Understanding Redis’s Unified Request/Response Protocol: A Deep Dive

Redis Communication Overview

Redis listens on TCP port 6379. Each client connection is a dedicated TCP socket, and every command sent to the server terminates with the CRLF sequence ( \r\n).

Unified Request Protocol (binary‑safe)

Introduced in Redis 1.2 and made the default in Redis 2.0, the protocol encodes each argument as a bulk string, making the whole request binary‑safe.

*<number_of_arguments>
$<bytes_of_argument_1>
<argument_1_data>
...$<bytes_of_argument_N>
<argument_N_data>

Explanation:

*<number_of_arguments> – asterisk followed by the count of arguments.

$<bytes_of_argument_i> – dollar sign followed by the byte length of the i‑th argument.

<argument_i_data> – the raw bytes of the argument.

Example with nc

nc localhost 6379
keys *

The client sends the following byte stream:

*2
$18
user:sign:5:202101
$18
seckill_vouchers:6

Response Types

Redis replies start with a single‑character prefix that identifies the response type:

Simple string – prefix +, e.g. +OK Error – prefix -, e.g. -ERR unknown command `keys*` Integer – prefix :, e.g. :1000 Bulk string – prefix $, followed by the length and the data, e.g. $5\r\nhello\r\n Array (multi‑bulk) – prefix *, followed by the number of elements, each element being any of the above types.

Bulk Reply Example

*2
$18
user:sign:5:202101
$18
seckill_vouchers:6

When an array contains multiple bulk strings, each element is encoded with its own $ length header, allowing clients to parse complex list results efficiently.