BLPOP key [key ...] timeout
BLPOP is a blocking list pop primitive. It is the blocking version of LPOP because it blocks the connection when there are no elements to pop from any of the given lists. An element is popped from the head of the first list that is non-empty, with the given keys being checked in the order that they are given.
When BLPOP is called, if at least one of the specified keys contains a non-empty list, an element is popped from the head of the list and returned to the caller together with the
key it was popped from.
Keys are checked in the order that they are given. Let's say that the key
list1 doesn't exist and
list3 hold non-empty lists. Consider the following command:
BLPOP list1 list2 list3 0
BLPOP guarantees to return an element from the list stored at
list2 (since it is the first non empty list when checking
list3 in that order).
Once new data is present on one of the lists, the client returns with the name of the key unblocking it and the popped value.
When BLPOP causes a client to block and a non-zero timeout is specified, the client will unblock returning a
nil multi-bulk value when the specified timeout has expired without a push operation against at least one of the specified keys.
The timeout argument is interpreted as a double value specifying the maximum number of seconds to block. A timeout of zero can be used to block indefinitely.
BLPOP key1 key2 key3 key4 0, assuming that both
key4are non-empty, will always return an element from
BLPOPwhen multiple elements are pushed inside a list.
There are times when a list can receive multiple elements in the context of the same conceptual command:
LPUSH mylist a b c.
When multiple elements are pushed inside a list where there are clients blocking, the behavior is different for Redis 2.4 and Redis 2.6 or newer.
For Redis 2.6 what happens is that the command performing multiple pushes is executed, and only after the execution of the command the blocked clients are served. Consider this sequence of commands.
Client A: BLPOP foo 0 Client B: LPUSH foo a b c
If the above condition happens using a Redis 2.6 server or greater, Client A will be served with the
c element, because after the LPUSH command the list contains
c,b,a, so taking an element from the left means to return
Instead Redis 2.4 works in a different way: clients are served in the context of the push operation, so as long as
LPUSH foo a b c starts pushing the first element to the list, it will be delivered to the Client A, that will receive
a (the first element pushed).
The behavior of Redis 2.4 creates a lot of problems when replicating or persisting data into the AOF file, so the much more generic and semantically simpler behavior was introduced into Redis 2.6 to prevent problems.
Note that for the same reason a Lua script or a
MULTI/EXEC block may push elements into a list and afterward delete the list. In this case the blocked clients will not be served at all and will continue to be blocked as long as no data is present on the list after the execution of a single command, transaction, or script.
BLPOP can be used with pipelining (sending multiple commands and reading the replies in batch), however this setup makes sense almost solely when it is the last command of the pipeline.
Using BLPOP inside a MULTI / EXEC block does not make a lot of sense as it would require blocking the entire server in order to execute the block atomically, which in turn does not allow other clients to perform a push operation. For this reason the behavior of BLPOP inside MULTI / EXEC when the list is empty is to return a
nil multi-bulk reply, which is the same thing that happens when the timeout is reached.
Array reply: specifically:
nilmulti-bulk when no element could be popped and the timeout expired.
timeoutis interpreted as a double instead of an integer.
redis> DEL list1 list2 (integer) 0 redis> RPUSH list1 a b c (integer) 3 redis> BLPOP list1 list2 0 1) "list1" 2) "a"
When BLPOP returns an element to the client, it also removes the element from the list. This means that the element only exists in the context of the client: if the client crashes while processing the returned element, it is lost forever.
This can be a problem with some application where we want a more reliable messaging system. When this is the case, please check the BRPOPLPUSH command, that is a variant of BLPOP that adds the returned element to a target list before returning it to the client.
Using blocking list operations it is possible to mount different blocking primitives. For instance for some application you may need to block waiting for elements into a Redis Set, so that as far as a new element is added to the Set, it is possible to retrieve it without resort to polling. This would require a blocking version of SPOP that is not available, but using blocking list operations we can easily accomplish this task.
The consumer will do:
LOOP forever WHILE SPOP(key) returns elements ... process elements ... END BRPOP helper_key END
While in the producer side we'll use simply:
MULTI SADD key element LPUSH helper_key x EXEC
© 2009–2020 Salvatore Sanfilippo
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