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  19 <b>ProtocolSpecification: Contents</b><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Networking layer">Networking layer</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Simple INLINE commands">Simple INLINE commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Bulk commands">Bulk commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Bulk replies">Bulk replies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multi-Bulk replies">Multi-Bulk replies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Nil elements in Multi-Bulk replies">Nil elements in Multi-Bulk replies</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Single line reply">Single line reply</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Integer reply">Integer reply</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multi bulk commands">Multi bulk commands</a><br>&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Multiple commands and pipelining">Multiple commands and pipelining</a>
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  22                 <h1 class="wikiname">ProtocolSpecification</h1>
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  29                     &iuml;&raquo;&iquest;= Protocol Specification =<br/><br/>The Redis protocol is a compromise between being easy to parse by a computer
  30 and being easy to parse by an human. Before reading this section you are
  31 strongly encouraged to read the &quot;REDIS TUTORIAL&quot; section of this README in order
  32 to get a first feeling of the protocol playing with it by TELNET.<h2><a name="Networking layer">Networking layer</a></h2>A client connects to a Redis server creating a TCP connection to the port 6379.
  33 Every redis command or data transmitted by the client and the server is
  34 terminated by &quot;\r\n&quot; (CRLF).<h2><a name="Simple INLINE commands">Simple INLINE commands</a></h2>The simplest commands are the inline commands. This is an example of a
  35 server/client chat (the server chat starts with S:, the client chat with C:)<br/><br/><pre class="codeblock python" name="code">
  36 C: PING
  37 S: +PONG
  38 </pre>An inline command is a CRLF-terminated string sent to the client. The server can reply to commands in different ways:
  39 <ul><li> With an error message (the first byte of the reply will be &quot;-&quot;)</li><li> With a single line reply (the first byte of the reply will be &quot;+)</li><li> With bulk data (the first byte of the reply will be &quot;$&quot;)</li><li> With multi-bulk data, a list of values (the first byte of the reply will be &quot;<code name="code" class="python">*</code>&quot;)</li><li> With an integer number (the first byte of the reply will be &quot;:&quot;)</li></ul>
  40 The following is another example of an INLINE command returning an integer:<br/><br/><pre class="codeblock python python" name="code">
  41 C: EXISTS somekey
  42 S: :0
  43 </pre>Since 'somekey' does not exist the server returned ':0'.<br/><br/>Note that the EXISTS command takes one argument. Arguments are separated
  44 simply by spaces.<h2><a name="Bulk commands">Bulk commands</a></h2>A bulk command is exactly like an inline command, but the last argument
  45 of the command must be a stream of bytes in order to send data to the server.
  46 the &quot;SET&quot; command is a bulk command, see the following example:<br/><br/><pre class="codeblock python python python" name="code">
  47 C: SET mykey 6
  48 C: foobar
  49 S: +OK
  50 </pre>The last argument of the commnad is '6'. This specify the number of DATA
  51 bytes that will follow (note that even this bytes are terminated by two
  52 additional bytes of CRLF).<br/><br/>All the bulk commands are in this exact form: instead of the last argument
  53 the number of bytes that will follow is specified, followed by the bytes,
  54 and CRLF. In order to be more clear for the programmer this is the string
  55 sent by the client in the above sample:<br/><br/><blockquote>&quot;SET mykey 6\r\nfoobar\r\n&quot;</blockquote>
  56 <h2><a name="Bulk replies">Bulk replies</a></h2>The server may reply to an inline or bulk command with a bulk reply. See
  57 the following example:<br/><br/><pre class="codeblock python python python python" name="code">
  58 C: GET mykey
  59 S: $6
  60 S: foobar
  61 </pre>A bulk reply is very similar to the last argument of a bulk command. The
  62 server sends as the first line a &quot;$&quot; byte followed by the number of bytes
  63 of the actual reply followed by CRLF, then the bytes are sent followed by
  64 additional two bytes for the final CRLF. The exact sequence sent by the
  65 server is:<br/><br/><blockquote>&quot;$6\r\nfoobar\r\n&quot;</blockquote>
  66 If the requested value does not exist the bulk reply will use the special
  67 value -1 as data length, example:<br/><br/><pre class="codeblock python python python python python" name="code">
  68 C: GET nonexistingkey
  69 S: $-1
  70 </pre>The client library API should not return an empty string, but a nil object, when the requested object does not exist.
  71 For example a Ruby library should return 'nil' while a C library should return
  72 NULL, and so forth.<h2><a name="Multi-Bulk replies">Multi-Bulk replies</a></h2>Commands similar to LRANGE needs to return multiple values (every element
  73 of the list is a value, and LRANGE needs to return more than a single element). This is accomplished using multiple bulk writes,
  74 prefixed by an initial line indicating how many bulk writes will follow.
