Commit fc879abe authored by antirez's avatar antirez
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Merge branch 'unstable'

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<b>ExistsCommand: Contents</b><br>&nbsp;&nbsp;<a href="#EXISTS _key_">EXISTS _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="EXISTS _key_">EXISTS _key_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Test if the specified key exists. The command returns&quot;0&quot; if the key exists, otherwise &quot;1&quot; is returned.Note that even keys set with an empty string as value willreturn &quot;1&quot;.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python" name="code">
1 if the key exists.
0 if the key does not exist.
</pre>
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<b>ExpireCommand: Contents</b><br>&nbsp;&nbsp;<a href="#EXPIRE _key_ _seconds_">EXPIRE _key_ _seconds_</a><br>&nbsp;&nbsp;<a href="#EXPIREAT _key_ _unixtime_ (Redis &gt;">EXPIREAT _key_ _unixtime_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#PERSIST _key_ (Redis &gt;">PERSIST _key_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#How the expire is removed from a key">How the expire is removed from a key</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Restrictions with write operations against volatile keys">Restrictions with write operations against volatile keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Restrictions for write operations with volatile keys as sources">Restrictions for write operations with volatile keys as sources</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Setting the timeout again on already volatile keys">Setting the timeout again on already volatile keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Enhanced Lazy Expiration algorithm">Enhanced Lazy Expiration algorithm</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Version 1.0">Version 1.0</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Version 1.1">Version 1.1</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#FAQ: Can you explain better why Redis &lt; 2.1.3 deletes keys with an EXPIRE on write operations?">FAQ: Can you explain better why Redis &lt; 2.1.3 deletes keys with an EXPIRE on write operations?</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#FAQ: How this limitations were solved in Redis versions &gt; 2.1.3?">FAQ: How this limitations were solved in Redis versions &gt; 2.1.3?</a>
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<h1 class="wikiname">ExpireCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="EXPIRE _key_ _seconds_">EXPIRE _key_ _seconds_</a></h1>
<h1><a name="EXPIREAT _key_ _unixtime_ (Redis &gt;">EXPIREAT _key_ _unixtime_ (Redis &gt;</a></h1> 1.1)=
<h1><a name="PERSIST _key_ (Redis &gt;">PERSIST _key_ (Redis &gt;</a></h1> 2.1.3) =
<i>Time complexity: O(1)</i><blockquote>Set a timeout on the specified key. After the timeout the key will beautomatically deleted by the server. A key with an associated timeout issaid to be <i>volatile</i> in Redis terminology.</blockquote>
<blockquote>Voltile keys are stored on disk like the other keys, the timeout is persistenttoo like all the other aspects of the dataset. Saving a dataset containingexpires and stopping the server does not stop the flow of time as Redisstores on disk the time when the key will no longer be available as Unixtime, and not the remaining seconds.</blockquote>
<blockquote>EXPIREAT works exctly like EXPIRE but instead to get the number of secondsrepresenting the Time To Live of the key as a second argument (that is arelative way of specifing the TTL), it takes an absolute one in the form ofa UNIX timestamp (Number of seconds elapsed since 1 Gen 1970).</blockquote>
<blockquote>EXPIREAT was introduced in order to implement <a href="AppendOnlyFileHowto.html">the Append Only File persistence mode</a>so that EXPIRE commands are automatically translated into EXPIREAT commands for the append only file. Of course EXPIREAT can alsoused by programmers that need a way to simply specify that a given key should expire at a given time in the future.</blockquote>
<blockquote>Since Redis 2.1.3 you can update the value of the timeout of a key alreadyhaving an expire set. It is also possible to undo the expire at allturning the key into a normal key using the PERSIST command.</blockquote>
<h2><a name="How the expire is removed from a key">How the expire is removed from a key</a></h2><blockquote>When the key is set to a new value using the SET command, or when a keyis destroied via DEL, the timeout is removed from the key.</blockquote>
<h2><a name="Restrictions with write operations against volatile keys">Restrictions with write operations against volatile keys</a></h2><blockquote>IMPORTANT: Since Redis 2.1.3 or greater, there are no restrictions aboutthe operations you can perform against volatile keys, however older versionsof Redis, including the current stable version 2.0.0, has the followinglimitations:</blockquote>
