The Redis protocol is a compromise between the following things:
- Simple to implement
- Fast to parse by a computer
- Easy enough to parse by a human
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
Redis accepts commands composed of different arguments. Once a command is received, it is processed and a reply is sent back to the client.
The new unified request protocol
The new unified protocol was introduced in Redis 1.2, but it became the standard way for talking with the Redis server in Redis 2.0.
In the unified protocol all the arguments sent to the Redis server are binary safe. This is the general form:
*<number of arguments> CR LF $<number of bytes of argument 1> CR LF <argument data> CR LF ... $<number of bytes of argument N> CR LF <argument data> CR LF
See the following example:
*3 $3 SET $5 mykey $7 myvalue
This is how the above command looks as a quoted string, so that it is possible to see the exact value of every byte in the query:
As you will see in a moment this format is also used in Redis replies. The
format used for every argument
$6\r\nmydata\r\n is called a Bulk Reply.
While the actual unified request protocol is what Redis uses to return list of
items, and is called a @multi-bulk-reply. It is just the sum of N different Bulk
Replies prefixed by a
*<argc>\r\n string where
<argc> is the number of
arguments (Bulk Replies) that will follow.
Redis will reply to commands with different kinds of replies. It is possible to check the kind of reply from the first byte sent by the server:
- With a single line reply the first byte of the reply will be "+"
- With an error message the first byte of the reply will be "-"
- With an integer number the first byte of the reply will be ":"
- With bulk reply the first byte of the reply will be "$"
- With multi-bulk reply the first byte of the reply will be "
A status reply (or: single line reply) is in the form of a single line string starting with "+" terminated by "\r\n". For example:
The client library should return everything after the "+", that is, the string "OK" in this example.
Errors are sent similar to status replies. The only difference is that the first byte is "-" instead of "+".
Error replies are only sent when something strange happened, for instance if you try to perform an operation against the wrong data type, or if the command does not exist and so forth. So an exception should be raised by the library client when an Error Reply is received.
This 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, SCARD
Bulk replies are used by the server in order to return a single binary safe string.
C: GET mykey S: $6\r\nfoobar\r\n
The server sends as the first line a "$" byte followed by the number of bytes of the actual reply, followed by CRLF, then the actual data bytes are sent, followed by additional two bytes for the final CRLF. The exact sequence sent by the server is:
If the requested value does not exist the bulk reply will use the special value -1 as data length, example:
C: GET nonexistingkey 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 (or set a special flag in the reply object), and so forth.
Commands like LRANGE need 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. The first byte of a multi bulk reply is
C: LRANGE mylist 0 3 s: *4 s: $3 s: foo s: $3 s: bar s: $5 s: Hello s: $5 s: World
As you can see the multi bulk reply is exactly the same format used in order to send commands to the Redis server using the unified protocol.
The first line the server sent is
*4\r\n in order to specify that four bulk
replies will follow. Then every bulk write is transmitted.
If the specified key does not exist, the key is considered to hold an empty
list and the value
0 is sent as multi bulk count. Example:
C: LRANGE nokey 0 1 S: *0
BLPOP command times out, it returns the nil multi bulk reply. This
type of multi bulk has count
-1 and should be interpreted as a nil value.
C: BLPOP key 1 S: *-1
A client library API SHOULD return a nil object and not an empty list when this
happens. This is necessary to distinguish between an empty list and an error
condition (for instance the timeout condition of the
Nil elements in Multi-Bulk replies
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: bar
The second element is nul. The client library should return something like this:
Multiple commands and pipelining
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.
The old protocol for sending commands
Before of the Unified Request Protocol Redis used a different protocol to send commands, that is still supported since it is simpler to type by hand via telnet. In this protocol there are two kind of commands:
- Inline commands: simple commands where arguments are just space separated strings. No binary safeness is possible.
- Bulk commands: bulk commands are exactly like inline commands, but the last argument is handled in a special way in order to allow for a binary-safe last argument.
The simplest way to send Redis a command is via inline commands. The following is an example of a server/client chat using an inline command (the server chat starts with S:, the client chat with C:)
C: PING S: +PONG
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 by spaces.
Some commands when sent as inline commands require a special form in order to support a binary safe last argument. This commands will use the last argument for a "byte count", then the bulk data is sent (that can be binary safe since the server knows how many bytes to read).
See for instance the following example:
C: SET mykey 6 C: foobar S: +OK
The last argument of the command is '6'. This specify the number of DATA bytes that will follow, that is, the string "foobar". Note that even this bytes are terminated by two additional bytes of CRLF.
All the bulk commands are in this exact form: instead of the last argument the number of bytes that will follow is specified, followed by the bytes composing the argument itself, and CRLF. In order to be more clear for the programmer this is the string sent by the client in the above sample:
"SET mykey 6\r\nfoobar\r\n"
Redis has an internal list of what command is inline and what command is bulk, so you have to send this commands accordingly. It is strongly suggested to use the new Unified Request Protocol instead.