Redis Serialization Protocol (RESP)

REdis Serialization Protocol is the protocol Redis servers & clients use to communicate with each other.

Data Types

RESP2 (the default) supports encoding 5 data types:

• Simple strings
• Simple errors
• Integers
• Bulk strings
• Arrays

Each data type encoding begins with a different prefix:

• + denotes a simple string
• - denotes a simple error
• : denotes an integer
• $ denotes a bulk string
• * denotes an array

Example

In the encoding +PONG\r\n:

• + denotes a simple string.
• PONG is the string value
• \r\n is the CRLF terminator

Simple strings

Used to send simple, human readable messages.

They are encoded as: +<data>\r\n

Examples:

• The string hello would be encoded as +hello\r\n
• The string hello world would be encoded as +hello world\r\n

Simple errors

Used to return error messages.

They are encoded as: -<error message>\r\n

Example:

• The error message ERR unknown command "SETT" would be encoded as -ERR unknown command "SETT"\r\n

Integers

Integers are encoded as :<value>\r\n

Examples:

• The integer 42 would be encoded as :42\r\n
• The integer -52 would be encoded as :-52\r\n

Bulk strings

Used to represent binary strings (aka "byte string").

Unlike simple strings, bulk strings can contain any type of character, including newlines and control characters.

Bulk strings are encoded as $<length>\r\n<data>\r\n.

Examples:

• The string hello would be encoded as $5\r\nhello\r\n
• The string hello world would be encoded as $11\r\nhello world\r\n$
• The empty string "" would be encoded as $0\r\n\r\n

Aside: RESP2 does not have a dedicated type for null values.

A special null bulk string can be used to represent a null value.

It is encoded as:

$-1\r\n

Arrays

Arrays are encoded as *<number-of-elements>\r\n<element-1>...<element-n> where <element-1>, <element-2> are the RESP encoded values of the elements in the list.

Examples:

• The array [42,52] would be encoded as *2\r\n:42\r\n:52\r\n
• The array ["hello",42], where hello is a bulk string, would be encoded as *2\r\n$5\r\nhello\r\n:42\r\n

Redis commands

All commands are sent to the server as a RESP Array of Bulk Strings. The first (and sometimes second) element of the RESP array is the command name and the rest are its arguments.

The server replies to clients with any valid RESP data type.

For e.g. the command SET foo bar

would have this structure:

[SET, "foo", "bar"]

and would be encoded as:

*3\r\n$3\r\nSET\r\n$3\r\nfoo\r\n$3\r\nbar\r\n

If the operation was successful, the server would respond with a simple OK encoded as:

+OK\r\n

Summary

We covered the 5 data types supported by RESP:

• Simple strings (example: hello -> +hello\r\n)
• Simple errors (example: ERR -> -ERR\r\n)
• Integers (example: 42 -> :42\r\n)
• Bulk strings (example: hello -> $5\r\nhello\r\n)
• Arrays (example: [42, 52] -> *2\r\n:42\r\n:52\r\n)

We also discussed how clients send commands to Redis servers and how the server responds to them:

• All commands are sent to the server as a RESP Array of Bulk Strings.
• The server may reply to clients with any valid RESP data type.

If you want to learn more about RESP, check out the official spec.