Active-Active / Active-Stand-by architectural pattern is commonly used when we consider fail-over in server deployment design.

Active-Active

Following is the simple example for your better understanding of Active-Active Architecture.

You may have some question how to assign job at Load Balancer above. There will be following different approaches

• Round Robin (sometimes called "Next in Loop")
• Weighted Round Robin - as Round Robin, but some servers get a larger share of the overall traffic
• Random
• Source IP hash - if someone try DDoS attach from same location, only hashed server will be damaged
• URL hash
• Least connections, weighted least connections
• Least traffic
• Least latency

Active-Stand-by

This approach is super simple, but little bit wasting resources, so I usually don't like. By the way, Active-Stand-by is commonly used to build DR(Disaster Recovery) Center.

When you see, there are two different entities - one for Active, and the other for Stand-by. Stand-by means it does not work at all as long as Active has no problem.