Shulou(Shulou.com)06/01 Report--

Optimistic lock

Optimistic locks are not included in the database and need to be implemented by ourselves. Optimistic lock refers to the operation of the database (update operation), the idea is very optimistic, that this operation will not lead to conflict, in the operation of the data, there is no other special processing (that is, no lock), and after the update, to determine whether there is a conflict.

The implementation usually goes like this: when the data in the table is manipulated (updated), add a version field to the data table first, and add 1 to the version number of that record each time you operate. That is, first query the record and get the version field. If you want to operate on that record (update), first determine whether the value of version is equal to the value of version when you just queried. If it is equal, it means that there are no other programs operating on it during this period, then you can update it and add 1 to the value of the version field. If it is found that the current version value is not equal to the version value just obtained during the update, it means that other programs have operated on it during this period, and no update operation will be carried out.

Pessimistic lock

Corresponding to the optimistic lock is the pessimistic lock. Pessimistic lock means that there will be data conflicts when operating data, so it is necessary to acquire a lock to operate on the same data every time. This is very similar to synchronized in java, so pessimistic locking takes a lot of time. In addition, corresponding to the optimistic lock, the pessimistic lock is realized by the database itself. when we want to use it, we can directly call the relevant statements of the database.

At this point, there are two other lock concepts involved in pessimistic locks, namely shared locks and exclusive locks. Shared lock and exclusive lock are different implementations of pessimistic lock, and they both belong to the category of pessimistic lock.

Summary:

Optimistic Lock-- Program-side implementation

Pessimistic Lock-- Database self-implementation

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