<blockquote>Write operations like LPUSH, LSET and every other command that has theeffect of modifying the value stored at a volatile key have a special semantic:basically a volatile key is destroyed when it is target of a write operation.See for example the following usage pattern:</blockquote>
<pre class="codeblock python" name="code">
% ./redis-cli lpush mylist foobar /Users/antirez/hack/redis
OK
% ./redis-cli lpush mylist hello /Users/antirez/hack/redis
OK
% ./redis-cli expire mylist 10000 /Users/antirez/hack/redis
1
% ./redis-cli lpush mylist newelement
OK
% ./redis-cli lrange mylist 0 -1 /Users/antirez/hack/redis
1. newelement
</pre><blockquote>What happened here is that LPUSH against the key with a timeout set deletedthe key before to perform the operation. There is so a simple rule, writeoperations against volatile keys will destroy the key before to perform theoperation. Why Redis uses this behavior? In order to retain an importantproperty: a server that receives a given number of commands in the samesequence will end with the same dataset in memory. Without the delete-on-writesemantic what happens is that the state of the server depends on the timethe commands were issued. This is not a desirable property in a distributed databasethat supports replication.</blockquote>
<h2><a name="Restrictions for write operations with volatile keys as sources">Restrictions for write operations with volatile keys as sources</a></h2>Even when the volatile key is not modified as part of a write operation, if it is
read in a composite write operation (such as SINTERSTORE) it will be cleared at the
start of the operation. This is done to avoid concurrency issues in replication.
Imagine a key that is about to expire and the composite operation is run against it.
On a slave node, this key might already be expired, which leaves you with a
desync in your dataset.<h2><a name="Setting the timeout again on already volatile keys">Setting the timeout again on already volatile keys</a></h2><blockquote>Trying to call EXPIRE against a key that already has an associated timeoutwill not change the timeout of the key, but will just return 0. If insteadthe key does not have a timeout associated the timeout will be set and EXPIREwill return 1.</blockquote>
<h2><a name="Enhanced Lazy Expiration algorithm">Enhanced Lazy Expiration algorithm</a></h2><blockquote>Redis does not constantly monitor keys that are going to be expired.Keys are expired simply when some client tries to access a key, andthe key is found to be timed out.</blockquote>
<blockquote>Of course this is not enough as there are expired keys that will neverbe accessed again. This keys should be expired anyway, so once everysecond Redis test a few keys at random among keys with an expire set.All the keys that are already expired are deleted from the keyspace. </blockquote>
<h3><a name="Version 1.0">Version 1.0</a></h3><blockquote>Each time a fixed number of keys where tested (100 by default). So ifyou had a client setting keys with a very short expire faster than 100for second the memory continued to grow. When you stopped to insertnew keys the memory started to be freed, 100 keys every second in thebest conditions. Under a peak Redis continues to use more and more RAMeven if most keys are expired in each sweep.</blockquote>
<h3><a name="Version 1.1">Version 1.1</a></h3><blockquote>Each time Redis:</blockquote>
<ol><li> Tests 100 random keys from expired keys set.</li><li> Deletes all the keys found expired.</li><li> If more than 25 keys were expired, it start again from 1.</li></ol>
<blockquote>This is a trivial probabilistic algorithm, basically the assumption isthat our sample is representative of the whole key space,and we continue to expire until the percentage of keys that are likelyto be expired is under 25%</blockquote>
<blockquote>This means that at any given moment the maximum amount of keys alreadyexpired that are using memory is at max equal to max setting operations per second divided by 4.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically:<br/><br/><pre class="codeblock python python" name="code">
1: the timeout was set.
0: the timeout was not set since the key already has an associated timeout
(this may happen only in Redis versions &lt; 2.1.3, Redis &gt;= 2.1.3 will
happily update the timeout), or the key does not exist.
</pre><h2><a name="FAQ: Can you explain better why Redis &lt; 2.1.3 deletes keys with an EXPIRE on write operations?">FAQ: Can you explain better why Redis &lt; 2.1.3 deletes keys with an EXPIRE on write operations?</a></h2>
Ok let's start with the problem:
<pre class="codeblock python python python" name="code">
redis&gt; set a 100
OK
redis&gt; expire a 360
(integer) 1
redis&gt; incr a
(integer) 1
</pre>
I set a key to the value of 100, then set an expire of 360 seconds, and then incremented the key (before the 360 timeout expired of course). The obvious result would be: 101, instead the key is set to the value of 1. Why?
