Unlocking Redis: How the RESP Protocol Powers Lightning‑Fast Data Access

In the world of Redis, speed reigns supreme, with QPS reaching the hundred‑thousand level.

Redis communicates with its clients using the RESP (REdis Serialization Protocol), designed with three principles: simple implementation, fast parsing for computers, and strong readability for humans.

Simple implementation

Fast parsing for computers

Human‑readable

A Redis command is packaged by the client, transmitted over the network, and then parsed and executed by the server.

The protocol works on top of TCP and defines a compact, line‑oriented format.

To observe the raw protocol messages, you can masquerade as a Redis server using a ServerSocket that listens on port 6379:

public static void server() throws IOException {
    ServerSocket serverSocket = new ServerSocket(6379);
    Socket socket = serverSocket.accept();
    byte[] bytes = new byte[1024];
    InputStream input = socket.getInputStream();
    while (input.read(bytes) != 0) {
        System.out.println(new String(bytes));
    }
}

Then launch the redis-cli client and send a command, e.g., set key1 value1. The mock server will print the following RESP message:

*3
$3
set
$4
key1
$6
value1

The message format is:

*<number of arguments> CRLF
$<byte length of argument 1> CRLF
<argument 1 data> CRLF
$<byte length of argument 2> CRLF
<argument 2 data> CRLF
... 
$<byte length of argument N> CRLF
<argument N data> CRLF

Here CRLF corresponds to the characters \r\n.

Redis defines several response types:

Simple Strings

Start with +, contain a single line, end with \r\n. Successful commands often return +OK.

Error Replies

Start with -, contain an error type and description, and end with \r\n. They are used for malformed commands or wrong argument counts.

Integer Replies

Start with :, contain a decimal integer, and end with \r\n. Used for commands like INCR, LLEN, or EXISTS.

Bulk Strings

Start with $, followed by the byte length, \r\n, the data, and another \r\n. A length of -1 indicates a null value.

Arrays (Multi‑Bulk Replies)

Start with *, followed by the number of elements, then a series of bulk string replies. This is used to return multiple values such as a list of elements.

Below is a Java example of a Redis client that builds and sends a RESP command and reads the server’s reply:

private static void client() throws IOException {
    String CRLF = "
";
    Socket socket = new Socket("localhost", 6379);
    try (OutputStream out = socket.getOutputStream()) {
        StringBuilder sb = new StringBuilder();
        sb.append("*3").append(CRLF)
          .append("$3").append(CRLF).append("set").append(CRLF)
          .append("$4").append(CRLF).append("key1").append(CRLF)
          .append("$6").append(CRLF).append("value1").append(CRLF);
        out.write(sb.toString().getBytes());
        out.flush();
        try (InputStream inputStream = socket.getInputStream()) {
            byte[] buff = new byte[1024];
            int len = inputStream.read(buff);
            if (len > 0) {
                String ret = new String(buff, 0, len);
                System.out.println("Recv:" + ret);
            }
        }
    }
}

Running this client prints +OK, confirming successful communication.