-
The Redis protocol is a compromise between being easy to parse by a computer
+ = Protocol Specification =
The Redis protocol is a compromise between being easy to parse by a computer
and being easy to parse by an human. Before reading this section you are
strongly encouraged to read the "REDIS TUTORIAL" section of this README in order
-to get a first feeling of the protocol playing with it by TELNET.
A client connects to a Redis server creating a TCP connection to the port 6973.
+to get a first feeling of the protocol playing with it by TELNET.
A client connects to a Redis server creating a TCP connection to the port 6379.
Every redis command or data transmitted by the client and the server is
terminated by "\r\n" (CRLF).
The simplest commands are the inline commands. This is an example of a
server/client chat (the server chat starts with S:, the client chat with C:)
C: PING
S: +PONG
-
An inline command is a CRLF-terminated string sent to the client. The server
-usually replies to inline commands with a single line that can be a number
-or a return code.
When the server replies with a status code (that is a one line reply just indicating if the operation succeeded or not), if the first character of the
-reply is a "+" then the command succeeded, if it is a "-" then the following
-part of the string is an error.
The following is another example of an INLINE command returning an integer:
+
An inline command is a CRLF-terminated string sent to the client. The server can reply to commands in different ways:
+
- With an error message (the first byte of the reply will be "-")
- With a single line reply (the first byte of the reply will be "+)
- With bulk data (the first byte of the reply will be "$")
- With multi-bulk data, a list of values (the first byte of the reply will be "
*") - With an integer number (the first byte of the reply will be ":")
+The following is another example of an INLINE command returning an integer:
C: EXISTS somekey
-S: 0
-
Since 'somekey' does not exist the server returned '0'.
Note that the EXISTS command takes one argument. Arguments are separated
+S: :0
+Since 'somekey' does not exist the server returned ':0'.
Note that the EXISTS command takes one argument. Arguments are separated
simply by spaces.
A bulk command is exactly like an inline command, but the last argument
of the command must be a stream of bytes in order to send data to the server.
the "SET" command is a bulk command, see the following example:
@@ -58,82 +56,83 @@ sent by the client in the above sample:
"SET mykey 6\r
The server may reply to an inline or bulk command with a bulk reply. See
the following example:
C: GET mykey
-S: 6
+S: $6
S: foobar
A bulk reply is very similar to the last argument of a bulk command. The
-server sends as the first line the number of bytes of the actual reply
-followed by CRLF, then the bytes are sent followed by additional two bytes
-for the final CRLF. The exact sequence sent by the server is:
"6\r\nfoobar\r\n"
+server sends as the first line a "$" byte followed by the number of bytes
+of the actual reply followed by CRLF, then the bytes are sent followed by
+additional two bytes for the final CRLF. The exact sequence sent by the
+server is:
"$6\r\nfoobar\r\n"
If the requested value does not exist the bulk reply will use the special
-value 'nil' instead to send the line containing the number of bytes to read.
-This is an example:
+value -1 as data length, example:
C: GET nonexistingkey
-S: nil
-
The client library API should not return an empty string, but a nil object.
+S: $-1
+
The client library API should not return an empty string, but a nil object, when the requested object does not exist.
For example a Ruby library should return 'nil' while a C library should return
-NULL.Bulk replies can signal errors, for example trying to use GET against a list
-value is not permitted. Bulk replies use a negative bytes count in order to
-signal an error. An error string of ABS(bytes_count) bytes will follow. See
-the following example:
-S: GET alistkey
-S: -38
-S: -ERR Requested element is not a string
-
-38 means: sorry your operation resulted in an error, but a 38 bytes string
-that explains this error will follow. Client APIs should abort on this kind
-of errors, for example a PHP client should call the die() function.
The following commands reply with a bulk reply: GET, KEYS, LINDEX, LPOP, RPOPCommands similar to LRANGE needs to return multiple values (every element
+NULL, and so forth.Commands similar to LRANGE needs to return multiple values (every element
of the list is a value, and LRANGE needs to return more than a single element). This is accomplished using multiple bulk writes,
prefixed by an initial line indicating how many bulk writes will follow.
-Example:
+The first byte of a multi bulk reply is always *. Example:
C: LRANGE mylist 0 3
-S: 4
-S: 3
+S: *4
+S: $3
S: foo
-S: 3
+S: $3
S: bar
-S: 5
+S: $5
S: Hello
-S: 5
+S: $5
S: World
-
The first line the server sent is "4\r\n" in order to specify that four bulk
+
The first line the server sent is "4\r\n" in order to specify that four bulk
write will follow. Then every bulk write is transmitted.
