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

parents e4f6b8c3 cf1eefa4
An updated list of client libraries for Redis can be found here:
http://redis.io/clients
All the links are in the front page.
Most mainstream (and less mainstream) languages are supported.
If you plan to write a new client library for a missing language, please
get in touch with the other developers writing a message to the Redis google
group that is located here: http://groups.google.com/group/redis-db
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<b>AppendCommand: Contents</b><br>&nbsp;&nbsp;<a href="#APPEND _key_ _value_">APPEND _key_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Examples">Examples</a>
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&iuml;&raquo;&iquest;#sidebar <a href="StringCommandsSidebar.html">StringCommandsSidebar</a><h1><a name="APPEND _key_ _value_">APPEND _key_ _value_</a></h1>
<i>Time complexity: O(1). The amortized time complexity is O(1) assuming the appended value is small and the already present value is of any size, since the dynamic string library used by Redis will double the free space available on every reallocation.</i><blockquote>If the <i>key</i> already exists and is a string, this command appends theprovided value at the end of the string.If the <i>key</i> does not exist it is created and set as an empty string, soAPPEND will be very similar to SET in this special case.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>, specifically the total length of the string after the append operation.<h2><a name="Examples">Examples</a></h2><pre class="codeblock python" name="code">
redis&gt; exists mykey
(integer) 0
redis&gt; append mykey &quot;Hello &quot;
(integer) 6
redis&gt; append mykey &quot;World&quot;
(integer) 11
redis&gt; get mykey
&quot;Hello World&quot;
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<b>AppendOnlyFileHowto: Contents</b><br>&nbsp;&nbsp;<a href="#Append Only File HOWTO">Append Only File HOWTO</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#General Information">General Information</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Log rewriting">Log rewriting</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Wait... but how does this work?">Wait... but how does this work?</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#How durable is the append only file?">How durable is the append only file?</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#What should I do if my Append Only File gets corrupted?">What should I do if my Append Only File gets corrupted?</a>
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&iuml;&raquo;&iquest;#sidebar <a href="RedisGuides.html">RedisGuides</a>
<h1><a name="Append Only File HOWTO">Append Only File HOWTO</a></h1><h2><a name="General Information">General Information</a></h2>Append only file is an alternative durability option for Redis. What this mean? Let's start with some fact:<br/><br/><ul><li> For default Redis saves snapshots of the dataset on disk, in a binary file called dump.rdb (by default at least). For instance you can configure Redis to save the dataset every 60 seconds if there are at least 100 changes in the dataset, or every 1000 seconds if there is at least a single change in the dataset. This is known as &quot;Snapshotting&quot;.</li><li> Snapshotting is not very durable. If your computer running Redis stops, your power line fails, or you write killall -9 redis-server for a mistake, the latest data written on Redis will get lost. There are applications where this is not a big deal. There are applications where this is not acceptable and Redis <b>was</b> not an option for this applications.</li></ul>
What is the solution? To use append only file as alternative to snapshotting. How it works?<br/><br/><ul><li> It is an 1.1 only feature.</li><li> You have to turn it on editing the configuration file. Just make sure you have &quot;appendonly yes&quot; somewhere.</li><li> Append only files work this way: every time Redis receive a command that changes the dataset (for instance a SET or LPUSH command) it appends this command in the append only file. When you restart Redis it will first <b>re-play</b> the append only file to rebuild the state.</li></ul>
<h2><a name="Log rewriting">Log rewriting</a></h2>As you can guess... the append log file gets bigger and bigger, every time there is a new operation changing the dataset. Even if you set always the same key &quot;mykey&quot; to the values of &quot;1&quot;, &quot;2&quot;, &quot;3&quot;, ... up to 10000000000 in the end you'll have just a single key in the dataset, just a few bytes! but how big will be the append log file? Very very big.<br/><br/>So Redis supports an interesting feature: it is able to rebuild the append log file, in background, without to stop processing client commands. The key is the command <a href="BGREWRITEAOF.html">BGREWRITEAOF</a>. This command basically is able to use the dataset in memory in order to rewrite the shortest sequence of commands able to rebuild the exact dataset that is currently in memory.<br/><br/>So from time to time when the log gets too big, try this command. It's safe as if it fails you will not lost your old log (but you can make a backup copy given that currently 1.1 is still in beta!).<h2><a name="Wait... but how does this work?">Wait... but how does this work?</a></h2>Basically it uses the same fork() copy-on-write trick that snapshotting already uses. This is how the algorithm works:<br/><br/><ul><li> Redis forks, so now we have a child and a parent.</li><li> The child starts writing the new append log file in a temporary file.</li><li> The parent accumulates all the new changes in an in-memory buffer (but at the same time it writes the new changes in the <b>old</b> append only file, so if the rewriting fails, we are safe).</li><li> When the child finished to rewrite the file, the parent gets a signal, and append the in-memory buffer at the end of the file generated by the child.</li><li> Profit! Now Redis atomically renames the old file into the new one, and starts appending new data into the new file.</li></ul>
<h2><a name="How durable is the append only file?">How durable is the append only file?</a></h2>Check redis.conf, you can configure how many times Redis will fsync() data on disk. There are three options:<br/><br/><ul><li> Fsync() every time a new command is appended to the append log file. Very very slow, very safe.</li><li> Fsync() one time every second. Fast enough, and you can lose 1 second of data if there is a disaster.</li><li> Never fsync(), just put your data in the hands of the Operating System. The faster and unsafer method.</li></ul>
The suggested (and default) policy is &quot;everysec&quot;. It is both very fast and pretty safe. The &quot;always&quot; policy is very slow in practice, even if it was improved in Redis 2.0.0 there is no way to make fsync() faster than it is.<h2><a name="What should I do if my Append Only File gets corrupted?">What should I do if my Append Only File gets corrupted?</a></h2>It is possible that the server crashes while writing the AOF file (this still should never lead to inconsistencies) corrupting the file in a way that is no longer loadable by Redis. When this happens you can fix this problem using the following procedure:<br/><br/><ul><li> Make a backup copy of your AOF file.</li><li> Fix the original file with: ./redis-check-aof --fix <code name="code" class="python">&lt;filename&gt;</code></li><li> Optionally use diff -u to check what is the difference between two files.</li><li> Restart the server with the fixed file.</li></ul>