  75 The first byte of a multi bulk reply is always <code name="code" class="python">*</code>. Example:<br/><br/><pre class="codeblock python python python python python python" name="code">
  76 C: LRANGE mylist 0 3
  77 S: *4
  78 S: $3
  79 S: foo
  80 S: $3
  81 S: bar
  82 S: $5
  83 S: Hello
  84 S: $5
  85 S: World
  86 </pre>The first line the server sent is &quot;<b>4\r\n&quot; in order to specify that four bulk
  87 write will follow. Then every bulk write is transmitted.<br/><br/>If the specified key does not exist instead of the number of elements in the
  88 list, the special value -1 is sent as count. Example:<br/><br/><pre class="codeblock python python python python python python python" name="code">
  89 C: LRANGE nokey 0 1
  90 S: *-1
  91 </pre>A client library API SHOULD return a nil object and not an empty list when this
  92 happens. This makes possible to distinguish between empty list and non existing ones.<h2><a name="Nil elements in Multi-Bulk replies">Nil elements in Multi-Bulk replies</a></h2>Single elements of a multi bulk reply may have -1 length, in order to signal that this elements are missing and not empty strings. This can happen with the SORT command when used with the GET <i>pattern</i> option when the specified key is missing. Example of a multi bulk reply containing an empty element:<br/><br/><pre class="codeblock python python python python python python python python" name="code">
  93 S: *3
  94 S: $3
  95 S: foo
  96 S: $-1
  97 S: $3
  98 S: bar
  99 </pre>The second element is nul. The client library should return something like this:<br/><br/><pre class="codeblock python python python python python python python python python" name="code">
 100 [&quot;foo&quot;,nil,&quot;bar&quot;]
 101 </pre><h2><a name="Single line reply">Single line reply</a></h2>As already seen a single line reply is in the form of a single line string
 102 starting with &quot;+&quot; terminated by &quot;\r\n&quot;. For example:<br/><br/><pre class="codeblock python python python python python python python python python python" name="code">
 103 +OK
 104 </pre>The client library should return everything after the &quot;+&quot;, that is, the string &quot;OK&quot; in the example.<br/><br/>The following commands reply with a status code reply:
 105 PING, SET, SELECT, SAVE, BGSAVE, SHUTDOWN, RENAME, LPUSH, RPUSH, LSET, LTRIM<h2><a name="Integer reply">Integer reply</a></h2>This type of reply is just a CRLF terminated string representing an integer, prefixed by a &quot;:&quot; byte. For example &quot;:0\r\n&quot;, or &quot;:1000\r\n&quot; are integer replies.<br/><br/>With commands like INCR or LASTSAVE using the integer reply to actually return a value there is no special meaning for the returned integer. It is just an incremental number for INCR, a UNIX time for LASTSAVE and so on.<br/><br/>Some commands like EXISTS will return 1 for true and 0 for false.<br/><br/>Other commands like SADD, SREM and SETNX will return 1 if the operation was actually done, 0 otherwise.<br/><br/>The following commands will reply with an integer reply: SETNX, DEL, EXISTS, INCR, INCRBY, DECR, DECRBY, DBSIZE, LASTSAVE, RENAMENX, MOVE, LLEN, SADD, SREM, SISMEMBER, SCARD<h2><a name="Multi bulk commands">Multi bulk commands</a></h2>As you can see with the protocol described so far there is no way to
 106 send multiple binary-safe arguments to a command. With bulk commands the
 107 last argument is binary safe, but there are commands where multiple binary-safe
 108 commands are needed, like the MSET command that is able to SET multiple keys
 109 in a single operation.<br/><br/>In order to address this problem Redis 1.1 introduced a new way of seding
 110 commands to a Redis server, that uses exactly the same protocol of the
 111 multi bulk replies. For instance the following is a SET command using the
 112 normal bulk protocol:<br/><br/><pre class="codeblock python python python python python python python python python python python" name="code">
 113 SET mykey 8
 114 myvalue
 115 </pre>While the following uses the multi bulk command protocol:<br/><br/><pre class="codeblock python python python python python python python python python python python python" name="code">
 116 *3
 117 $3
 118 SET
 119 $5
 120 mykey
 121 $8
 122 myvalue
 123 </pre>Commands sent in this format are longer, so currently they are used only in
 124 order to transmit commands containing multiple binary-safe arguments, but
 125 actually this protocol can be used to send every kind of command, without to
 126 know if it's an inline, bulk or multi-bulk command.<br/><br/>It is possible that in the future Redis will support only this format.<br/><br/>A good client library may implement unknown commands using this
 127 command format in order to support new commands out of the box without
 128 modifications.<h2><a name="Multiple commands and pipelining">Multiple commands and pipelining</a></h2>A client can use the same connection in order to issue multiple commands.
 129 Pipelining is supported so multiple commands can be sent with a single
 130 write operation by the client, it is not needed to read the server reply
 131 in order to issue the next command. All the replies can be read at the end.<br/><br/>Usually Redis server and client will have a very fast link so this is not
 132 very important to support this feature in a client implementation, still
 133 if an application needs to issue a very large number of commands in short
 134 time to use pipelining can be much faster.
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