Shulou(Shulou.com)05/31 Report--

Today, I will talk to you about the implementation of distributed locks in Redis Template, which may not be well understood by many people. in order to make you understand better, the editor has summarized the following for you. I hope you can get something according to this article.

Reliability.

First, to ensure that distributed locks are available, we need to ensure that the implementation of the lock meets at least the following four conditions:

1. Repulsion. At any given time, only one client can hold the lock.

two。 There is no deadlock. Even if one client crashes while holding the lock without actively unlocking it, it is guaranteed that other clients will be able to lock it.

3. It is fault tolerant. As long as most of the Redis nodes are functioning properly, the client can lock and unlock.

4. The person who unlocks the bell must also tie the bell. Locking and unlocking must be the same client, and the client cannot unlock the lock added by others.

To acquire a distributed lock using the SETNX command of Redis:

The C1 and C2 threads simultaneously check the timestamp to acquire the lock, execute the SETNX command and both return 0, when the lock is still held by C3 and C3 has crashed

C1 DEL lock

C1 uses the SETNX command to acquire the lock and is successful

C2 DEL lock

C2 uses the SETNX command to acquire the lock and succeeds

ERROR: both C1 and C2 have acquired the lock due to race conditions

Fortunately, this can be avoided by following these steps to see how the C4 thread operates

C4 uses the SETNX command to acquire the lock

C3 has crashed but still holds the lock, so Redis returns 0 to C4

C4 uses the GET command to acquire the lock and check if the lock has expired, and if not, wait for a while and try again

If the lock has expired, C4 tries to GETSET lock.foo

Using the get syntax, C4 can check whether the old time is still the expiration time, and if so, acquire the lock

If another client, C5, acquires the lock first, C4 returns a non-expiration time after executing the GETSET command, and then C4 continues to retry to acquire the lock from scratch. This operation C4 will extend the expiration time of locks acquired by C5 a little bit, but this is not a big problem.

Next, we show it in the form of code:

Package com.shuige.components.cache.redis;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.data.redis.core.RedisCallback;import org.springframework.data.redis.core.RedisTemplate;import org.springframework.data.redis.core.ValueOperations;import org.springframework.stereotype.Component;import java.util.Objects;import java.util.concurrent.TimeUnit / * Description: general Redis help Class * User: zhouzhou * Date: 2018-09-05 * Time: 15:39 * / @ Componentpublic class CommonRedisHelper {public static final String LOCK_PREFIX = "redis_lock"; public static final int LOCK_EXPIRE = 300; / / ms @ Autowired RedisTemplate redisTemplate / * * finally strengthen the distributed lock * * @ param key key value * @ return whether to get * / public boolean lock (String key) {String lock = LOCK_PREFIX + key; / / using the lambda expression return (Boolean) redisTemplate.execute ((RedisCallback) connection-> {long expireAt = System.currentTimeMillis () + LOCK_EXPIRE + 1) Boolean acquire = connection.setNX (lock.getBytes (), String.valueOf (expireAt). GetBytes ()); if (acquire) {return true;} else {byte [] value = connection.get (lock.getBytes ()); if (Objects.nonNull (value) & & value.length > 0) {long expireTime = Long.parseLong (new String (value)) If (expireTime < System.currentTimeMillis ()) {/ / if the lock has expired byte [] oldValue = connection.getSet (lock.getBytes (), String.valueOf (System.currentTimeMillis () + LOCK_EXPIRE + 1). GetBytes ()); / / prevent deadlock return Long.parseLong (new String (oldValue)) < System.currentTimeMillis () } return false;});} / * * remove lock * * @ param key * / public void delete (String key) {redisTemplate.delete (key);}}

How to use it? after importing the tool class:

Boolean lock = redisHelper.lock (key); if (lock) {/ / perform logical operation redisHelper.delete (key);} else {/ / set a counter for the number of failures. When 5 times are reached, int failCount = 1 is returned; while (failCount)

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