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<b>AuthCommand: Contents</b><br>&nbsp;&nbsp;<a href="#AUTH _password_">AUTH _password_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ConnectionHandlingSidebar.html">ConnectionHandlingSidebar</a><h1><a name="AUTH _password_">AUTH _password_</a></h1><blockquote>Request for authentication in a password protected Redis server.A Redis server can be instructed to require a password before to allow clientsto issue commands. This is done using the <i>requirepass</i> directive in theRedis configuration file.</blockquote>
<blockquote>If the password given by the client is correct the server replies withan OK status code reply and starts accepting commands from the client.Otherwise an error is returned and the clients needs to try a new password.Note that for the high performance nature of Redis it is possible to trya lot of passwords in parallel in very short time, so make sure to generatea strong and very long password so that this attack is infeasible.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>Benchmarks: Contents</b><br>&nbsp;&nbsp;<a href="#How Fast is Redis?">How Fast is Redis?</a><br>&nbsp;&nbsp;<a href="#Latency percentiles">Latency percentiles</a>
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<h1 class="wikiname">Benchmarks</h1>
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<h1><a name="How Fast is Redis?">How Fast is Redis?</a></h1>Redis includes the <code name="code" class="python">redis-benchmark</code> utility that simulates <a href="SETs.html">SETs</a>/GETs done by N clients at the same time sending M total queries (it is similar to the Apache's <code name="code" class="python">ab</code> utility). Below you'll find the full output of the benchmark executed against a Linux box.<br/><br/><ul><li> The test was done with 50 simultaneous clients performing 100000 requests.</li><li> The value SET and GET is a 256 bytes string.</li><li> The Linux box is running <b>Linux 2.6</b>, it's <b>Xeon X3320 2.5Ghz</b>.</li><li> Text executed using the loopback interface (127.0.0.1).</li></ul>
Results: <b>about 110000 <a href="SETs.html">SETs</a> per second, about 81000 GETs per second.</b><h1><a name="Latency percentiles">Latency percentiles</a></h1><pre class="codeblock python" name="code">
./redis-benchmark -n 100000
====== SET ======
100007 requests completed in 0.88 seconds
50 parallel clients
3 bytes payload
keep alive: 1
58.50% &lt;= 0 milliseconds
99.17% &lt;= 1 milliseconds
99.58% &lt;= 2 milliseconds
99.85% &lt;= 3 milliseconds
99.90% &lt;= 6 milliseconds
100.00% &lt;= 9 milliseconds
114293.71 requests per second
====== GET ======
100000 requests completed in 1.23 seconds
50 parallel clients
3 bytes payload
keep alive: 1
43.12% &lt;= 0 milliseconds
96.82% &lt;= 1 milliseconds
98.62% &lt;= 2 milliseconds
100.00% &lt;= 3 milliseconds
81234.77 requests per second
====== INCR ======
100018 requests completed in 1.46 seconds
50 parallel clients
3 bytes payload
keep alive: 1
32.32% &lt;= 0 milliseconds
96.67% &lt;= 1 milliseconds
99.14% &lt;= 2 milliseconds
99.83% &lt;= 3 milliseconds
99.88% &lt;= 4 milliseconds
99.89% &lt;= 5 milliseconds
99.96% &lt;= 9 milliseconds
100.00% &lt;= 18 milliseconds
68458.59 requests per second
====== LPUSH ======
100004 requests completed in 1.14 seconds
50 parallel clients
3 bytes payload
keep alive: 1
62.27% &lt;= 0 milliseconds
99.74% &lt;= 1 milliseconds
99.85% &lt;= 2 milliseconds
99.86% &lt;= 3 milliseconds
99.89% &lt;= 5 milliseconds
99.93% &lt;= 7 milliseconds
99.96% &lt;= 9 milliseconds
100.00% &lt;= 22 milliseconds
100.00% &lt;= 208 milliseconds
88109.25 requests per second
====== LPOP ======
100001 requests completed in 1.39 seconds
50 parallel clients
3 bytes payload
keep alive: 1
54.83% &lt;= 0 milliseconds
97.34% &lt;= 1 milliseconds
99.95% &lt;= 2 milliseconds
99.96% &lt;= 3 milliseconds
99.96% &lt;= 4 milliseconds
100.00% &lt;= 9 milliseconds
100.00% &lt;= 208 milliseconds
71994.96 requests per second
</pre>Notes: changing the payload from 256 to 1024 or 4096 bytes does not change the numbers significantly (but reply packets are glued together up to 1024 bytes so GETs may be slower with big payloads). The same for the number of clients, from 50 to 256 clients I got the same numbers. With only 10 clients it starts to get a bit slower.<br/><br/>You can expect different results from different boxes. For example a low profile box like <b>Intel core duo T5500 clocked at 1.66Ghz running Linux 2.6</b> will output the following:
<pre class="codeblock python python" name="code">
./redis-benchmark -q -n 100000
SET: 53684.38 requests per second
GET: 45497.73 requests per second
INCR: 39370.47 requests per second
LPUSH: 34803.41 requests per second
LPOP: 37367.20 requests per second
</pre>Another one using a 64 bit box, a Xeon L5420 clocked at 2.5 Ghz:<br/><br/><pre class="codeblock python python python" name="code">
./redis-benchmark -q -n 100000
PING: 111731.84 requests per second
SET: 108114.59 requests per second
GET: 98717.67 requests per second
INCR: 95241.91 requests per second
LPUSH: 104712.05 requests per second
LPOP: 93722.59 requests per second
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<b>BgrewriteaofCommand: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a href="#BGREWRITEAOF">BGREWRITEAOF</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h3><a name="BGREWRITEAOF">BGREWRITEAOF</a></h3>
<blockquote>Please for detailed information about the Redis Append Only File check<a href="AppendOnlyFileHowto.html">the Append Only File Howto</a>.</blockquote>
<blockquote>BGREWRITEAOF rewrites the Append Only File in background when it gets toobig. The Redis Append Only File is a Journal, so every operation modifyingthe dataset is logged in the Append Only File (and replayed at startup).This means that the Append Only File always grows. In order to rebuildits content the BGREWRITEAOF creates a new version of the append only filestarting directly form the dataset in memory in order to guarantee thegeneration of the minimal number of commands needed to rebuild the database.</blockquote>
<blockquote>The <a href="AppendOnlyFileHowto.html">Append Only File Howto</a> contains further details.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>BgsaveCommand: Contents</b><br>&nbsp;&nbsp;<a href="#BGSAVE">BGSAVE</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="BGSAVE">BGSAVE</a></h1>
<blockquote>Save the DB in background. The OK code is immediately returned.Redis forks, the parent continues to server the clients, the childsaves the DB on disk then exit. A client my be able to check if theoperation succeeded using the <a href="LastsaveCommand.html">LASTSAVE</a> command.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Status code reply</a>
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<b>BlpopCommand: Contents</b><br>&nbsp;&nbsp;<a href="#BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Non blocking behavior">Non blocking behavior</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Blocking behavior">Blocking behavior</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multiple clients blocking for the same keys">Multiple clients blocking for the same keys</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#blocking POP inside a MULTI/EXEC transaction">blocking POP inside a MULTI/EXEC transaction</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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&iuml;&raquo;&iquest;#sidebar <a href="ListCommandsSidebar.html">ListCommandsSidebar</a><h1><a name="BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BLPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a></h1> 1.3.1) =
<h1><a name="BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;">BRPOP _key1_ _key2_ ... _keyN_ _timeout_ (Redis &gt;</a></h1> 1.3.1) =
