary server does not exist anymore.
7. The request is then routed by the plugin to another weblogic server which is part of the same cluster. This new weblogic server becomes the new primary.
And......
8. The new weblogic primary server gets all the session information from the secondary server.
And that is how the user never loses his shopping cart items.
Question for you : Do we need anything special on the application code to achieve this, or is it all an application server's effort ?
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