There is a very important reason involving the Append Only File and Replication. Let's rework a bit our example adding the notion of time to the mix:
<pre class="codeblock python python python python" name="code">
SET a 100
EXPIRE a 5
... wait 10 seconds ...
INCR a
</pre>
Imagine a Redis version that does not implement the &quot;Delete keys with an expire set on write operation&quot; semantic.
Running the above example with the 10 seconds pause will lead to 'a' being set to the value of 1, as it no longer exists when INCR is called 10 seconds later.<br/><br/>Instead if we drop the 10 seconds pause, the result is that 'a' is set to 101.<br/><br/>And in the practice timing changes! For instance the client may wait 10 seconds before INCR, but the sequence written in the Append Only File (and later replayed-back as fast as possible when Redis is restarted) will not have the pause. Even if we add a timestamp in the AOF, when the time difference is smaller than our timer resolution, we have a race condition.<br/><br/>The same happens with master-slave replication. Again, consider the example above: the client will use the same sequence of commands without the 10 seconds pause, but the replication link will slow down for a few seconds due to a network problem. Result? The master will contain 'a' set to 101, the slave 'a' set to 1.<br/><br/>The only way to avoid this but at the same time have reliable non time dependent timeouts on keys is to destroy volatile keys when a write operation is attempted against it.<br/><br/>After all Redis is one of the rare fully persistent databases that will give you EXPIRE. This comes to a cost :)<h2><a name="FAQ: How this limitations were solved in Redis versions &gt; 2.1.3?">FAQ: How this limitations were solved in Redis versions &gt; 2.1.3?</a></h2>Since Redis 2.1.3 there are no longer restrictions in the use you can do of write commands against volatile keys, still the replication and AOF file are guaranteed to be fully consistent.<br/><br/>In order to obtain a correct behavior without sacrificing consistency now when a key expires, a DEL operation is synthesized in both the AOF file and against all the attached slaves. This way the expiration process is centralized in the master instance, and there is no longer a chance of consistency errors.<br/><br/>However while the slaves while connected to a master will not expire keys independently, they'll still take the full state of the expires existing in the dataset, so when a slave is elected to a master it will be able to expire the keys independently, fully acting as a master.
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<b>Features: Contents</b><br>&nbsp;&nbsp;<a href="#Features (DRAFT)">Features (DRAFT)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Speed">Speed</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Persistence">Persistence</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Support for Data Structures">Support for Data Structures</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Atomic Operations">Atomic Operations</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Variety of Supported Languages">Variety of Supported Languages</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Master/Slave Replication">Master/Slave Replication</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Sharding">Sharding</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Hot Backups">Hot Backups</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Simple to Install, Setup and Manage">Simple to Install, Setup and Manage</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Portable">Portable</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Liberal Licensing">Liberal Licensing</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#What's next?">What's next?</a>
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<h1 class="wikiname">Features</h1>
