web server vs web container vs application server

web server : is something that is capable of handling http requests (recieving http requesting, processing them, creating http responses and sending them to the clients). Apache Web Server

web container : is a J2EE compliant implementation which provides an environment for the Servlets and JSPs to run. Tomcat typically a web container

application server : is a complete environment for running business components. It contains the web server capabilities as well as web container capabilities. eg: IBM WebSphere, Oracle Application Server

What is failover?

what is failover?

In computing, failover is automatic switching to a redundant or standby computer server, system, hardware component or network upon the failure or abnormal termination of the previously active application,server, system, hardware component, or network. Failover and switchover are essentially the same operation, except that failover is automatic and usually operates without warning, while switchover requires human intervention.

taken from http://en.wikipedia.org/wiki/Failover

How ever when the main server recovers, requests are then again sent to the main server.

There are 3 main type of fail overs

1. clod failover : Should do manually
2. warm failover : failover happens automatically. But the transaction may abort due to the failure of data synchronization.
3. hot failover : happens automatically. Data synchronization also happens. Therefore can continue operation without a problem.