How to use redis to implement distributed locks

Distributed locks can actually be understood as: controlling the distributed system to operate shared resources in an orderly manner, and maintaining consistency through mutual exclusion.

To give an inappropriate example: Suppose the shared resource is a house with various books in it, the distributed system is the people who want to enter the house to read, and the distributed lock ensures that the house only has A door and only one person can enter at a time, and the door has only one key. (Recommended learning: Redis video tutorial)

Use redis to implement distributed locks                          

Use the redis command set key value NX EX max -lock-time implements locking

Use redis command EVAL to implement unlocking

Lock:

Jedis jedis = new Jedis("127.0.0.1", 6379);
 
 private static final String SUCCESS = "OK";
 
 /**
 
  * 加锁操作
 
  * @param key 锁标识
 
  * @param value 客户端标识
 
  * @param timeOut 过期时间
 
  */
 
 public Boolean lock(String key,String value,Long timeOut){
 
     String var1 = jedis.set(key,value,"NX","EX",timeOut);
 
     if(LOCK_SUCCESS.equals(var1)){
 
         return true;
 
     }
 
     return false;
 
 }

Interpretation:

Lock operation: jedis.set(key,value,"NX","EX",timeOut)[Guaranteed atomic operation of locking]

The key is the key value of redis as the identification of the lock, and the value is used as the identification of the client here. Only when the key-value matches can you have the right to delete the lock [Guaranteed security]

Setting the expiration time through timeOut is guaranteed not to happen Deadlock will occur [Avoid deadlock]

NX, what does EX mean?

NX: The operation will only be performed when the key does not exist, if not exists;

EX: Set the expiration time of the key to seconds, the specific time is determined by the fifth Parameter decision

Unlock

Jedis jedis = new Jedis("127.0.0.1", 6379);
 
 private static final Long UNLOCK_SUCCESS = 1L;
 
 /**
 
  * 解锁操作
 
  * @param key 锁标识
 
  * @param value 客户端标识
 
  * @return
 
  */
 
 public static Boolean unLock(String key,String value){
 
     String luaScript = "if redis.call(\"get\",KEYS[1]) == ARGV[1] then return redis.call(\"del\",KEYS[1]) else  return 0 end";
 
     Object var2 = jedis.eval(luaScript,Collections.singletonList(key), Collections.singletonList(value));
 
     if (UNLOCK_SUCCESS == var2) {
 
         return true;
 
     }
    return false;
 }

Interpretation:

luaScript This string is a lua script, which means if according to If the value obtained by key is the same as the value passed in, execute del, otherwise it will return 0 [Ensure security]

jedis.eval(String, list, list); This command is to execute the lua script, KEYS The set of ARGV is the second parameter, and the set of ARGV is the third parameter [Atomic operation to ensure unlocking]

The above realizes how to use redis to correctly implement distributed locks, but there is a small flaw in the lock The appropriate expiration time should be set to an appropriate value. This actually needs to be considered based on the business scenario.

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