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<h1><a name="Features (DRAFT)">Features (DRAFT)</a></h1>Checking Redis for the first time? Here your will find the most important features, and pointers to a lot more information.<h2><a name="Speed">Speed</a></h2>Redis is written in ANSI C, and loads the whole dataset in memory, so it is wicked <i><b>fast</b>!</i> Up to 110,000 <a href="SETs.html">SETs</a>/second, 81,000 GETs/second can be achieved in an entry level Linux box. Read more about Redis <a href="Speed.html">Speed</a>.<br/><br/>Also Redis supports <a href="Pipelining.html">Pipelining</a> of commands and <a href="MultiBulkCommands.html">getting and setting m&Atilde;&ordm;ltiple values in a single command</a> to speed up communication with the client libraries.<h2><a name="Persistence">Persistence</a></h2>While all the data lives in memory, changes are <i>asynchronously</i> saved on disk using flexible policies based on elapsed time and/or number of updates since last save. <br/><br/>If you can't afford losing some data, starting on version 1.1 (currently in beta but you can download it from the Git repository) Redis supports an append-only file persistence mode. Check more on <a href="Persistence.html">Persistence</a>, or read the <a href="AppendOnlyFileHowto.html">AppendOnlyFileHowto</a> for more information.<h2><a name="Support for Data Structures">Support for Data Structures</a></h2>Values in Redis can be <a href="Strings.html">Strings</a> as in a conventional key-value store, but also <a href="Lists.html">Lists</a>, <a href="Sets.html">Sets</a>, and <a href="SortedSets.html">SortedSets</a> (to be support in <a href="RoadMap.html">version 1.1</a>). This data types allow pushing/poping elements, or adding/removing them, also perform server side union, intersection, difference between sets, and so forth depending on the types. Redis supports different kind of sorting abilities for <a href="Sets.html">Sets</a> and <a href="Lists.html">Lists</a>.<br/><br/>You can think in Redis as a <b>Data Structures Server</b>, that allows you to model non trivial problems. Read <a href="DataTypes.html">Data Types</a> to learn more about the way Redis handle <a href="Strings.html">Strings</a>, and the <a href="Commands.html">Commands</a> supported by <a href="Lists.html">Lists</a>, <a href="Sets.html">Sets</a> and <a href="SortedSets.html">SortedSets</a><h2><a name="Atomic Operations">Atomic Operations</a></h2>Redis operations working on the different Data Types are <b>atomic</b>, so setting or increasing a key, adding and removing elements from a set, increasing a counter will all be accomplished safely.<h2><a name="Variety of Supported Languages">Variety of Supported Languages</a></h2>Ruby, Python, Twisted Python, PHP, Erlang, Tcl, Perl, Lua, Java, Scala, Clojure, choose your poison. Check the list of <a href="SupportedLanguages.html">Supported Languages</a> for all the details.<br/><br/>If your favorite language is not supported yet, you can write your own client library, as the <a href="ProtocolSpecification.html">Protocol</a> is pretty simple.<h2><a name="Master/Slave Replication">Master/Slave Replication</a></h2>Redis supports a very simple and fast Master/Slave replication. Is so simple it takes only one line in the <a href="Configuration.html">configuration file</a> to set it up, and 21 seconds for a Slave to complete the initial sync of 10 MM key set in a Amazon EC2 instance.<br/><br/>Read more about Master/Slave <a href="Replication.html">Replication</a>. <h2><a name="Sharding">Sharding</a></h2>Distributing the dataset across multiple Redis instances is easy in Redis, as in any other key-value store. And this depends basically on the <a href="Supported.html">Languages</a> client libraries being able to do so. <br/><br/>Read more about <a href="Sharding.html">Sharding</a> if you want to know more abour distributing data and workload in Redis.<h2><a name="Hot Backups">Hot Backups</a></h2>TODO<h2><a name="Simple to Install, Setup and Manage">Simple to Install, Setup and Manage</a></h2>Installing Redis requires little more than downloading it, uncompressing it and running make. Management is near zero, so you can start using Redis in a matter of minutes.<br/><br/>Go on and read about Redis <a href="Installation.html">installation</a>, its <a href="Setup.html">Setup</a> and <a href="Management.html">Management</a>.<h2><a name="Portable">Portable</a></h2>Redis is written in ANSI C and works in most POSIX systems like Linux, BSD, Mac OS X, Solaris, and so on. Redis is reported to compile and work under WIN32 if compiled with Cygwin, but there is no official support for Windows currently.<h2><a name="Liberal Licensing">Liberal Licensing</a></h2>Redis is free software released under the very liberal BSD license.<h2><a name="What's next?">What's next?</a></h2>Want to get started with Redis? Try the <a href="QuickStart.html">Quick Start</a> you will be up and running in just a matter of minutes. <br/><br/>Check the <a href="CodeSamples.html">Code Samples</a> and find how you can use Redis with your favorite programming language.<br/><br/><a href="Comparisons.html">Compare</a> Redis with other key-value stores, like Tokyo Cabinet or Memcached.
