BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis >

BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis >

BLPOP (and BRPOP) is a blocking list pop primitive. You can see this commandsas blocking versions of LPOP and RPOP able toblock if the specified keys don't exist or contain empty lists.

The following is a description of the exact semantic. We describe BLPOP butthe two commands are identical, the only difference is that BLPOP pops theelement from the left (head) of the list, and BRPOP pops from the right (tail).

Non blocking behavior

When BLPOP is called, if at least one of the specified keys contain a nonempty list, an element is popped from the head of the list and returned tothe caller together with the name of the key (BLPOP returns a two elementsarray, the first element is the key, the second the popped value).

Keys are scanned from left to right, so for instance if youissue BLPOP list1 list2 list3 0 against a dataset where list1 does notexist but list2 and list3 contain non empty lists, BLPOP guaranteesto return an element from the list stored at list2 (since it is the firstnon empty list starting from the left).

Blocking behavior

If none of the specified keys exist or contain non empty lists, BLPOPblocks until some other client performs a LPUSH oran RPUSH operation against one of the lists.

Once new data is present on one of the lists, the client finally returnswith the name of the key unblocking it and the popped value.

When blocking, if a non-zero timeout is specified, the client will unblockreturning a nil special value if the specified amount of seconds passedwithout a push operation against at least one of the specified keys.

The timeout argument is interpreted as an integer value. A timeout of zero means instead to block forever.

Multiple clients blocking for the same keys

Multiple clients can block for the same key. They are put intoa queue, so the first to be served will be the one that started to waitearlier, in a first-blpopping first-served fashion.

blocking POP inside a MULTI/EXEC transaction

BLPOP and BRPOP can be used with pipelining (sending multiple commands and reading the replies in batch), but it does not make sense to use BLPOP or BRPOP inside a MULTI/EXEC block (a Redis transaction).

The behavior of BLPOP inside MULTI/EXEC when the list is empty is to return a multi-bulk nil reply, exactly what happens when the timeout is reached. If you like science fiction, think at it like if inside MULTI/EXEC the time will flow at infinite speed :)

Return value

BLPOP returns a two-elements array via a multi bulk reply in order to returnboth the unblocking key and the popped value.

When a non-zero timeout is specified, and the BLPOP operation timed out,the return value is a nil multi bulk reply. Most client values will returnfalse or nil accordingly to the programming language used.