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-<b>README: Contents</b><br> <a href="#
Introduction">Introduction</a><br> <a href="#Beyond key-value databases">Beyond key-value databases</a><br> <a href="#What are the differences between Redis and Memcached?">What are the differences between Redis and Memcached?</a><br> <a href="#What are the differences between Redis and Tokyo Cabinet / Tyrant?">What are the differences between Redis and Tokyo Cabinet / Tyrant?</a><br> <a href="#Does Redis support locking?">Does Redis support locking?</a><br> <a href="#Multiple databases support">Multiple databases support</a><br> <a href="#Redis Data Types">Redis Data Types</a><br> <a href="#Implementation Details">Implementation Details</a><br> <a href="#Redis Tutorial">Redis Tutorial</a><br> <a href="#License">License</a><br> <a href="#Credits">Credits</a>
+<b>README: Contents</b><br> <a href="#
All data in memory, but saved on disk">All data in memory, but saved on disk</a><br> <a href="#Master-Slave replication made trivial">Master-Slave replication made trivial</a><br> <a href="#It's persistent but supports expires">It's persistent but supports expires</a><br> <a href="#Beyond key-value databases">Beyond key-value databases</a><br> <a href="#Multiple databases support">Multiple databases support</a><br> <a href="#Know more about Redis!">Know more about Redis!</a><br> <a href="#Redis Tutorial">Redis Tutorial</a><br> <a href="#License">License</a><br> <a href="#Credits">Credits</a>
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<h1 class="wikiname">README</h1>
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- <h1><a name="Introduction">Introduction</a></h1>Redis is a database. To be more specific redis is a very simple database
-implementing a dictionary where keys are associated with values. For example
-I can set the key "surname_1992" to the string "Smith".<br/><br/>Redis takes the whole dataset in memory, but the dataset is persistent
-since from time to time Redis writes a dump of the dataset on disk asynchronously. The dump is loaded every time the server is restarted. This means that if a system crash occurs the last few queries can get lost (that is acceptable in many applications), so we supported master-slave replication from the early days.<h1><a name="Beyond key-value databases">Beyond key-value databases</a></h1>In most key-value databases keys and values are simple strings. In Redis keys are just strings too, but the associated values can be Strings, Lists and Sets, and there are commands to perform complex atomic operations against this data types, so you can think at Redis as a data structures server.<br/><br/>For example you can append elements to a list stored at the key "mylist" using the LPUSH or RPUSH operation in O(1). Later you'll be able to get a range of elements with LRANGE or trim the list with LTRIM. Sets are very flexible too, it is possible to add and remove elements from Sets (unsorted collections of strings), and then ask for server-side intersection of Sets.<br/><br/>All this features, the support for sorting Lists and Sets, 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="What are the differences between Redis and Memcached?">What are the differences between Redis and Memcached?</a></h1>In the following ways:<br/><br/><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. Redis instead can be used as the main DB for the application. We <a href="TwitterAlikeExample.html">wrote a simple Twitter clone</a> using only Redis as database.</li></ul>
-<ul><li> Like memcached Redis uses a key-value model, but while keys can just be strings, values in Redis can be lists and sets, and complex operations like intersections, set/get n-th element of lists, pop/push of elements, can be performed against sets and lists. It is possible to use lists as message queues.</li></ul>
-<h1><a name="What are the differences between Redis and Tokyo Cabinet / Tyrant?">What are the differences between Redis and Tokyo Cabinet / Tyrant?</a></h1>Redis and Tokyo can be used for the same applications, but actually they are <b>ery</b> different beasts:<br/><br/><ul><li> Tokyo is purely key-value, everything beyond key-value storing of strings is delegated to an embedded Lua interpreter. AFAIK there is no way to guarantee atomicity of operations like pushing into a list, and every time you want to have data structures inside a Tokyo key you have to perform some kind of object serialization/de-serialization.</li></ul>
-<ul><li> Tokyo stores data on disk, synchronously, this means you can have datasets bigger than memory, but that under load, like every kind of process that relay on the disk I/O for speed, the performances may start to degrade. With Redis you don't have this problems but you have another problem: the dataset in every single server must fit in your memory.</li></ul>
-<ul><li> Redis is generally an higher level beast in the operations supported. Things like SORTing, Server-side set-intersections, can't be done with Tokyo. But 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 remember that in many scalable applications you need multiple servers talking with multiple servers, so the server-client model is almost always needed.</li></ul>
-<h1><a name="Does Redis support locking?">Does Redis support locking?</a></h1>No, the idea is to provide atomic primitives in order to make the programmer
-able to use redis with locking free algorithms. For example imagine you have
-10 computers and 1 redis server. You want to count words in a very large text.