If the specified key does not exist instead of the number of elements in the
-list, the special value 'nil' is sent. Example:
+list, the special value -1 is sent as count. Example:
C: LRANGE nokey 0 1
-S: nil
+S: *-1
A client library API SHOULD return a nil object and not an empty list when this
-happens. This makes possible to distinguish between empty list and non existing ones.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 pattern option when the specified key is missing. Example of a multi bulk reply containing an empty element:
-S: 3
-S: 3
+happens. This makes possible to distinguish between empty list and non existing ones.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 pattern option when the specified key is missing. Example of a multi bulk reply containing an empty element:
+S: *3
+S: $3
S: foo
-S: -1
-S: 3
+S: $-1
+S: $3
S: bar
-
The second element is nul. The client library should return something like this:
+
The second element is nul. The client library should return something like this:
["foo",nil,"bar"]
-
Like bulk reply errors Multi-bulk reply errors are reported using a negative
-count. Example:
-C: LRANGE stringkey 0 1
-S: -38
-S: -ERR Requested element is not a string
-
The following commands reply with a multi-bulk reply: LRANGE, LINTER
Check the Bulk replies errors section for more information.As already seen a status code reply is in the form of a single line string
-terminated by "\r\n". For example:
+
As already seen a single line reply is in the form of a single line string
+starting with "+" terminated by "\r\n". For example:
+OK
-
and
--ERR no suck key
-
are two examples of status code replies. The first character of a status code reply is always "+" or "-".
The following commands reply with a status code reply:
-PING, SET, SELECT, SAVE, BGSAVE, SHUTDOWN, RENAME, LPUSH, RPUSH, LSET, LTRIMThis type of reply is just a CRLF terminated string representing an integer. For example "0\r\n", or "1000\r\n" are integer replies.
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.
Some commands like EXISTS will return 1 for true and 0 for false.
Other commands like SADD, SREM and SETNX will return 1 if the operation was actually done, 0 otherwise, and a negative value if the operation is invalid (for example SADD against a non-set value), accordingly to this table:
-
--1 no such key
--2 operation against the a key holding a value of the wrong type
--3 source and destiantion objects/dbs are the same
--4 argument out of range
-
-In all this cases it is mandatory that the client raises an error instead to pass the negative value to the caller. Please check the commands documentation for the exact behaviour.
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
The commands that will never return a negative integer (commands that can't fail) are: INCR, DECR, INCRBY, DECRBY, LASTSAVE, EXISTS, SETNX, DEL, DBSIZE.This replies are just single line strings terminated by CRLF. Only two commands reply in this way currently, RANDOMKEY and TYPE.A client can use the same connection in order to issue multiple commands.
+
The client library should return everything after the "+", that is, the string "OK" in the example.
The following commands reply with a status code reply:
+PING, SET, SELECT, SAVE, BGSAVE, SHUTDOWN, RENAME, LPUSH, RPUSH, LSET, LTRIMThis type of reply is just a CRLF terminated string representing an integer, prefixed by a ":" byte. For example ":0\r\n", or ":1000\r\n" are integer replies.
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.
Some commands like EXISTS will return 1 for true and 0 for false.
Other commands like SADD, SREM and SETNX will return 1 if the operation was actually done, 0 otherwise.
The following commands will reply with an integer reply: SETNX, DEL, EXISTS, INCR, INCRBY, DECR, DECRBY, DBSIZE, LASTSAVE, RENAMENX, MOVE, LLEN, SADD, SREM, SISMEMBER, SCARDAs you can see with the protocol described so far there is no way to
+send multiple binary-safe arguments to a command. With bulk commands the
+last argument is binary safe, but there are commands where multiple binary-safe
+commands are needed, like the MSET command that is able to SET multiple keys
+in a single operation.
In order to address this problem Redis 1.1 introduced a new way of seding
+commands to a Redis server, that uses exactly the same protocol of the
+multi bulk replies. For instance the following is a SET command using the
+normal bulk protocol:
+SET mykey 8
+myvalue
+
While the following uses the multi bulk command protocol:
+*3
+$3
+SET
+$5
+mykey
+$8
+myvalue
+
Commands sent in this format are longer, so currently they are used only in
+order to transmit commands containing multiple binary-safe arguments, but
+actually this protocol can be used to send every kind of command, without to
+know if it's an inline, bulk or multi-bulk command.
It is possible that in the future Redis will support only this format.
A good client library may implement unknown commands using this
+command format in order to support new commands out of the box without
+modifications.A client can use the same connection in order to issue multiple commands.
Pipelining is supported so multiple commands can be sent with a single
write operation by the client, it is not needed to read the server reply
in order to issue the next command. All the replies can be read at the end.
Usually Redis server and client will have a very fast link so this is not
very important to support this feature in a client implementation, still
if an application needs to issue a very large number of commands in short
-time to use pipelining can be much faster.
+time to use pipelining can be much faster.
+