<i>Time complexity: O(1)</i><blockquote>BLPOP (and BRPOP) is a blocking list pop primitive. You can see this commandsas blocking versions of <a href="LpopCommand.html">LPOP</a> and <a href="LpopCommand.html">RPOP</a> able toblock if the specified keys don't exist or contain empty lists.</blockquote>
<blockquote>The following is a description of the exact semantic. We describe BLPOP butthe two commands are identical, the only difference is that BLPOP pops theelement from the left (head) of the list, and BRPOP pops from the right (tail).</blockquote>
<h2><a name="Non blocking behavior">Non blocking behavior</a></h2><blockquote>When BLPOP is called, if at least one of the specified keys contain a nonempty list, an element is popped from the head of the list and returned tothe caller together with the name of the key (BLPOP returns a two elementsarray, the first element is the key, the second the popped value).</blockquote>
<blockquote>Keys are scanned from left to right, so for instance if youissue <b>BLPOP list1 list2 list3 0</b> against a dataset where <b>list1</b> does notexist but <b>list2</b> and <b>list3</b> contain non empty lists, BLPOP guaranteesto return an element from the list stored at <b>list2</b> (since it is the firstnon empty list starting from the left).</blockquote>
<h2><a name="Blocking behavior">Blocking behavior</a></h2><blockquote>If none of the specified keys exist or contain non empty lists, BLPOPblocks until some other client performs a <a href="RpushCommand.html">LPUSH</a> oran <a href="RpushCommand.html">RPUSH</a> operation against one of the lists.</blockquote>
<blockquote>Once new data is present on one of the lists, the client finally returnswith the name of the key unblocking it and the popped value.</blockquote>
<blockquote>When blocking, if a non-zero timeout is specified, the client will unblockreturning a nil special value if the specified amount of seconds passedwithout a push operation against at least one of the specified keys.</blockquote>
<blockquote>The timeout argument is interpreted as an integer value. A timeout of zero means instead to block forever.</blockquote>
<h2><a name="Multiple clients blocking for the same keys">Multiple clients blocking for the same keys</a></h2><blockquote>Multiple clients can block for the same key. They are put intoa queue, so the first to be served will be the one that started to waitearlier, in a first-blpopping first-served fashion.</blockquote>
<h2><a name="blocking POP inside a MULTI/EXEC transaction">blocking POP inside a MULTI/EXEC transaction</a></h2><blockquote>BLPOP and BRPOP can be used with pipelining (sending multiple commands and reading the replies in batch), but it does not make sense to use BLPOP or BRPOP inside a MULTI/EXEC block (a Redis transaction).</blockquote>
<blockquote>The behavior of BLPOP inside MULTI/EXEC when the list is empty is to return a multi-bulk nil reply, exactly what happens when the timeout is reached. If you like science fiction, think at it like if inside MULTI/EXEC the time will flow at infinite speed :) </blockquote>
<h2><a name="Return value">Return value</a></h2><blockquote>BLPOP returns a two-elements array via a multi bulk reply in order to returnboth the unblocking key and the popped value.</blockquote>
<blockquote>When a non-zero timeout is specified, and the BLPOP operation timed out,the return value is a nil multi bulk reply. Most client values will returnfalse or nil accordingly to the programming language used.</blockquote>
<a href="ReplyTypes.html">Multi bulk reply</a>
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<b>BrpoplpushCommand: Contents</b><br>&nbsp;&nbsp;<a href="#BRPOPLPUSH _srckey_ _dstkey_ _timeout_ (Redis &gt;">BRPOPLPUSH _srckey_ _dstkey_ _timeout_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Return value">Return value</a>
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<h1><a name="BRPOPLPUSH _srckey_ _dstkey_ _timeout_ (Redis &gt;">BRPOPLPUSH _srckey_ _dstkey_ _timeout_ (Redis &gt;</a></h1> 2.1.8) =
<i>Time complexity: O(1)</i><blockquote>Blocking version of the <a href="RpoplpushCommand.html">RPOPLPUSH</a> command. Atomically removes and returnsthe last element (tail) of the source list at <i>srckey</i>, and as a side effect pushes the returned element in the head of the list at <i>dstkey</i>.</blockquote>
If the source list is empty, the client blocks until another client pushes against the source list. Of course in such a case the push operation against the destination list will be performed after the command unblocks detecting a push against the source list.<br/><br/>Note that the command returns an error if the target key already exists but is not a list. The error is delayed at the time the push operation is attempted, that is, immediately if the source list is not empty, or when the first push against the source list happens in the case the command would block.<br/><br/>The timeout value can be 0 or a positive integer value. When it is zero the command will block forever, until something is pushed against <i>srckey</i>. Otherwise the command will wait the specified number of seconds at max, returning an nil value when the timeout expires.<br/><br/>The source and destination of the list can be the same, having the effect of rotating the list. Please check <a href="RpoplpushCommand.html">RPOPLPUSH</a> for more information.<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Bulk reply</a>
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<b>CommandReference: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Categorized Command List">Categorized Command List</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Connection handling">Connection handling</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on all value types">Commands operating on all value types</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on string values">Commands operating on string values</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on lists">Commands operating on lists</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on sets">Commands operating on sets</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on sorted zsets (sorted sets)">Commands operating on sorted zsets (sorted sets)</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Commands operating on hashes">Commands operating on hashes</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Sorting">Sorting</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Transactions">Transactions</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Publish/Subscribe">Publish/Subscribe</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Persistence control commands">Persistence control commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Remote server control commands">Remote server control commands</a>
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<h1 class="wikiname">CommandReference</h1>
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&iuml;&raquo;&iquest;= Redis Command Reference =<br/><br/>Every command name links to a specific wiki page describing the behavior of the command.<h2><a name="Categorized Command List">Categorized Command List</a></h2><h2><a name="Connection handling">Connection handling</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="QuitCommand.html">QUIT</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> close the connection </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="AuthCommand.html">AUTH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>password</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> simple password authentication if enabled </td></tr></table>
