BLPOP

Syntax

  1. BLPOP key [key ...] timeout

Time complexity: O(N) where N is the number of provided keys.

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.

Non-blocking behavior

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 list2 and list3 hold non-empty lists. Consider the following command:

  1. 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 list1, list2 and list3 in that order).

Blocking behavior

If none of the specified keys exist, BLPOP blocks the connection until another client performs an LPUSH or RPUSH operation against one of the keys.

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.

What key is served first? What client? What element? Priority ordering details.

  • If the client tries to blocks for multiple keys, but at least one key contains elements, the returned key / element pair is the first key from left to right that has one or more elements. In this case the client is not blocked. So for instance BLPOP key1 key2 key3 key4 0, assuming that both key2 and key4 are non-empty, will always return an element from key2.
  • If multiple clients are blocked for the same key, the first client to be served is the one that was waiting for more time (the first that blocked for the key). Once a client is unblocked it does not retain any priority, when it blocks again with the next call to BLPOP it will be served accordingly to the number of clients already blocked for the same key, that will all be served before it (from the first to the last that blocked).
  • When a client is blocking for multiple keys at the same time, and elements are available at the same time in multiple keys (because of a transaction or a Lua script added elements to multiple lists), the client will be unblocked using the first key that received a push operation (assuming it has enough elements to serve our client, as there may be other clients as well waiting for this key). Basically after the execution of every command, Dragonfly will run a list of all the keys that received data AND that have at least a client blocked. The list is ordered by new element arrival time, from the first key that received data to the last. For every key processed, Dragonfly will serve all the clients waiting for that key in a FIFO fashion, as long as there are elements in this key. When the key is empty or there are no longer clients waiting for this key, the next key that received new data in the previous command / transaction / script is processed, and so forth.

Behavior of !BLPOP when 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:

  • Variadic push operations such as LPUSH mylist a b c.
  • After an EXEC of a MULTI block with multiple push operations against the same list.
  • Executing a Lua Script.

If a command performing multiple pushes is executed, only after the execution of the command, the blocked clients are served. Consider this sequence of commands.

  1. Client A:   BLPOP foo 0
  2. Client B:   LPUSH foo a b c

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 c.

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 inside a !MULTI / !EXEC transaction

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.

If you like science fiction, think of time flowing at infinite speed inside a MULTI / EXEC block…

Return

Array reply: specifically:

  • A nil multi-bulk when no element could be popped and the timeout expired.
  • A two-element multi-bulk with the first element being the name of the key where an element was popped and the second element being the value of the popped element.

Examples

  1. dragonfly> DEL list1 list2
  2. (integer) 0
  3. dragonfly> RPUSH list1 a b c
  4. (integer) 3
  5. dragonfly> BLPOP list1 list2 0
  6. 1) "list1"
  7. 2) "a"

Reliable queues

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.

Pattern: Event notification

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 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:

  1. LOOP forever
  2.     WHILE SPOP(key) returns elements
  3.         ... process elements ...
  4.     END
  5.     BRPOP helper_key
  6. END

While in the producer side we’ll use simply:

  1. MULTI
  2. SADD key element
  3. LPUSH helper_key x
  4. EXEC