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<b>FlushallCommand: Contents</b><br>&nbsp;&nbsp;<a href="#FLUSHALL">FLUSHALL</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="FLUSHALL">FLUSHALL</a></h1>
<blockquote>Delete all the keys of all the existing databases, not just the currently selected one. This command never fails.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="FLUSHDB">FLUSHDB</a></h1>
<blockquote>Delete all the keys of the currently selected DB. This command never fails.</blockquote>
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<b>FromSqlToDataStructures: Contents</b><br>&nbsp;&nbsp;<a href="#Introduction (IDEA MORE THAN A DRAFT)">Introduction (IDEA MORE THAN A DRAFT)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Data Structures">Data Structures</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Dude where is my SELECT statement?">Dude where is my SELECT statement?</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#LISTs">LISTs</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SETs">SETs</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SORT to the rescue">SORT to the rescue</a><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#SORT BY">SORT BY</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#HASHEs">HASHEs</a>
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<h1><a name="Introduction (IDEA MORE THAN A DRAFT)">Introduction (IDEA MORE THAN A DRAFT)</a></h1><b>&Acirc;&iquest;Coming from SQLand?</b> <i>&Acirc;&iquest;Who doesn't?</i> Redis is simple, <i>primitive</i> when comapred to the world you are used to in the world of Relational Database Managers (RDBMS) and Structure Query Language (SQL), here you will find insight to build bridges between both worlds to model real life problems.<h2><a name="Data Structures">Data Structures</a></h2>When I was young, happy and single ;) I studied <b>Data Structures</b> at the university, actually I learnt Data Structures and Algorithms <i>before</i> learning anything about Databases, and particularly RDBMS and SQL. This is natural because you need to know about Data Structures and Algorithms to understand a Database.<br/><br/>Redis can be seen as a <b>Data Structures Server</b>, a very simple interface to a extremly fast and efficient <h2><a name="Dude where is my SELECT statement?">Dude where is my SELECT statement?</a></h2><h2><a name="LISTs">LISTs</a></h2>In SQL there is no such thing as a &quot;natural&quot; order, a SELECT statement without a ORDER BY clause will return data in a undefined order. In Redis <a href="LISTs.html">LISTs</a> address the problem of natural ordering, ...<h2><a name="SETs">SETs</a></h2>So you have a bunch of unordered data, <h2><a name="SORT to the rescue">SORT to the rescue</a></h2>But sometimes we <i>need</i> to actually sort a LIST in a order different from its natural or take a SET and have it ordered, there is where the <i>fast</i> SORT commands comes handy... <h3><a name="SORT BY">SORT BY</a></h3>Just SORTing keys would be kind of boring, sometimes useless right? Well, you can SORT...<h2><a name="HASHEs">HASHEs</a></h2>Umm, sorry you will have to wait for a <a href="RoadMap.html">upcoming version of Redis</a> to have Hashes, but here are Idioms you should house to manage Dictionary like data...
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&iuml;&raquo;&iquest;== Generic Commands ==<br/><br/><ul><li> <a href="ExistsCommand.html">EXISTS</a></li><li> <a href="DelCommand.html">DEL</a></li><li> <a href="TypeCommand.html">TYPE</a></li><li> <a href="KeysCommand.html">KEYS</a></li><li> <a href="RandomkeyCommand.html">RANDOMKEY</a></li><li> <a href="RenameCommand.html">RENAME</a></li><li> <a href="RenamenxCommand.html">RENAMENX</a></li><li> <a href="DbsizeCommand.html">DBSIZE</a></li><li> <a href="ExpireCommand.html">EXPIRE</a></li><li> <a href="ExpireCommand.html">PERSIST</a></li><li> <a href="TtlCommand.html">TTL</a></li><li> <a href="SelectCommand.html">SELECT</a></li><li> <a href="MoveCommand.html">MOVE</a></li><li> <a href="FlushdbCommand.html">FLUSHDB</a></li><li> <a href="FlushallCommand.html">FLUSHALL</a></li><li> <a href="MultiExecCommand.html">Redis Transactions</a></li></ul>
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<b>GetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#GET _key_">GET _key_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">GetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="GET _key_">GET _key_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Get the value of the specified key. If the keydoes not exist the special value 'nil' is returned.If the value stored at <i>key</i> is not a string an erroris returned because GET can only handle string values.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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<b>GetbitCommand: Contents</b><br>&nbsp;&nbsp;<a href="#GETBIT _key_ _offset_ (Redis &gt;">GETBIT _key_ _offset_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">GetbitCommand</h1>
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<h1><a name="GETBIT _key_ _offset_ (Redis &gt;">GETBIT _key_ _offset_ (Redis &gt;</a></h1> 2.1.8) =
<i>Time complexity: O(1)</i><blockquote>Returns the bit value at <i>offset</i> in the string value stored at <i>key</i>.</blockquote>
When <i>offset</i> is beyond the string length, the string is assumed to be a contiguous space with 0 bits. When <i>key</i> does not exist it is assumed to be an empty string, so <i>offset</i> is always out of range and the value is also assumed to be a contiguous space with 0 bits.<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically: the bit value stored at <i>offset</i>.