<h2><a name="Commands operating on all value types">Commands operating on all value types</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ExistsCommand.html">EXISTS</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> test if a key exists </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="DelCommand.html">DEL</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> delete a key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="TypeCommand.html">TYPE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> return the type of the value stored at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="KeysCommand.html">KEYS</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>pattern</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> return all the keys matching a given pattern </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="RandomkeyCommand.html">RANDOMKEY</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> return a random key from the key space </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="RenameCommand.html">RENAME</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>oldname</i> <i>newname</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> rename the old key in the new one, destroying the newname key if it already exists </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="RenamenxCommand.html">RENAMENX</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>oldname</i> <i>newname</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> rename the <i>oldname</i> key to <i>newname</i>, if the <i>newname</i> key does not already exist </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="DbsizeCommand.html">DBSIZE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> return the number of keys in the current db </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ExpireCommand.html">EXPIRE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> set a time to live in seconds on a key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ExpireCommand.html">PERSIST</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> remove the expire from a key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="TtlCommand.html">TTL</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> get the time to live in seconds of a key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SelectCommand.html">SELECT</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>index</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Select the DB with the specified index </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="MoveCommand.html">MOVE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>dbindex</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Move the key from the currently selected DB to the <i>dbindex</i> DB </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="FlushdbCommand.html">FLUSHDB</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove all the keys from the currently selected DB </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="FlushallCommand.html">FLUSHALL</a></td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove all the keys from all the databases </td></tr></table>
<h2><a name="Commands operating on string values">Commands operating on string values</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SetCommand.html">SET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set a <i>key</i> to a string <i>value</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="GetCommand.html">GET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the string value of the <i>key</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="GetsetCommand.html">GETSET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set a key to a string returning the old value of the key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SetnxCommand.html">SETNX</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set a key to a string value if the key does not exist </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SetexCommand.html">SETEX</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>time</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set+Expire combo command </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SetbitCommand.html">SETBIT</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>offset</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set bit at <i>offset</i> to <i>value</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="GetbitCommand.html">GETBIT</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>offset</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return bit value at <i>offset</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="MsetCommand.html">MSET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>value1</i> <i>key2</i> <i>value2</i> ... <i>keyN</i> <i>valueN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set multiple keys to multiple values in a single atomic operation </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="MsetCommand.html">MSETNX</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>value1</i> <i>key2</i> <i>value2</i> ... <i>keyN</i> <i>valueN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set multiple keys to multiple values in a single atomic operation if none of the keys already exist </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="MgetCommand.html">MGET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Multi-get, return the strings values of the keys </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="IncrCommand.html">INCR</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Increment the integer value of key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="IncrCommand.html">INCRBY</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>integer</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Increment the integer value of <i>key</i> by <i>integer</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="IncrCommand.html">DECR</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Decrement the integer value of key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="IncrCommand.html">DECRBY</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>integer</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Decrement the integer value of <i>key</i> by <i>integer</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="AppendCommand.html">APPEND</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Append the specified string to the string stored at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SubstrCommand.html">SUBSTR</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>start</i> <i>end</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return a substring of a larger string </td></tr></table>
<h2><a name="Commands operating on lists">Commands operating on lists</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="RpushCommand.html">RPUSH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Append an element to the tail of the List value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="RpushCommand.html">LPUSH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Append an element to the head of the List value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LlenCommand.html">LLEN</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the length of the List value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LrangeCommand.html">LRANGE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>start</i> <i>end</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return a range of elements from the List at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LtrimCommand.html">LTRIM</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>start</i> <i>end</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Trim the list at key to the specified range of elements </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LindexCommand.html">LINDEX</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>index</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the