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Today, I will talk to you about what the mysql distributed lock in java is. Many people may not know much about it. In order to make you understand better, the editor has summarized the following content for you. I hope you can get something according to this article.

problem

(1) what is a distributed lock?

(2) Why do you need distributed locks?

(3) how does mysql implement distributed locks?

(4) the advantages and disadvantages of mysql distributed lock?

Brief introduction

With the continuous increase of concurrency, stand-alone services will evolve to multi-node or micro-services sooner or later. At this time, the synchronized or ReentrantLock used in the stand-alone mode will no longer be applicable. We urgently need a solution to ensure thread safety in the distributed environment. Today we will learn how to achieve distributed thread safety with mysql distributed locks.

Basic knowledge

Mysql provides two functions-- get_lock ('key', timeout) and release_lock (' key')-- to implement distributed locks, which can be locked according to key, which is a string that sets the timeout (in seconds) and releases the lock when release_lock ('key') is called or the client is disconnected.

They are used as follows:

Mysql > select get_lock ('user_1', 10);-> 1mysql > select release_lock (' user_1');-> 1

Get_lock ('user_1', 10) returns 1 if the lock is acquired within 10 seconds, otherwise 0

Release_lock ('user_1') returns 1 if the lock is held by the current client, 0 if the lock is held by another client, and null if the lock is not held by any client

Multi-client case

In order to give an example, this article is originally created by the public serial number "Tong GE read the source code", where the timeout is all set to 0, that is, return immediately.

Moment client A client B1get_lock ('user_1', 0)-> 1-2-get_lock (' user_1', 0)-> 03-release_lock ('user_1', 0)-> 04release_lock (' user_1', 0)-> 1-5release_lock ('user_2', 0)-> null-6-get_lock (' user_1', 0)-> 17-release_lock ('user_1', 0)-> 1Java implementation

To facilitate quick implementation, the springboot2.1 + mybatis implementation is used here, and the configuration of spring is omitted, listing only a few major classes.

Define Locker Interfac

There is only one method in the interface. Input parameter 1 is the locked key, and input parameter 2 is the executed command.

Public interface Locker {void lock (String key, Runnable command);} mysql distributed lock implementation

Note the following two points in the implementation of mysql:

(1) locking and releasing locks must be in the same session (the same client), so the Mapper API cannot be called here, because the Mapper API may result in not being in the same session.

(2) reentrancy is guaranteed by ThreadLocal.

@ Slf4j@Componentpublic class MysqlLocker implements Locker {private static final ThreadLocal localSession = new ThreadLocal (); @ Autowired private SqlSessionFactory sqlSessionFactory; @ Override public void lock (String key, Runnable command) {/ / lock / release lock must use the same session SqlSessionWrapper sqlSessionWrapper = localSession.get (); if (sqlSessionWrapper = = null) {/ / acquire lock localSession.set (new SqlSessionWrapper (sqlSessionFactory.openSession ()) for the first time) } try {/ / this article by the public number "Tongge read source code" original / /-1 means that the lock has not been acquired and has been waiting for if (getLock (key,-1)) {command.run ();}} catch (Exception e) {log.error ("lock error", e) } finally {releaseLock (key);}} private boolean getLock (String key, long timeout) {Map param = new HashMap (); param.put ("key", key); param.put ("timeout", timeout); SqlSessionWrapper sqlSessionWrapper = localSession.get (); Integer result = sqlSessionWrapper.sqlSession.selectOne ("LockerMapper.getLock", param) If (result! = null & & result.intValue () = 1) {/ / got the lock, state plus 1 sqlSessionWrapper.state++; return true;} return false;} private boolean releaseLock (String key) {SqlSessionWrapper sqlSessionWrapper = localSession.get (); Integer result = sqlSessionWrapper.sqlSession.selectOne ("LockerMapper.releaseLock", key) If (result! = null & & result.intValue () = 1) {/ / the lock is released successfully, state minus 1 sqlSessionWrapper.state--; / / when state is reduced to 0, all locks acquired by the current thread have been released, then close session and remove if (sqlSessionWrapper.state = = 0) {sqlSessionWrapper.sqlSession.close () from ThreadLocal LocalSession.remove ();} return true;} return false;} private static class SqlSessionWrapper {int state; SqlSession sqlSession; public SqlSessionWrapper (SqlSession sqlSession) {this.state = 0; this.sqlSession = sqlSession;}} LockerMapper.xml

Statements that define get_lock () and release_lock ().

Select get_lock (# {key}, # {timeout}); select release_lock (# {key}) test class

Here 1000 threads are started, each printing a sentence and sleeping for 2 seconds.

@ RunWith (SpringRunner.class) @ SpringBootTest (classes = Application.class) public class MysqlLockerTest {@ Autowired private Locker locker; @ Test public void testMysqlLocker () throws IOException {for (int I = 0; I)

< 1000; i++) { // 多节点测试 try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } new Thread(()->

{locker.lock ("lock", ()-> {/ / reentrant test locker.lock ("lock", ()-> {System.out.println (String.format ("time:% d, threadName:% s", System.currentTimeMillis (), Thread.currentThread (). GetName () Try {Thread.sleep (2000);} catch (InterruptedException e) {e.printStackTrace ();}});}), "Thread-" + I) .start () } System.in.read ();}} run the result

Looking at the running results, it is found that the information of a thread is printed every 2 seconds, indicating that the lock is valid, and the verification in the distributed environment is also very simple, with multiple MysqlLockerTest instances.

Time: 1568715905952, threadName: Thread-3time: 1568715907955, threadName: Thread-4time: 1568715909966, threadName: Thread-8time: 1568715911967, threadName: Thread-0time: 1568715913969, threadName: Thread-1time: 1568715915972, threadName: Thread-9time: 1568715917975, threadName: Thread-6time: 1568715919997, threadName: Thread-5time: 1568715921999, threadName: Thread-7time: 1568715924001, threadName: Thread-2 summary

(1) distributed locks are required in a distributed environment, and thread safety cannot be guaranteed by the lock of a single machine.

(2) mysql distributed lock is based on get_lock ('key', timeout) and release_lock (' key') functions.

(3) mysql distributed locks are reentrant locks.

Colored egg

What do you need to pay attention to when using mysql distributed locks?

A: it is necessary to ensure that multiple service nodes use the same mysql library. This article is originally created by the public number "Tong GE read the source code".

What are the advantages of mysql distributed locks?

Answer: 1) convenient and fast, because almost every service connects to the database, but not every service uses redis or zookeeper

2) if the client is disconnected, the lock will be released automatically, which will not cause the lock to be occupied all the time

3) the mysql distributed lock is a reentrant lock, and the cost of retrofitting the old code is low.

What are the disadvantages of mysql distributed locks?

Answer: 1) lock directly to the database, increasing the pressure on the database

2) the locked thread will occupy a session, that is, a number of connections. If the concurrency is large, the normally executed sql statement may not get the connection.

3) after the service split, it is not appropriate for each service to use its own database

4) compared with redis or zookeeper distributed locks, the efficiency is relatively low.

After reading the above, do you have any further understanding of what mysql distributed locks are in java? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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