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<b>GetsetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#GETSET _key_ _value_">GETSET _key_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Design patterns">Design patterns</a>
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<h1 class="wikiname">GetsetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="GETSET _key_ _value_">GETSET _key_ _value_</a></h1>
<i>Time complexity: O(1)</i><blockquote>GETSET is an atomic <i>set this value and return the old value</i> command.Set <i>key</i> to the string <i>value</i> and return the old value stored at <i>key</i>.The string can't be longer than 1073741824 bytes (1 GB).</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a><h2><a name="Design patterns">Design patterns</a></h2><blockquote>GETSET can be used together with INCR for counting with atomic reset whena given condition arises. For example a process may call INCR against thekey <i>mycounter</i> every time some event occurred, but from time totime we need to get the value of the counter and reset it to zero atomicallyusing <code name="code" class="python">GETSET mycounter 0</code>.</blockquote>
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<b>HackingStrings: Contents</b><br>&nbsp;&nbsp;<a href="#Hacking Strings">Hacking Strings</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Creating Redis Strings">Creating Redis Strings</a>
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<h1 class="wikiname">HackingStrings</h1>
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<h1><a name="Hacking Strings">Hacking Strings</a></h1>The implementation of Redis strings is contained in <b></b>sds.c<b></b> ( sds stands for Simple Dynamic Strings ).<br/><br/>The C structure <i>sdshdr</i> declared in <b>sds.h</b> represents a Redis string:<br/><br/><pre class="codeblock python" name="code">
struct sdshdr {
long len;
long free;
char buf[];
};
</pre>The <i>buf</i> character array stores the actual string.<br/><br/>The <i>len</i> field stores the length of <i>buf</i>. This makes obtaining the length
of a Redis string an O(1) operation.<br/><br/>The <i>free</i> field stores the number of additional bytes available for use.<br/><br/>Together the <i>len</i> and <i>free</i> field can be thought of as holding the metadata of the
<i>buf</i> character array.<h2><a name="Creating Redis Strings">Creating Redis Strings</a></h2>A new data type named <code name="code" class="python">sds</code> is defined in <b>sds.h</b> to be a synonymn for a character pointer:<br/><br/><pre class="codeblock python python" name="code">
typedef char *sds;
</pre><code name="code" class="python">sdsnewlen</code> function defined in <b>sds.c</b> creates a new Redis String: <br/><br/><pre class="codeblock python python python" name="code">
sds sdsnewlen(const void *init, size_t initlen) {
struct sdshdr *sh;
sh = zmalloc(sizeof(struct sdshdr)+initlen+1);
#ifdef SDS_ABORT_ON_OOM
if (sh == NULL) sdsOomAbort();
#else
if (sh == NULL) return NULL;
#endif
sh-&gt;len = initlen;
sh-&gt;free = 0;
if (initlen) {
if (init) memcpy(sh-&gt;buf, init, initlen);
else memset(sh-&gt;buf,0,initlen);
}
sh-&gt;buf[initlen] = '\0';
return (char*)sh-&gt;buf;
}
</pre>Remember a Redis string is a variable of type <code name="code" class="python">struct sdshdr</code>. But <code name="code" class="python">sdsnewlen</code> returns a character pointer!!<br/><br/>That's a trick and needs some explanation.<br/><br/>Suppose I create a Redis string using <code name="code" class="python">sdsnewlen</code> like below:<br/><br/><pre class="codeblock python python python python" name="code">
sdsnewlen(&quot;redis&quot;, 5);
</pre>This creates a new variable of type <code name="code" class="python">struct sdshdr</code> allocating memory for <i>len</i> and <i>free</i>
fields as well as for the <i>buf</i> character array.<br/><br/><pre class="codeblock python python python python python" name="code">
sh = zmalloc(sizeof(struct sdshdr)+initlen+1); // initlen is length of init argument.