element at index position from the List at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LsetCommand.html">LSET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>index</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set a new value as the element at index position of the List at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LremCommand.html">LREM</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>count</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove the first-N, last-N, or all the elements matching value from the List at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LpopCommand.html">LPOP</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return and remove (atomically) the first element of the List at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LpopCommand.html">RPOP</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return and remove (atomically) the last element of the List at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="BlpopCommand.html">BLPOP</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>key2</i> ... <i>keyN</i> <i>timeout</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Blocking LPOP </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="BlpopCommand.html">BRPOP</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>key2</i> ... <i>keyN</i> <i>timeout</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Blocking RPOP </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="RpoplpushCommand.html">RPOPLPUSH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>srckey</i> <i>dstkey</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return and remove (atomically) the last element of the source List stored at <i>srckey</i> and push the same element to the destination List stored at <i>dstkey</i> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="BrpoplpushCommand.html">BRPOPLPUSH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>srckey</i> <i>dstkey</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Like RPOPLPUSH but blocking of source key is empty </td></tr></table>
<h2><a name="Commands operating on sets">Commands operating on sets</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SaddCommand.html">SADD</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Add the specified member to the Set value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SremCommand.html">SREM</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove the specified member from the Set value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SpopCommand.html">SPOP</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove and return (pop) a random element from the Set value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SmoveCommand.html">SMOVE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>srckey</i> <i>dstkey</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Move the specified member from one Set to another atomically </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ScardCommand.html">SCARD</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the number of elements (the cardinality) of the Set at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SismemberCommand.html">SISMEMBER</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Test if the specified value is a member of the Set at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SinterCommand.html">SINTER</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the intersection between the Sets stored at key1, key2, ..., keyN </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SinterstoreCommand.html">SINTERSTORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>dstkey</i> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Compute the intersection between the Sets stored at key1, key2, ..., keyN, and store the resulting Set at dstkey </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SunionCommand.html">SUNION</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the union between the Sets stored at key1, key2, ..., keyN </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SunionstoreCommand.html">SUNIONSTORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>dstkey</i> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Compute the union between the Sets stored at key1, key2, ..., keyN, and store the resulting Set at dstkey </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SdiffCommand.html">SDIFF</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the difference between the Set stored at key1 and all the Sets key2, ..., keyN </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SdiffstoreCommand.html">SDIFFSTORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>dstkey</i> <i>key1</i> <i>key2</i> ... <i>keyN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Compute the difference between the Set key1 and all the Sets key2, ..., keyN, and store the resulting Set at dstkey </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SmembersCommand.html">SMEMBERS</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return all the members of the Set value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SrandmemberCommand.html">SRANDMEMBER</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return a random member of the Set value at key </td></tr></table>
<h2><a name="Commands operating on sorted zsets (sorted sets)">Commands operating on sorted zsets (sorted sets)</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZaddCommand.html">ZADD</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>score</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Add the specified member to the Sorted Set value at key or update the score if it already exist </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZremCommand.html">ZREM</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove the specified member from the Sorted Set value at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZincrbyCommand.html">ZINCRBY</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>increment</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> If the member already exists increment its score by <i>increment</i>, otherwise add the member setting <i>increment</i> as score </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZrankCommand.html">ZRANK</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the rank (or index) or <i>member</i> in the sorted set at <i>key</i>, with scores being ordered from low to high </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZrankCommand.html">ZREVRANK</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>member</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the rank (or index) or <i>member</i> in the sorted set at <i>key</i>, with scores being ordered from high to low </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZrangeCommand.html">ZRANGE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>start</i> <i>end</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return a range of elements from the sorted set at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZrangeCommand.html">ZREVRANGE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>start</i> <i>end</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return a range of elements from the sorted set at key, exactly like ZRANGE, but the sorted set is ordered in traversed in reverse order, from the greatest to the smallest score </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZrangebyscoreCommand.html">ZRANGEBYSCORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>min</i> <i>max</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return all the elements with score &gt;= min and score &lt;= max (a range query) from the sorted set </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZrangebyscoreCommand.html">ZCOUNT</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>min</i> <i>max</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the number of elements with score &gt;= min and score &lt;= max in the sorted set </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZcardCommand.html">ZCARD</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the cardinality (number of elements) of the sorted set at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZscoreCommand.html">ZSCORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>element</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the score associated with the specified element of the sorted set at key </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZremrangebyrankCommand.html">ZREMRANGEBYRANK</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>min</i> <i>max</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove all the elements with rank &gt;= min and rank &lt;= max from the sorted set </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZremrangebyscoreCommand.html">ZREMRANGEBYSCORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>min</i> <i>max</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove all the elements with score &gt;= min and score &lt;= max from the sorted set </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ZunionstoreCommand.html">ZUNIONSTORE / ZINTERSTORE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>dstkey</i> <i>N</i> <i>key1</i> ... <i>keyN</i> WEIGHTS <i>w1</i> ... <i>wN</i> AGGREGATE SUM|MIN|MAX </td><td style="border: 1px solid #aaa; padding: 5px;"> Perform a union or intersection over a number of sorted sets with optional weight and aggregate </td></tr></table>
<h2><a name="Commands operating on hashes">Commands operating on hashes</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HsetCommand.html">HSET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field</i> <i>value</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set the hash field to the specified value. Creates the hash if needed. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HgetCommand.html">HGET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Retrieve the value of the specified hash field. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HmgetCommand.html">HMGET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field1</i> ... <i>fieldN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Get the hash values associated to the specified fields. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HmsetCommand.html">HMSET</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field1</i> <i>value1</i> ... <i>fieldN</i> <i>valueN</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Set the hash fields to their respective values. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HincrbyCommand.html">HINCRBY</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field</i> <i>integer</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Increment the integer value of the hash at <i>key</i> on <i>field</i> with <i>integer</i>. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HexistsCommand.html">HEXISTS</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Test for existence of a specified field in a hash </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HdelCommand.html">HDEL</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> <i>field</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Remove the specified field from a hash </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HlenCommand.html">HLEN</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the number of items in a hash. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HgetallCommand.html">HKEYS</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return all the fields in a hash. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HgetallCommand.html">HVALS</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return all the values in a hash. </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="HgetallCommand.html">HGETALL</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> </td><td style="border: 1px solid #aaa; padding: 5px;"> Return all the fields and associated values in a hash. </td></tr></table>
<h2><a name="Sorting">Sorting</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SortCommand.html">SORT</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> <i>key</i> BY <i>pattern</i> LIMIT <i>start</i> <i>end</i> GET <i>pattern</i> ASC|DESC ALPHA </td><td style="border: 1px solid #aaa; padding: 5px;"> Sort a Set or a List accordingly to the specified parameters </td></tr></table>
<h2><a name="Transactions">Transactions</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="MultiExecCommand.html">MULTI/EXEC/DISCARD/WATCH/UNWATCH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Redis atomic transactions </td></tr></table>
<h2><a name="Publish/Subscribe">Publish/Subscribe</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="PublishSubscribe.html">SUBSCRIBE/UNSUBSCRIBE/PUBLISH</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Redis Public/Subscribe messaging paradigm implementation </td></tr></table>
<h2><a name="Persistence control commands">Persistence control commands</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SaveCommand.html">SAVE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Synchronously save the DB on disk </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="BgsaveCommand.html">BGSAVE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Asynchronously save the DB on disk </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="LastsaveCommand.html">LASTSAVE</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Return the UNIX time stamp of the last successfully saving of the dataset on disk </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ShutdownCommand.html">SHUTDOWN</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Synchronously save the DB on disk, then shutdown the server </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="BgrewriteaofCommand.html">BGREWRITEAOF</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Rewrite the append only file in background when it gets too big </td></tr></table>
<h2><a name="Remote server control commands">Remote server control commands</a></h2><table><tr><td style="border: 1px solid #aaa; padding: 5px;"> <b>Command</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Parameters</b> </td><td style="border: 1px solid #aaa; padding: 5px;"> <b>Description</b> </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="InfoCommand.html">INFO</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Provide information and statistics about the server </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="MonitorCommand.html">MONITOR</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Dump all the received requests in real time </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="SlaveofCommand.html">SLAVEOF</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Change the replication settings </td></tr><tr><td style="border: 1px solid #aaa; padding: 5px;"> <a href="ConfigCommand.html">CONFIG</a> </td><td style="border: 1px solid #aaa; padding: 5px;"> - </td><td style="border: 1px solid #aaa; padding: 5px;"> Configure a Redis server at runtime </td></tr></table>
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<b>Comparisons: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Memcached">Memcached</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Tokyo Cabinet / Toyo Tyrant">Tokyo Cabinet / Toyo Tyrant</a>
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<h1 class="wikiname">Comparisons</h1>
