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Kenneth Reitz
100 lines
4.3 KiB
ReStructuredText
100 lines
4.3 KiB
ReStructuredText
`|Redis Documentation| <index.html>`_
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**BlpopCommand: Contents**
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`BLPOP \_key1\_ \_key2\_ ... \_keyN\_ \_timeout\_ (Redis > <#BLPOP%20_key1_%20_key2_%20...%20_keyN_%20_timeout_%20(Redis%20%3E>`_
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`BRPOP \_key1\_ \_key2\_ ... \_keyN\_ \_timeout\_ (Redis > <#BRPOP%20_key1_%20_key2_%20...%20_keyN_%20_timeout_%20(Redis%20%3E>`_
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`Non blocking behavior <#Non%20blocking%20behavior>`_
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`Blocking behavior <#Blocking%20behavior>`_
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`Multiple clients blocking for the same keys <#Multiple%20clients%20blocking%20for%20the%20same%20keys>`_
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`blocking POP inside a MULTI/EXEC transaction <#blocking%20POP%20inside%20a%20MULTI/EXEC%20transaction>`_
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`Return value <#Return%20value>`_
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BlpopCommand
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============
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#sidebar `ListCommandsSidebar <ListCommandsSidebar.html>`_
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BLPOP \_key1\_ \_key2\_ ... \_keyN\_ \_timeout\_ (Redis >
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=========================================================
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1.3.1) =
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BRPOP \_key1\_ \_key2\_ ... \_keyN\_ \_timeout\_ (Redis >
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=========================================================
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1.3.1) = *Time complexity: O(1)*
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BLPOP (and BRPOP) is a blocking list pop primitive. You can see
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this commandsas blocking versions of `LPOP <LpopCommand.html>`_ and
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`RPOP <LpopCommand.html>`_ able toblock if the specified keys don't
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exist or contain empty lists.
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The following is a description of the exact semantic. We describe
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BLPOP butthe two commands are identical, the only difference is
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that BLPOP pops theelement from the left (head) of the list, and
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BRPOP pops from the right (tail).
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Non blocking behavior
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---------------------
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When BLPOP is called, if at least one of the specified keys contain
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a nonempty list, an element is popped from the head of the list and
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returned tothe caller together with the name of the key (BLPOP
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returns a two elementsarray, the first element is the key, the
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second the popped value).
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Keys are scanned from left to right, so for instance if youissue
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**BLPOP list1 list2 list3 0** against a dataset where **list1**
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does notexist but **list2** and **list3** contain non empty lists,
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BLPOP guaranteesto return an element from the list stored at
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**list2** (since it is the firstnon empty list starting from the
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left).
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Blocking behavior
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-----------------
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If none of the specified keys exist or contain non empty lists,
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BLPOPblocks until some other client performs a
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`LPUSH <RpushCommand.html>`_ oran `RPUSH <RpushCommand.html>`_
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operation against one of the lists.
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Once new data is present on one of the lists, the client finally
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returnswith the name of the key unblocking it and the popped
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value.
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When blocking, if a non-zero timeout is specified, the client will
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unblockreturning a nil special value if the specified amount of
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seconds passedwithout a push operation against at least one of the
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specified keys.
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The timeout argument is interpreted as an integer value. A timeout
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of zero means instead to block forever.
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Multiple clients blocking for the same keys
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-------------------------------------------
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Multiple clients can block for the same key. They are put intoa
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queue, so the first to be served will be the one that started to
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waitearlier, in a first-blpopping first-served fashion.
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blocking POP inside a MULTI/EXEC transaction
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--------------------------------------------
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BLPOP and BRPOP can be used with pipelining (sending multiple
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commands and reading the replies in batch), but it does not make
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sense to use BLPOP or BRPOP inside a MULTI/EXEC block (a Redis
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transaction).
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The behavior of BLPOP inside MULTI/EXEC when the list is empty is
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to return a multi-bulk nil reply, exactly what happens when the
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timeout is reached. If you like science fiction, think at it like
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if inside MULTI/EXEC the time will flow at infinite speed :)
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Return value
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------------
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BLPOP returns a two-elements array via a multi bulk reply in order
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to returnboth the unblocking key and the popped value.
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When a non-zero timeout is specified, and the BLPOP operation timed
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out,the return value is a nil multi bulk reply. Most client values
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will returnfalse or nil accordingly to the programming language
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used.
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`Multi bulk reply <ReplyTypes.html>`_
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.. |Redis Documentation| image:: redis.png |