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  18 <!-- This is a (PRE) block.  Make sure it's left aligned or your toc title will be off. -->
  19 <b>README: Contents</b><br>&nbsp;&nbsp;<a href="#All data in memory, but saved on disk">All data in memory, but saved on disk</a><br>&nbsp;&nbsp;<a href="#Master-Slave replication made trivial">Master-Slave replication made trivial</a><br>&nbsp;&nbsp;<a href="#It's persistent but supports expires">It's persistent but supports expires</a><br>&nbsp;&nbsp;<a href="#Beyond key-value databases">Beyond key-value databases</a><br>&nbsp;&nbsp;<a href="#Multiple databases support">Multiple databases support</a><br>&nbsp;&nbsp;<a href="#Know more about Redis!">Know more about Redis!</a><br>&nbsp;&nbsp;<a href="#Redis Tutorial">Redis Tutorial</a><br>&nbsp;&nbsp;<a href="#License">License</a><br>&nbsp;&nbsp;<a href="#Credits">Credits</a>
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  22                 <h1 class="wikiname">README</h1>
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  29                     &iuml;&raquo;&iquest;= Introduction =<br/><br/>Redis is an extremely fast and powerful key-value store database and server implemented in ANSI C.  Redis offers many different ways to do one straightforward thing: store a value (&quot;antirez&quot;) to a key (&quot;redis&quot;).  While the format of keys must always be simple strings, the power is with the values, which support the following data types:<br/><br/><ul><li> <a href="Strings.html">Strings</a></li><li> <a href="Lists.html">Lists</a></li><li> <a href="Sets.html">Sets</a></li><li> <a href="SortedSets.html">Sorted Sets (zsets)</a></li><li> <a href="Hashes.html">Hashes</a></li></ul>
  30 Each value type has an associated list of commands which can operate on them, and the <a href="CommandReference.html">The Redis Command Reference</a> contains an up to date list of these commands, organized primarily by data type.  The Redis source also includes a <a href="RedisCLI.html">Redis command line interface</a> which allows you to interact directly with the server, and is the means by which this introduction will provide examples.  Once you walk through the <a href="QuickStart.html">Redis Quick Start Guide</a> to get your instance of Redis running, you can follow along.  <br/><br/>One of the most powerful aspects of Redis is the wide range of commands which are optimized to work with specific data value types and executed as atomic server-side operations.  The <a href="Lists.html">List</a> type is a great example - Redis implements O(1) operations such as <a href="RpushCommand.html">LPUSH</a> or <a href="RpushCommand.html">RPUSH</a>, which have accompanying <a href="LpopCommand.html">LPOP</a> and <a href="LpopCommand.html">RPOP</a> methods:<br/><br/><pre class="codeblock python" name="code">
  31 redis&gt; lpush programming_languages C
  32 OK
  33 redis&gt; lpush programming_languages Ruby
  34 OK
  35 redis&gt; rpush programming_languages Python
  36 OK
  37 redis&gt; rpop programming_languages
  38 Python
  39 redis&gt; lpop programming_languages
  40 Ruby
  41 </pre>More complex operations are available for each data type as well.  Continuing with lists, you can get a range of elements with <a href="LrangeCommand.html">LRANGE</a> (O(start+n)) or trim the list with <a href="LtrimCommand.html">LTRIM</a> (O(n)):<br/><br/><pre class="codeblock python python" name="code">
  42 redis&gt; lpush cities NYC
  43 OK
  44 redis&gt; lpush cities SF
  45 OK
  46 redis&gt; lpush cities Tokyo
  47 OK
  48 redis&gt; lpush cities London
  49 OK
  50 redis&gt; lpush cities Paris
  51 OK
  52 redis&gt; lrange cities 0 2
  53 1. Paris
  54 2. London
  55 3. Tokyo
  56 redis&gt; ltrim cities 0 1
  57 OK
  58 redis&gt; lpop cities
  59 Paris
  60 redis&gt; lpop cities
  61 London
  62 redis&gt; lpop cities
  63 (nil)
  64 </pre>You can also add and remove elements from a set, and perform intersections, unions, and differences. <br/><br/>Redis can also be looked at as a data structures server. A Redis user is virtually provided with an interface to  <a href="http://en.wikipedia.org/wiki/Abstract_data_type" target="_blank">Abstract Data Types</a>, saving them from the responsibility of implementing concrete data structures and algorithms -- indeed both algorithms and data structures in Redis are properly chosen in order to obtain the best performance.<h1><a name="All data in memory, but saved on disk">All data in memory, but saved on disk</a></h1>Redis loads and mantains the whole dataset into memory, but the dataset is persistent, since at the same time it is saved on disk, so that when the server is restarted data can be loaded back in memory.