</pre>After <code name="code" class="python">sdsnewlen</code> succesfully creates a Redis string the result is something like:<br/><br/><pre class="codeblock python python python python python python" name="code">
-----------
|5|0|redis|
-----------
^ ^
sh sh-&gt;buf
</pre><code name="code" class="python">sdsnewlen</code> returns sh-&gt;buf to the caller.<br/><br/>What do you do if you need to free the Redis string pointed by <code name="code" class="python">sh</code>?<br/><br/>You want the pointer <code name="code" class="python">sh</code> but you only have the pointer <code name="code" class="python">sh-&gt;buf</code>.<br/><br/>Can you get the pointer <code name="code" class="python">sh</code> from <code name="code" class="python">sh-&gt;buf</code>?<br/><br/>Yes. Pointer arithmetic. Notice from the above ASCII art that if you subtract
the size of two longs from <code name="code" class="python">sh-&gt;buf</code> you get the pointer <code name="code" class="python">sh</code>. <br/><br/>The sizeof two longs happens to be the size of <code name="code" class="python">struct sdshdr</code>.<br/><br/>Look at <code name="code" class="python">sdslen</code> function and see this trick at work:<br/><br/><pre class="codeblock python python python python python python python" name="code">
size_t sdslen(const sds s) {
struct sdshdr *sh = (void*) (s-(sizeof(struct sdshdr)));
return sh-&gt;len;
}
</pre>Knowing this trick you could easily go through the rest of the functions in <b>sds.c</b>.<br/><br/>The Redis string implementation is hidden behind an interface that accepts only character pointers. The users of Redis strings need not care about how its implemented and treat Redis strings as a character pointer.
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<h1 class="wikiname">HashCommandsSidebar</h1>
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&iuml;&raquo;&iquest;== Hash Commands ==<br/><br/><ul><li> <a href="HsetCommand.html">HSET</a></li><li> <a href="HgetCommand.html">HGET</a></li><li> <a href="HsetnxCommand.html">HSETNX</a></li><li> <a href="HmsetCommand.html">HMSET</a></li><li> <a href="HmgetCommand.html">HMGET</a></li><li> <a href="HincrbyCommand.html">HINCRBY</a></li><li> <a href="HexistsCommand.html">HEXISTS</a></li><li> <a href="HdelCommand.html">HDEL</a></li><li> <a href="HlenCommand.html">HLEN</a></li><li> <a href="HgetallCommand.html">HKEYS</a></li><li> <a href="HgetallCommand.html">HVALS</a></li><li> <a href="HgetallCommand.html">HGETALL</a></li></ul>
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<b>Hashes: Contents</b><br>&nbsp;&nbsp;<a href="#Redis Hash Type">Redis Hash Type</a><br>&nbsp;&nbsp;<a href="#Implementation details">Implementation details</a>
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<h1 class="wikiname">Hashes</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="Redis Hash Type">Redis Hash Type</a></h1>Redis Hashes are unordered maps of <a href="String.html">Redis Strings</a> between fields and values. It is possible to add, remove, test for existence of fields in O(1) amortized time. It is also possible to enumerate all the keys, values, or both, in O(N) (where N is the number of fields inside the hash).<br/><br/>Redis Hashes are interesting because they are very well suited to represent objects. For instance web applications users can be represented by a Redis Hash containing fields such username, encrpypted_password, lastlogin, and so forth.<br/><br/>Another very important property of Redis Hashes is that they use very little memory for hashes composed of a small number of fields (configurable, check redis.conf for details), compared to storing every field as a top level Redis key. This is obtained using a different specialized representation for small hashes. See the implementation details paragraph below for more information.<br/><br/>Commands operating on hashes try to make a good use of the return value in order to signal the application about previous existence of fields. For instance the <a href="HsetCommand.html">HSET</a> command will return 1 if the field set was not already present in the hash, otherwise will return 0 (and the user knows this was just an update operation).<br/><br/>The max number of fields in a set is 232-1 (4294967295, more than 4 billion of members per hash).<h1><a name="Implementation details">Implementation details</a></h1>The obvious internal representation of hashes is indeed an hash table, as the name of the data structure itself suggests. Still the drawback of this representation is that there is a lot of space overhead for hash table metadata.<br/><br/>Because one of the most interesting uses of Hashes is object encoding, and objects are often composed of a few fields each, Redis uses a different internal representation for small hashes (for Redis to consider a hash small, this must be composed a limited number of fields, and each field and value can't exceed a given number of bytes. All this is user-configurable).<br/><br/>Small hashes are thus encoded using a data structure called zipmap (is not something you can find in a CS book, the name is a Redis invention), that is a very memory efficient data structure to represent string to string maps, at the cost of being O(N) instead of O(1) for most operations. Since the constant times of this data structure are very small, and the zipmaps are converted into real hash tables once they are big enough, the amortized time of Redis hashes is still O(1), and in the practice small zipmaps are not slower than small hash tables because they are designed for good cache locality and fast access.<br/><br/>The result is that small hashes are both memory efficient and fast, while bigger hashes are fast but not as memory efficient than small hashes.