-This large text is split among the 10 computers, every computer will process
-its part and use Redis's INCR command to atomically increment a counter
-for every occurrence of the word found.<br/><br/>INCR/DECR are not the only atomic primitives, there are others like PUSH/POP
-on lists, POP RANDOM KEY operations, UPDATE and so on. For example you can
-use Redis like a Tuple Space (<a href="http://en.wikipedia.org/wiki/Tuple_space" target="_blank">http://en.wikipedia.org/wiki/Tuple_space</a>) in
-order to implement distributed algorithms.<br/><br/>(News: locking with key-granularity is now planned)<h1><a name="Multiple databases support">Multiple databases support</a></h1>Another synchronization primitive is the support for multiple DBs. 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' or 'POPRANDOMKEY' commands.<h1><a name="Redis Data Types">Redis Data Types</a></h1>Redis supports the following three data types as values:<br/><br/><ul><li> Strings: just any sequence of bytes. Redis strings are binary safe so they can not just hold text, but images, compressed data and everything else.</li><li> Lists: lists of strings, with support for operations like append a new string on head, on tail, list length, obtain a range of elements, truncate the list to a given length, sort the list, and so on.</li><li> Sets: an unsorted set of strings. It is possible to add or delete elements from a set, to perform set intersection, union, subtraction, and so on.</li></ul>
-Values can be Strings, Lists or Sets. Keys can be a subset of strings not containing newlines ("\n") and spaces (" ").<br/><br/>Note that sometimes strings may hold numeric vaules that must be parsed by
-Redis. An example is the INCR command that atomically increments the number
-stored at the specified key. In this case Redis is able to handle integers
-that can be stored inside a 'long long' type, that is a 64-bit signed integer.<h2><a name="Implementation Details">Implementation Details</a></h2>Strings are implemented as dynamically allocated strings of characters.
-Lists are implemented as doubly linked lists with cached length.
-Sets are implemented using hash tables that use chaining to resolve collisions.<h1><a name="Redis Tutorial">Redis Tutorial</a></h1>(note, you can skip this section if you are only interested in "formal" doc.)<br/><br/>Later in this document you can find detailed information about Redis commands,
+ = Introduction =<br/><br/>Redis is a database. To be specific, Redis is a database implementing a dictionary, where every key is associated with a value. For example I can set the key "surname_1992" to the string "Smith".
+What makes Redis different from many other key-value stores, is that every single value has a type. The following types are supported:<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 Set</a> (since version 1.1)</li></ul>
+The type of a value determines what operations (called commands) are available for the value itself.
+For example you can append elements to a list stored at the key "mylist" using the LPUSH or RPUSH command in O(1). Later you'll be able to get a range of elements with LRANGE or trim the list with LTRIM. Sets are very flexible too, it is possible to add and remove elements from Sets (unsorted collections of strings), and then ask for server-side intersection, union, difference of Sets. Each command is performed through server-side atomic operations.
+Please refer to the <a href="CommandReference.html">Command Reference</a> to see the full list of operations associated to these data types.<br/><br/>In other words, you can look at Redis 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 her from the responsibility to implement concrete data structures and algorithms. Indeed both algorithms and data structures in Redis are properly choosed 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 kind of persistence supported: the first one is called snapshotting. In this mode Redis, from time to time, writes a dump on 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 "formal" doc.)<br/><br/>Later in this document you can find detailed information about Redis commands,
the protocol specification, and so on. This kind of documentation is useful
but... if you are new to Redis it is also BORING! The Redis protocol is designed
so that is both pretty efficient to be parsed by computers, but simple enough
the "+OK" sent lets us know that the server received everything and
the command was actually executed.<br/><br/>Let's try to get the key content now:<br/><br/><pre class="codeblock python python" name="code">
GET foo
-3
+$3
bar
</pre>Ok that's very similar to 'set', just the other way around. We sent "get foo",
-the server replied with a first line that is just a number of bytes the value
-stored at key contained, followed by the actual bytes. Again "\r\n" are appended
-b
oth to the bytes count and the actual data.<br/><br/>What about requesting a non existing key?<br/><br/><pre class="codeblock python python python" name="code">
+the server replied with a first line that is just the $ character follwed by
+the number of bytes the value stored at key contained, followed by the actual
+b
ytes. Again "\r\n" 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" name="code">
GET blabla
-nil
-</pre>When the key does not exist instead of the length just the "nil" string is sent.
-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" name="code">
+$-1
+</pre>When the key does not exist instead of the length, just the "$-1" 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" name="code">
EXISTS nokey
-0
+:0
EXISTS foo
-1
-</pre>As you can see the server replied '0' the first time since 'nokey' does not
-exist, and '
1' for 'foo', a key that actually exists.<br/><br/>Ok... now you know the basics, read the <a href="CommandReference.html">REDIS COMMAND REFERENCE</a> section to
+:1
+</pre>As you can see the server replied ':0' the first time since 'nokey' does not
+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
learn all the commands supported by Redis and the <a href="ProtocolSpecification.html">PROTOCOL SPECIFICATION</a>
section for more details about the protocol used if you plan to implement one
-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'.<br/><br/>Enjoy,
-antirez
-
+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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