I hear a lot about pools of load-balanced servers. But, doesn't it mean that you have multiple web-servers which only do is delivering requests to the single poor database? The database server is the heaviest element of the system and single point of failure. How do you unload it with multiple servers?

@ValentinTihomirov Why would you think there'd be only one database server?

There are many different ways to handle database loads in a HA scenario. A good place to start reading is to do a web search for the CAP theorem.

@JennyD Thank you very much. CAP says about distributed systems. But, how can you have it distributed actually? I mean that database is something which stores consistent data. If you make reserve tickets for a plane then it must be possible to allocate the same seat for two people. There must be a single source of truth, a single database. You may one database managing one half of the plane and another managing another half and distribute the database this way.

But, this is not what work balancing is about. Work balancing says that you have two independent planes and we send first client to one database and another to another database. But they will be in conflict soon.