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&iuml;&raquo;&iquest;if your are asking yourself how is Redis different fom other key-value stores here you will find it compared to some of the most popular contendors (all great software) in this category. <h2><a name="Memcached">Memcached</a></h2><ul><li> Memcached is not persistent, it just holds everything in memory without saving since its main goal is to be used as a cache, while Redis is <a href="Persistence.html">persistent</a>.</li></ul>
<ul><li> Like memcached Redis uses a key-value model, but while keys can just be strings, values in Redis can be <a href="Lists.html">Lists</a>, <a href="Sets.html">Sets</a> or <a href="SortedSets.html">SortedSets</a> and complex operations like intersections, set/get n-th element of lists, pop/push of elements, can be performed against sets and lists.</li></ul>
<h2><a name="Tokyo Cabinet / Toyo Tyrant">Tokyo Cabinet / Toyo Tyrant</a></h2>Redis and Tokyo Cabinet can be used for the same applications, but actually they are <i>very different</i> beasts. If you read Twitter messages of people involved in scalable things both products are reported to work well, but surely there are times where one or the other can be the best choice.<br/><br/><ul><li> Tokyo Cabinet writes synchronously on disk, Redis takes the whole dataset on memory and writes on disk asynchronously. Tokyo Cabinet is safer and probably a better idea if your dataset is going to be bigger than RAM, but Redis is faster (note that Redis supports master-slave replication that is trivial to setup, so you are safe anyway if you want a setup where data can't be lost even after a disaster). </li></ul>
<ul><li> Redis supports higher level operations and data structures. Tokyo Cabinet supports a kind of database that is able to organize data into rows with named fields (in a way very similar to Berkeley DB) but can't do things like server side List and Set operations Redis is able to do: pushing or popping from Lists in an atomic way, in O(1) time complexity, server side Set intersections, Sorting of schema free data in complex ways (By the way TC supports sorting in the table-based database format). Redis on the other hand does not support the abstraction of tables with fields, the idea is that you can build this stuff in software easily if you really need a table-alike approach. </li></ul>
<ul><li> Tokyo Cabinet does not implement a networking layer. You have to use a networking layer called Tokyo Tyrant that interfaces to Tokyo Cabinet so you can talk to Tokyo Cabinet in a client-server fashion. In Redis the networking support is built-in inside the server, and is basically the only interface between the external world and the dataset. </li></ul>
<ul><li> Redis is reported to be much faster, especially if you plan to access Tokyo Cabinet via Tokyo Tyrant. Here I can only say that with Redis you can expect 100,000 operations/seconds with a normal Linux box and 50 concurrent clients. You should test Redis, Tokyo, and the other alternatives with your specific work load to get a feeling about performances for your application. </li></ul>
<ul><li> Redis is not an on-disk DB engine like Tokyo: the latter can be used as a fast DB engine in your C project without the networking overhead just linking to the library. Still in many scalable applications you need multiple servers talking with multiple clients, so the client-server model is almost always needed, this is why in Redis this is built-in. </li></ul>
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<b>ConfigCommand: Contents</b><br>&nbsp;&nbsp;<a href="#CONFIG GET _pattern_ (Redis &gt;">CONFIG GET _pattern_ (Redis &gt;</a><br>&nbsp;&nbsp;<a href="#CONFIG SET _parameter_ _value_ (Redis &gt;">CONFIG SET _parameter_ _value_ (Redis &gt;</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#CONFIG GET _pattern_">CONFIG GET _pattern_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#CONFIG SET _parameter_ _value_">CONFIG SET _parameter_ _value_</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Parameters value format">Parameters value format</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#See Also">See Also</a>
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<h1 class="wikiname">ConfigCommand</h1>
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&iuml;&raquo;&iquest;#sidebar <a href="ControlCommandsSidebar.html">ControlCommandsSidebar</a><h1><a name="CONFIG GET _pattern_ (Redis &gt;">CONFIG GET _pattern_ (Redis &gt;</a></h1> 2.0)=
<h1><a name="CONFIG SET _parameter_ _value_ (Redis &gt;">CONFIG SET _parameter_ _value_ (Redis &gt;</a></h1> 2.0)=<br/><br/><blockquote>The CONFIG command is able to retrieve or alter the configuration of a runningRedis server. Not all the configuration parameters are supported.</blockquote>
<blockquote>CONFIG has two sub commands, GET and SET. The GET command is used to readthe configuration, while the SET command is used to alter the configuration.</blockquote>
<h2><a name="CONFIG GET _pattern_">CONFIG GET _pattern_</a></h2><blockquote>CONFIG GET returns the current configuration parameters. This sub commandonly accepts a single argument, that is glob style pattern. All theconfiguration parameters matching this parameter are reported as alist of key-value pairs. Example:</blockquote><pre class="codeblock python" name="code">
$ redis-cli config get '*'
1. &quot;dbfilename&quot;
2. &quot;dump.rdb&quot;
3. &quot;requirepass&quot;
4. (nil)
5. &quot;masterauth&quot;
6. (nil)
7. &quot;maxmemory&quot;
8. &quot;0\n&quot;
9. &quot;appendfsync&quot;
10. &quot;everysec&quot;
11. &quot;save&quot;
12. &quot;3600 1 300 100 60 10000&quot;
$ redis-cli config get 'm*'
1. &quot;masterauth&quot;
2. (nil)
3. &quot;maxmemory&quot;
4. &quot;0\n&quot;
</pre>The return type of the command is a <a href="ReplyTypes.html">bulk reply</a>.<h2><a name="CONFIG SET _parameter_ _value_">CONFIG SET _parameter_ _value_</a></h2><blockquote>CONFIG SET is used in order to reconfigure the server, setting a specificconfiguration parameter to a new value.</blockquote>
<blockquote>The list of configuration parameters supported by CONFIG SET can beobtained issuing a <code name="code" class="python">CONFIG GET *</code> command.</blockquote>
<blockquote>The configuration set using CONFIG SET is immediately loaded by the Redisserver that will start acting as specified starting from the next command.</blockquote>
<blockquote>Example:</blockquote><pre class="codeblock python python" name="code">
$ ./redis-cli
redis&gt; set x 10
OK
redis&gt; config set maxmemory 200
OK
redis&gt; set y 20
(error) ERR command not allowed when used memory &gt; 'maxmemory'
redis&gt; config set maxmemory 0
OK
redis&gt; set y 20
OK
</pre><h2><a name="Parameters value format">Parameters value format</a></h2><blockquote>The value of the configuration parameter is the same as the one of thesame parameter in the Redis configuration file, with the following exceptions:</blockquote>
<ul><li> The <code name="code" class="python">save</code> paramter is a list of space-separated integers. Every pair of integers specify the time and number of changes limit to trigger a save. For instance the command <code name="code" class="python">CONFIG SET save &quot;3600 10 60 10000&quot;</code> will configure the server to issue a background saving of the RDB file every 3600 seconds if there are at least 10 changes in the dataset, and every 60 seconds if there are at least 10000 changes. To completely disable automatic snapshots just set the parameter as an empty string.</li><li> All the integer parameters representing memory are returned and accepted only using bytes as unit.</li></ul>
<h2><a name="See Also">See Also</a></h2>The <a href="InfoCommand.html">INFO</a> command can be used in order to read configuriaton parameters that are not available in the CONFIG command.
