Two issuses about concurrency that’re difficult to handle: 

   1. offline concurrency(transcations among several databases or serveral requests)

   2. multi-thread concurrency in application server side

Two basic ways of controlling concurrency

   1. isolation

   2. to make some data immutable

Optimistic Lock: detect conflictions and then solve them。 The lock is only held during the commit

Pessimistic Lock:  prevent conflictions

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