Large distributed systems run into a problem which smaller systems don’t usually have to worry about. “ Brewers CAP Theorem †[ Ref 1 ] [ Ref 2 ] [ Ref 3 ] defines this problem in a very simple way. It states, that though its desirable to have Consistency, High-Availability and Partition-tolerance in every system, unfortunately no system can achieve all three at the same time . C onsistent : A fully Consistent system is one where the system can guarantee that once you store a state (lets say “x=yâ€) in the system, it will report the same state in every subsequent operation until the state is explicitly changed by something outside the system. [ Example 1 ] A single MySQL database instance is automatically fully consistent since there is only one node keeping the state. [ Example 2 ] If two MySQL servers are involved, and if the system is designed in such a way that all keys starting “a†to “m†is kept on server 1, and keys “n†to “z†are kept on server
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