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<b>Configuration: Contents</b>
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The <code name="code" class="python">redis.conf</code> file included in the source code distribution is a starting point, you should be able to modify it in order do adapt it to your needs without troubles reading the comments inside the file.<br/><br/>In order to start Redis using a configuration file just pass the file name as the sole argument when starting the server:<br/><br/><pre class="codeblock python" name="code">
$ ./redis-server redis.conf
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&iuml;&raquo;&iquest;== Connection handling ==<br/><br/><ul><li> <a href="QuitCommand.html">QUIT</a></li><li> <a href="AuthCommand.html">AUTH</a></li></ul>
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&iuml;&raquo;&iquest;== Control Commands ==<br/><br/><ul><li> <a href="SaveCommand.html">SAVE</a></li><li> <a href="BgsaveCommand.html">BGSAVE</a></li><li> <a href="BgrewriteaofCommand.html">BGREWRITEAOF</a></li><li> <a href="LastsaveCommand.html">LASTSAVE</a></li><li> <a href="ShutdownCommand.html">SHUTDOWN</a></li><li> <a href="InfoCommand.html">INFO</a></li><li> <a href="MonitorCommand.html">MONITOR</a></li><li> <a href="SlaveofCommand.html">SLAVEOF</a></li></ul>
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<h1><a name="Credits">Credits</a></h1><ul><li> The Redis server was designed and written by <a href="http://invece.org" target="_blank">Salvatore Sanfilippo (aka antirez)</a></li><li> <a href="http://brainspl.at/" target="_blank">Ezra Zygmuntowicz (aka ezmobius)</a> - Ruby client lib initial version and hacking</li><li> <a href="http://qix.it" target="_blank">Ludovico Magnocavallo (aka ludo)</a> - Python clinet lib</li><li> <a href="http://www.adroll.com/" target="_blank">Valentino Volonghi of Adroll</a> - Erlang client lib</li><li> <b>brettbender</b> - found and fixed a bug in sds.c that caused the server to crash at least on 64 bit systems, and anyway to be buggy since we used the same vararg thing against vsprintf without to call va_start and va_end every time.</li><li> <a href="http://www.rot13.org/~dpavlin" target="_blank">Dobrica Pavlinusic</a> - Perl client lib</li><li> Brian Hammond - AUTH command implementation, C++ client lib</li><li> <a href="http://www.clorophilla.net/" target="_blank">Daniele Alessandri</a> - Lua client lib</li><li> Corey Stup - C99 cleanups</li><li> Taylor Weibley - Ruby client improvements</li><li> Bob Potter - Rearrange redisObject struct to reduce memory usage in 64bit environments</li><li> Luca Guidi and Brian McKinney - Ruby client improvements</li><li> Aman Gupta - SDIFF / SDIFFSTORE, other Set operations improvements, ability to disable clients timeout.</li><li> Diego Rosario Brogna - Code and ideas about dumping backtrace on sigsegv and similar error conditions.</li></ul>
p.s. sorry to take this file in sync is hard in this early days. Please drop me an email if I forgot to add your name here!
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&iuml;&raquo;&iquest;#sidebar <a href="GenericCommandsSidebar.html">GenericCommandsSidebar</a><h1><a name="DBSIZE">DBSIZE</a></h1><blockquote>Return the number of keys in the currently selected database.</blockquote>
<h2><a name="Return value">Return value</a></h2><a href="ReplyTypes.html">Integer reply</a>
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<b>DelCommand: Contents</b><br>&nbsp;&nbsp;<a href="#DEL _key1_ _key2_ ... _keyN_">DEL _key1_ _key2_ ... _keyN_</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="DEL _key1_ _key2_ ... _keyN_">DEL _key1_ _key2_ ... _keyN_</a></h1>
<i>Time complexity: O(1)</i><blockquote>Remove the specified keys. If a given key does not existno operation is performed for this key. The command returns the number ofkeys removed.</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">
an integer greater than 0 if one or more keys were removed
0 if none of the specified key existed
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Use random keys instead of incremental keys in order to avoid a single-key that gets incremented by many servers. This can can't be distributed among servers.
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<b>EventLibray: Contents</b><br>&nbsp;&nbsp;<a href="#Event Library">Event Library</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Why is an Event Library needed at all?">Why is an Event Library needed at all?</a>
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<h1><a name="Event Library">Event Library</a></h1><h2><a name="Why is an Event Library needed at all?">Why is an Event Library needed at all?</a></h2>Let us figure it out through a series of Q&amp;As.<br/><br/>Q: What do you expect a network server to be doing all the time? &lt;br/&gt;
A: Watch for inbound connections on the port its listening and accept them.<br/><br/>Q: Calling <a href="http://man.cx/accept%282%29" target="_blank">accept</a> yields a descriptor. What do I do with it?&lt;br/&gt;
A: Save the descriptor and do a non-blocking read/write operation on it.<br/><br/>Q: Why does the read/write have to be non-blocking?&lt;br/&gt;
A: If the file operation ( even a socket in Unix is a file ) is blocking how could the server for example accept other connection requests when its blocked in a file I/O operation.<br/><br/>Q: I guess I have to do many such non-blocking operations on the socket to see when it's ready. Am I right?&lt;br/&gt;
A: Yes. That is what an event library does for you. Now you get it.<br/><br/>Q: How do Event Libraries do what they do?&lt;br/&gt;
A: They use the operating system's <a href="http://www.devshed.com/c/a/BrainDump/Linux-Files-and-the-Event-Poll-Interface/" target="_blank">polling</a> facility along with timers.<br/><br/>Q: So are there any open source event libraries that do what you just described? &lt;br/&gt;
A: Yes. Libevent and Libev are two such event libraries that I can recall off the top of my head.<br/><br/>Q: Does Redis use such open source event libraries for handling socket I/O?&lt;br/&gt;
A: No. For various <a href="http://groups.google.com/group/redis-db/browse_thread/thread/b52814e9ef15b8d0/" target="_blank">reasons</a> Redis uses its own event library.
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