A Beginner's Guide to the Redis Protocol

One of Redis’s best unsung features is its wire protocol. It’s the reason that Redis has one of the largest ecosystems of high-quality client libraries. The Redis wire protocol is remarkably simple, which makes it easy to build a client that implements all of Redis’s major features. It’s also designed in a way that makes it easy to write fast and efficient client libraries.

What is RESP?

RESP (REdis Serialization Protocol) is the name of the text-based protocol that Redis clients and servers use to communicate with each other over TCP. All communication between server and client consists of five basic types:

  • Simple strings are used for common server replies such as “OK” (after a successful write command) or “PONG” (the successful response to the PING command).
  • Bulk strings are returned for almost all single-value read commands such as GET, LPOP, and HGET. Bulk strings are different from simple strings in that they can contain anything — newlines, control characters, or even valid RESP, all without being escaped or encoded.
  • Integers are used as the reply for any kind of counting command such as STRLEN, HLEN, or BITCOUNT.
  • Arrays can contain any number of RESP objects, including other arrays. This is used to send commands to the server, as well as for any reply that returns more than one element such as HGETALL, LRANGE, or MGET.
  • Errors are returned whenever Redis encounters an error while handling your command, such as when trying to run a command against a key holding the wrong type of data.

When writing and reading RESP, we don’t have to do any escaping or encoding of our keys or values. The only part of RESP we need to actually parse is the simple metadata included with each RESP object.

What does RESP look like?

All RESP objects begin with a prefix character and end with a line terminator (except arrays, which don’t include their own line terminator). The simplest example of a RESP object is the OK simple string response:

+OK\r\n

(RESP is a human-readable protocol but for clarity’s sake I’ll explicitly write out out the line breaks (\r\n) in all RESP examples.)

In the above simple string, + is the simple string prefix, OK is the body of the simple string, and \r\n is the line terminator that marks the end of this simple string.

To read a simple string, we read up to the next \r\n line terminator, returning the preceding bytes (up to +) as the returned reply string. This works because RESP simple strings cannot include any newline characters.

Errors and integers are formatted similarly to simple strings, but they use different prefixes. Errors are prefixed with -:

-ERR unknown command 'GETT'\r\n

And integers are prefixed with ::

:99\r\n

Bulk strings are unique in that they have two parts. A length specification and a body:

$13\r\nHello, World!\r\n

$ is the bulk string prefix, 13 is the number of bytes in the actual string body, and then \r\n terminates the length specification. Hello, World! is the 13 byte string body, and that is also terminated with \r\n (which is not part of the string body).

Because the bulk string container provides an exact length for the body, we never have to parse the actual string body to find the end — we can read another 13 bytes (plus the final line terminator) from the socket without inspecting the contents of the string. This means we can use any data that we want in the body of the string without encoding or escaping the contents.

Arrays also begin with a length specification, except the length field indicates the number of objects in the array rather than the number of bytes in the array’s contents.

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

In the above example, we have an array that contains two bulk strings (foo and bar). While the array length specification doesn’t allow us to skip ahead the way that we can when reading a bulk string, it does make it easy to implement arrays in our client. We read the array size, read that many more objects, and then return all of those objects in the final array.

All Redis commands are sent as arrays of bulk strings. For example, the command “SET mykey ‘my value’” would be written and sent as:

*3\r\n$3\r\nSET\r\n$5\r\nmykey\r\n$8\r\nmy value\r\n

Further RESP details

The prefixed lengths used in RESP allow parsers to be implemented without needing to do complicated state modeling or multiple passes through the data, allowing for very fast parsers to be built.

There’s a number of details I haven’t covered here, such as null bulk strings and null arrays. For more information, the official documentation (as usual) has a very readable and comprehensive documentation page for RESP.

A great way to understand the protocol better is to try implementing a simple client yourself. Check out our guide to reading and writing the Redis protocol in Go next.

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Last updated 30 Apr 2015. Originally posted 16 May 2014 by Tyson Mote

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