This is a case when you want to do remove operations like RPOP/LPOP, but the list is already empty. In this case, remove operation will return null does nothing. In this case a consumer is forced to wait some time and retry again with RPOP.
So Redis implements commands called BRPOP and BLPOP which are versions of RPOP and LPOP able to block if the list is empty: This will return to the caller only when a new element is added to the list, or when a user-specified timeout is reached.
Example : If we want pop the elements from the list A, which is empty right now;
127.0.0.1:6379> brpop A {timeout}
...
...
waits until new elements is added to list key A
timeout : No of seconds to wait until; 0 indicates wait forever
