What is the reason why the Redis list concurrent pop operation returns empty value?

Exploration on the reason for returning empty value under redis list operation

When using redis for list operations, you often encounter popping elements (pop) from the list. This article will analyze a problem in which an element pops up from the redis list using pipelines in a concurrent environment but gets null values.

Problem description:

A piece of code uses redis's pipeline mechanism to pop up 100 elements from a list called prizelist_xxx. The code is as follows:

 $prizes = $this->redisObject->pipeline(function ($pipe) use ($drawCount) {
    for ($i = 0; $i lpop($this->cachePrefix . "prizeList_" . $this->tag);
    }
});

In a non-concurrent environment, this code works properly. However, in concurrency environments, an empty array is sometimes returned, even if sufficient data exists in the list is confirmed in advance.

Analysis of the cause of the problem:

The key to the problem lies in the word "concurrency". The answer has pointed out the reason: in a concurrent environment, multiple coroutines (or threads) access and operate the same redis list at the same time. If other coroutines have taken all the elements in the list before the current coroutine performs the lpop operation, the current coroutine will naturally get a null value. Although pipelines can be operated in batches, they cannot guarantee atomic acquisition of data. Multiple pipeline operations are still concurrent, and there is no locking mechanism between them to ensure data consistency. Therefore, even if the list initially contains enough data, in the case of concurrent access, a coroutine may obtain a null value. This is not a code error, but a typical manifestation of data competition in a concurrent environment.

The above is the detailed content of What is the reason why the Redis list concurrent pop operation returns empty value?. For more information, please follow other related articles on the PHP Chinese website!