Consider the following code
redis.get('key', function(err, value) {
redis.set('key', value * 2);
});
The above code simply multiply a redis key by 2. However, if two users run concurrency, the execution order may like this
user1: redis.get('key') -> 1
user2: redis.get('key') -> 1
user1: redis.set('key', 1 x 2) -> 2
user2: redis.set('key', 1 x 2) -> 2