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Kenneth Reitz
63 lines
2.9 KiB
ReStructuredText
63 lines
2.9 KiB
ReStructuredText
`|Redis Documentation| <index.html>`_
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**RpoplpushCommand: Contents**
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`RPOPLPUSH \_srckey\_ \_dstkey\_ (Redis > <#RPOPLPUSH%20_srckey_%20_dstkey_%20(Redis%20%3E>`_
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`Programming patterns: safe queues <#Programming%20patterns:%20safe%20queues>`_
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`Programming patterns: server-side O(N) list traversal <#Programming%20patterns:%20server-side%20O(N)%20list%20traversal>`_
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`Return value <#Return%20value>`_
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RpoplpushCommand
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================
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#sidebar `ListCommandsSidebar <ListCommandsSidebar.html>`_
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RPOPLPUSH \_srckey\_ \_dstkey\_ (Redis >
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========================================
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1.1) = *Time complexity: O(1)*
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Atomically return and remove the last (tail) element of the
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*srckey* list,and push the element as the first (head) element of
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the *dstkey* list. Forexample if the source list contains the
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elements "a","b","c" and thedestination list contains the elements
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"foo","bar" after an RPOPLPUSH commandthe content of the two lists
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will be "a","b" and "c","foo","bar".
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If the *key* does not exist or the list is already empty the
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specialvalue 'nil' is returned. If the *srckey* and *dstkey* are
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the same theoperation is equivalent to removing the last element
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from the list and pusingit as first element of the list, so it's a
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"list rotation" command.
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Programming patterns: safe queues
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---------------------------------
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Redis lists are often used as queues in order to exchange messages
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betweendifferent programs. A program can add a message performing
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an `LPUSH <RpushCommand.html>`_ operationagainst a Redis list (we
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call this program a Producer), while another program(that we call
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Consumer) can process the messages performing an
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`RPOP <LpopCommand.html>`_ commandin order to start reading the
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messages from the oldest.
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Unfortunately if a Consumer crashes just after an
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`RPOP <LpopCommand.html>`_ operation the messagegets lost.
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RPOPLPUSH solves this problem since the returned message isadded to
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another "backup" list. The Consumer can later remove the
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messagefrom the backup list using the `LREM <LremCommand.html>`_
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command when the message was correctlyprocessed.
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Another process, called Helper, can monitor the "backup" list to
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check fortimed out entries to repush against the main queue.
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Programming patterns: server-side O(N) list traversal
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-----------------------------------------------------
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Using RPOPPUSH with the same source and destination key a process
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canvisit all the elements of an N-elements List in O(N) without to
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transferthe full list from the server to the client in a single
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`LRANGE <LrangeCommand.html>`_ operation.Note that a process can
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traverse the list even while other processesare actively RPUSHing
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against the list, and still no element will be skipped.
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Return value
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------------
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`Bulk reply <ReplyTypes.html>`_
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.. |Redis Documentation| image:: redis.png |