Redis distributed lock

There are three implementation methods of distributed lock ：

1. Implementation of distributed lock based on Database ;
2. Cache based （Redis etc. ） Implement distributed locks ;
3. be based on Zookeeper Implement distributed locks ;
4. article ：https://www.cnblogs.com/xiaozengzeng/p/12641153.html

Cache based （Redis etc. ） Implement distributed locks

1. Introduction to using commands ：
   （1）SETNX
   SETNX key val： If and only if key When there is no ,set One key by val String , return 1; if key There is , Then do nothing , return 0.
   （2）expire
   expire key timeout： by key Set a timeout , Unit is second, After this time, the lock will release automatically , Avoid deadlock .
   （3）delete
   delete key： Delete key

In the use of Redis When implementing distributed locks , These three commands are mainly used .

2. Realization thought ：
   （1） When getting the lock , Use setnx Lock , And use expire Command to add a timeout to the lock , After that time, the lock will be released automatically , The lock value Value is a randomly generated UUID, Judge when releasing the lock through this .
   （2） When acquiring the lock, a timeout for acquiring is also set , If it exceeds this time, give up acquiring lock .
   （3） When you release the lock , adopt UUID Decide whether to lock , If it's time to lock , execute delete Lock release .

Redis Locks mainly use Redis Of setnx command .

• Lock command ：SETNX key value, When the key doesn't exist , Set the key and return success , Otherwise return to failure .KEY It's the only sign of the lock , Generally, the name is decided by business .
• Unlock command ：DEL key, Release the lock by deleting key value pairs , So that other threads can pass through SETNX Command to get the lock .
• Lock timeout ：EXPIRE key timeout, Set up key Timeout for , To ensure that even if the lock is not explicitly released , The lock can also be released automatically after a certain period of time , Avoid locking resources forever .

if (setnx(key, 1) == 1){
    
    expire(key, 30)
    try {
    
        //TODO  Business logic 
    } finally {
    
        del(key)
    }
}

One 、 What is distributed lock ？

To introduce distributed locks , First of all, it should be mentioned that the thread lock is corresponding to the distributed phase lock 、 Process lock .

Thread lock ： Mainly used to give methods 、 Code block lock . When a method or code uses a lock , Only one thread executes the method or the code segment at the same time . Thread lock only in the same JVM Effective , Because the implementation of thread lock depends on the shared memory between threads , such as synchronized Is the shared object header , Display lock Lock Is to share a variable （state）.

Process lock ： In order to control multiple processes in the same operating system to access a shared resource , Because the process is independent , Individual processes cannot access resources of other processes , So it can't pass synchronized Implement process lock with equal thread lock .

Distributed lock ： When multiple processes are not in the same system , Using distributed lock to control the access of multiple processes to resources .

When we modify existing data in the system , Need to read first , Then modify and save , It's easy to run into concurrency problems at this point . Because modifying and saving are not atomic operations , In a concurrent scenario , Some operations on data may be lost . In a single server system, we often use local locks to avoid concurrency problems , However , When services are deployed in a cluster mode , Local lock cannot work between multiple servers , In this case, the distributed lock is needed to ensure the data consistency .

Atomicity ： That is, an operation or multiple operations are all executed and the execution process will not be interrupted by any factors , Or they don't do it . stay Java in , Reading and assigning variables of basic data type is atomic operation , That is, these operations cannot be interrupted , Or execute , Or not .

When we modify existing data in the system , Need to read first , Then modify and save , It's easy to run into concurrency problems at this point . Because modifying and saving are not atomic operations ,** In a concurrent scenario , Some operations on data may be lost .** In a single server system, we often use local locks to avoid concurrency problems , However , When services are deployed in a cluster mode , Local lock cannot work between multiple servers , In this case, the distributed lock is needed to ensure the data consistency .
https://segmentfault.com/a/1190000038330434

Redis Distributed lock

Redis Locks mainly use Redis Of setnx command .

• Lock command ：SETNX key value, When the key doesn't exist , Set the key and return success , Otherwise return to failure .KEY It's the only sign of the lock , Generally, the name is decided by business .
• Unlock command ：DEL key, Release the lock by deleting key value pairs , So that other threads can pass through SETNX Command to get the lock .
• Lock timeout ：EXPIRE key timeout, Set up key Timeout for , To ensure that even if the lock is not explicitly released , The lock can also be released automatically after a certain period of time , Avoid locking resources forever .

if (setnx(key, 1) == 1){
    
    expire(key, 30)
    try {
    
        //TODO  Business logic 
    } finally {
    
        del(key)
    }
}