Redis pessimistic lock handling in PHP applications

Redis is a high-performance in-memory database that is widely used in various web applications. Its superior performance and support for multiple data types make Redis the database of choice for many PHP applications. In PHP applications, we often need to control the concurrent access of multiple processes or threads to a shared resource. Shared resources include cache, logs, configurations, etc. The number of processes or threads that need to be accessed at the same time may be large, so concurrent access control mechanisms and locks are introduced to manage them. This article will introduce Redis's pessimistic lock handling in PHP applications.

Pessimistic lock is the most commonly used lock method. Its implementation is to assume that multiple processes or threads want to read or write a resource at the same time. Pessimistic lock will think that only one process can perform the operation. Other processes need to wait. Pessimistic locking can effectively avoid inconsistency problems caused by multiple concurrent processes or threads accessing the same resource at the same time.

In PHP applications, we can implement the pessimistic locking mechanism through Redis. Redis provides a mechanism to implement locks based on the SETNX and EXPIRE commands. The SETNX command can set a key and value in Redis. It can only be set successfully if the key does not exist. The EXPIRE command can set an expiration time for the key. Through the combination of SETNX and EXPIRE, we can implement a lock mechanism.

The following is a demonstration of the implementation of Redis pessimistic lock through PHP code:

$redis = new Redis();
$redis->connect('127.0.0.1', 6379);
$key = 'lock_key';
$timeout = 10; //设置超时时间为10秒
while(true){
    $lock = $redis->setnx($key, time() + $timeout);
    if($lock){ //如果成功设置了键值，表示锁可用
        break;
    }
    $expire = $redis->get($key);
    //判断当前时间是否已经超过超时时间
    if($expire < time()){
        //锁已过期，删除该键值重新获取锁
        $redis->del($key);
        continue;
    }
    //锁未过期，等待一段时间后尝试获取锁
    sleep(1);
}
//成功获取锁后，执行需要锁保护的代码
//执行结束后，删除该键值释放锁
$redis->del($key);

In the above code, we use a while (true) loop to obtain the lock. First try to set a key and value in Redis using the SETNX command, and set the expiration time of this lock to the current time plus a timeout timeout. If the lock is successfully set, you can enter the code that needs to be protected. If the lock is already occupied by another process or thread, the lock cannot be obtained and the lock will be tried again after waiting for a period of time. If the lock has expired, delete the key value and reacquire the lock.

In general, Redis's pessimistic locking mechanism is relatively simple, but the lock timeout and retry logic need to be carefully handled. At the same time, in order to avoid deadlock problems, we also need to pay attention to the release of locks. In actual use, we can simplify the use of pessimistic locks by encapsulating PHP functions or using third-party libraries.

In the face of high concurrency scenarios, pessimistic locking is a more effective locking mechanism. Using Redis's pessimistic locking mechanism in PHP applications can improve the concurrency performance of the system while ensuring data consistency.

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