Analysis of distributed lock redistribution principle

1. be based on setnx The distributed lock implemented has the following problems

• Do not reenter : The same thread cannot get the same handle more than once
• Can't try again : Only one attempt to acquire a lock returns false, No retry mechanism
• Time out release : Lock timeout release can avoid deadlock , But if the business takes a long time to execute , It will also cause the lock to release , There are safety risks
• Master slave consistency : If Redis Provides a master-slave cluster , There is a delay in master-slave synchronization , When the Lord outage , If you slave and synchronize the lock data in the master , Lock implementation will appear

Redisson It's a Redis On the basis of implementation Java In memory data grid （In-Memory Data Grid）. It not only provides a series of distributions Type Java Common objects , There are also many distributed services , It includes the implementation of various distributed locks . Use Redission Can solve the above mentioned 4 A question .

2. Code implementation

2.1 Import dependence

2.2 To configure Redisson client

2.3 Use Redisson Distributed locks for

3.Redisson Reentrant lock principle

3.1 structure

adopt hash structure ：

When value yes 0 The description can be deleted

3.2 flow chart

Source code

Lock source code ：

The source code parsing ：
Release the lock ：

The source code parsing ：

Redisson The principle of distributed lock

Principle summary ：

• Reentrant ： utilize hash Structure record thread id And reentry times
• Can try again （ Try again when you have time left , That is, publish and subscribe to realize lock retry ）： Use semaphores and PubSub Function implementation wait 、 Wake up the 、 Retry mechanism for failed lock acquisition
• Overtime renewal （ Renew the contract continuously through scheduled tasks ）： utilize watchDog, Every once in a while （releaseTime/3）, Reset timeout