<br/><br/>There are two kinds of persistence supported: the first one is called snapshotting. In this mode Redis periodically writes to disk asynchronously. The dataset is loaded from the dump every time the server is (re)started.<br/><br/>Redis can be configured to save the dataset when a certain number of changes is reached and after a given number of seconds elapses. For example, you can configure Redis to save after 1000 changes and at most 60 seconds since the last save. You can specify any combination for these numbers.<br/><br/>Because data is written asynchronously, when a system crash occurs, the last few queries can get lost (that is acceptable in many applications but not in all).  In order to make this a non issue Redis supports another, safer persistence mode, called <a href="AppendOnlyFileHowto.html">Append Only File</a>, where every command received altering the dataset (so not a read-only command, but a write command) is written on an append only file ASAP. This commands are <i>replayed</i> when the server is restarted in order to rebuild the dataset in memory.<br/><br/>Redis Append Only File supports a very handy feature: the server is able to safely rebuild the append only file in background in a non-blocking fashion when it gets too long. You can find <a href="AppendOnlyFileHowto.html">more details in the Append Only File HOWTO</a>.<h1><a name="Master-Slave replication made trivial">Master-Slave replication made trivial</a></h1>Whatever will be the persistence mode you'll use Redis supports master-slave replications if you want to stay really safe or if you need to scale to huge amounts of reads.<br/><br/><b>Redis Replication is trivial to setup</b>. So trivial that all you need to do in order to configure a Redis server to be a slave of another one, with automatic synchronization if the link will go down and so forth, is the following config line: <code name="code" class="python">slaveof 192.168.1.100 6379</code>. <a href="ReplicationHowto.html">We provide a Replication Howto</a> if you want to know more about this feature.<h1><a name="It's persistent but supports expires">It's persistent but supports expires</a></h1>Redis can be used as a <b>memcached on steroids</b> because is as fast as memcached but with a number of features more. Like memcached, Redis also supports setting timeouts to keys so that this key will be automatically removed when a given amount of time passes.<h1><a name="Beyond key-value databases">Beyond key-value databases</a></h1>All these features allow to use Redis as the sole DB for your scalable application without the need of any relational database. <a href="TwitterAlikeExample.html">We wrote a simple Twitter clone in PHP + Redis</a> to show a real world example, the link points to an article explaining the design and internals in very simple words.<h1><a name="Multiple databases support">Multiple databases support</a></h1>Redis supports multiple databases with commands to atomically move keys from one database to the other. By default DB 0 is selected for every new connection, but using the SELECT command it is possible to select a different database. The MOVE operation can move an item from one DB to another atomically. This can be used as a base for locking free algorithms together with the 'RANDOMKEY' commands.<h1><a name="Know more about Redis!">Know more about Redis!</a></h1>To really get a feeling about what Redis is and how it works please try reading <a href="IntroductionToRedisDataTypes.html">A fifteen minutes introduction to Redis data types</a>.<br/><br/>To know a bit more about how Redis works <i>internally</i> continue reading.<h1><a name="Redis Tutorial">Redis Tutorial</a></h1>(note, you can skip this section if you are only interested in &quot;formal&quot; doc.)<br/><br/>Later in this document you can find detailed information about Redis commands,
  65 the protocol specification, and so on. This kind of documentation is useful
  66 but... if you are new to Redis it is also BORING! The Redis protocol is designed
  67 so that is both pretty efficient to be parsed by computers, but simple enough
  68 to be used by humans just poking around with the 'telnet' command, so this
  69 section will show to the reader how to play a bit with Redis to get an initial
  70 feeling about it, and how it works.<br/><br/>To start just compile redis with 'make' and start it with './redis-server'.