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<b>HdelCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HDEL _key_ _field_ (Redis &gt;">HDEL _key_ _field_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HdelCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HDEL _key_ _field_ (Redis &gt;">HDEL _key_ _field_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>Remove the specified <i>field</i> from an hash stored at <i>key</i>.</blockquote>
<blockquote>If the <i>field</i> was present in the hash it is deleted and 1 is returned, otherwise 0 is returned and no operation is performed.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>HexistsCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HEXISTS _key_ _field_ (Redis &gt;">HEXISTS _key_ _field_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HexistsCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HEXISTS _key_ _field_ (Redis &gt;">HEXISTS _key_ _field_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>Return 1 if the hash stored at <i>key</i> contains the specified <i>field</i>.</blockquote>
<blockquote>Return 0 if the <i>key</i> is not found or the <i>field</i> is not present.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>HgetCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HGET _key_ _field_ (Redis &gt;">HGET _key_ _field_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HgetCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HGET _key_ _field_ (Redis &gt;">HGET _key_ _field_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>If <i>key</i> holds a hash, retrieve the value associated to the specified <i>field</i>.</blockquote>
<blockquote>If the <i>field</i> is not found or the <i>key</i> does not exist, a special 'nil' value is returned.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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<b>HgetallCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HKEYS _key_ (Redis &gt;">HKEYS _key_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#HVALS _key_ (Redis &gt;">HVALS _key_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#HGETALL _key_ (Redis &gt;">HGETALL _key_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HgetallCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HKEYS _key_ (Redis &gt;">HKEYS _key_ (Redis &gt;</a></h1> 1.3.10)=
<h1><a name="HVALS _key_ (Redis &gt;">HVALS _key_ (Redis &gt;</a></h1> 1.3.10)=
<h1><a name="HGETALL _key_ (Redis &gt;">HGETALL _key_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(N), where N is the total number of fields in the hash</i><blockquote>HKEYS returns all the fields names contained into a hash, HVALS all the associated values, while HGETALL returns both the fields and values in the form of <i>field1</i>, <i>value1</i>, <i>field2</i>, <i>value2</i>, ..., <i>fieldN</i>, <i>valueN</i>.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Multi Bulk Reply</a>
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<b>HincrbyCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HINCRBY _key_ _field_ _value_ (Redis &gt;">HINCRBY _key_ _field_ _value_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Examples">Examples</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HincrbyCommand</h1>
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<h1><a name="HINCRBY _key_ _field_ _value_ (Redis &gt;">HINCRBY _key_ _field_ _value_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>Increment the number stored at <i>field</i> in the hash at <i>key</i> by <i>value</i>. If <i>key</i> does not exist, a new key holding a hash is created. If <i>field</i> does not exist or holds a string, the value is set to 0 before applying the operation.</blockquote>
<blockquote>The range of values supported by HINCRBY is limited to 64 bit signed integers.</blockquote><h2><a name="Examples">Examples</a></h2>
Since the <i>value</i> argument is signed you can use this command to perform both increments and decrements:<br/><br/><pre class="codeblock python" name="code">
HINCRBY key field 1 (increment by one)
HINCRBY key field -1 (decrement by one, just like the DECR command)
HINCRBY key field -10 (decrement by 10)
</pre>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a> The new value at <i>field</i> after the increment operation.
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<b>HlenCommand: Contents</b><br>&nbsp;&nbsp;<a href="#HLEN _key_ (Redis &gt;">HLEN _key_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1 class="wikiname">HlenCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="HashCommandsSidebar.html">HashCommandsSidebar</a><h1><a name="HLEN _key_ (Redis &gt;">HLEN _key_ (Redis &gt;</a></h1> 1.3.10)=
<i>Time complexity: O(1)</i><blockquote>Return the number of entries (fields) contained in the hash stored at <i>key</i>. If the specified <i>key</i> does not exist, 0 is returned assuming an empty hash. </blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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