  71 The server will start and log stuff on the standard output, if you want
  72 it to log more edit redis.conf, set the loglevel to debug, and restart it.<br/><br/>You can specify a configuration file as unique parameter:<br/><br/><blockquote>./redis-server /etc/redis.conf</blockquote>
  73 This is NOT required. The server will start even without a configuration file
  74 using a default built-in configuration.<br/><br/>Now let's try to set a key to a given value:<br/><br/><pre class="codeblock python python python" name="code">
  75 $ telnet localhost 6379
  76 Trying 127.0.0.1...
  77 Connected to localhost.
  78 Escape character is '^]'.
  79 SET foo 3
  80 bar
  81 +OK
  82 </pre>The first line we sent to the server is &quot;set foo 3&quot;. This means &quot;set the key
  83 foo with the following three bytes I'll send you&quot;. The following line is
  84 the &quot;bar&quot; string, that is, the three bytes. So the effect is to set the
  85 key &quot;foo&quot; to the value &quot;bar&quot;. Very simple!<br/><br/>(note that you can send commands in lowercase and it will work anyway,
  86 commands are not case sensitive)<br/><br/>Note that after the first and the second line we sent to the server there
  87 is a newline at the end. The server expects commands terminated by &quot;\r\n&quot;
  88 and sequence of bytes terminated by &quot;\r\n&quot;. This is a minimal overhead from
  89 the point of view of both the server and client but allows us to play with
  90 Redis with the telnet command easily.<br/><br/>The last line of the chat between server and client is &quot;+OK&quot;. This means
  91 our key was added without problems. Actually SET can never fail but
  92 the &quot;+OK&quot; sent lets us know that the server received everything and
  93 the command was actually executed.<br/><br/>Let's try to get the key content now:<br/><br/><pre class="codeblock python python python python" name="code">
  94 GET foo
  95 $3
  96 bar
  97 </pre>Ok that's very similar to 'set', just the other way around. We sent &quot;get foo&quot;,
  98 the server replied with a first line that is just the $ character follwed by
  99 the number of bytes the value stored at key contained, followed by the actual
 100 bytes. Again &quot;\r\n&quot; are appended both to the bytes count and the actual data. In Redis slang this is called a bulk reply.<br/><br/>What about requesting a non existing key?<br/><br/><pre class="codeblock python python python python python" name="code">
 101 GET blabla
 102 $-1
 103 </pre>When the key does not exist instead of the length, just the &quot;$-1&quot; string is sent. Since a -1 length of a bulk reply has no meaning it is used in order to specifiy a 'nil' value and distinguish it from a zero length value. Another way to check if a given key exists or not is indeed the EXISTS command:<br/><br/><pre class="codeblock python python python python python python" name="code">
 104 EXISTS nokey
 105 :0
 106 EXISTS foo
 107 :1
 108 </pre>As you can see the server replied ':0' the first time since 'nokey' does not
 109 exist, and ':1' for 'foo', a key that actually exists. Replies starting with the colon character are integer reply.<br/><br/>Ok... now you know the basics, read the <a href="CommandReference.html">REDIS COMMAND REFERENCE</a> section to
 110 learn all the commands supported by Redis and the <a href="ProtocolSpecification.html">PROTOCOL SPECIFICATION</a>
 111 section for more details about the protocol used if you plan to implement one
 112 for a language missing a decent client implementation.<h1><a name="License">License</a></h1>Redis is released under the BSD license. See the COPYING file for more information.<h1><a name="Credits">Credits</a></h1>Redis is written and maintained by Salvatore Sanfilippo, Aka 'antirez'.
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