43. Capstone: a mini Redis-like key-value store with a wire protocol

This capstone combines nearly every earlier Part: Section 21's sockets, Section 18.2's hash table, Section 23.2's event-loop style, Section 16.3's parsing technique (applied to a wire protocol instead of JSON), and Section 40's API discipline.

The protocol — a simplified line-based version of Redis's real RESP protocol:

SET key value\r\n   -> +OK\r\n
GET key\r\n         -> $value\r\n   (or $-1\r\n if missing)
DEL key\r\n         -> :1\r\n       (or :0\r\n if it wasn't present)

// kvserver.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/select.h>
#include "hashtable_str.h"   // Section 18.2's hash table, adapted to store string values

#define MAX_CLIENTS 64
#define BUF_SIZE 4096

static void handle_command(HashTable *db, char *line, char *response, size_t resp_cap) {
    char cmd[16], key[128], value[BUF_SIZE];
    int n = sscanf(line, "%15s %127s %4095[^\r\n]", cmd, key, value);

    if (strcmp(cmd, "SET") == 0 && n == 3) {
        ht_set_str(db, key, value);
        snprintf(response, resp_cap, "+OK\r\n");
    } else if (strcmp(cmd, "GET") == 0 && n >= 2) {
        const char *val = ht_get_str(db, key);
        if (val) snprintf(response, resp_cap, "$%s\r\n", val);
        else     snprintf(response, resp_cap, "$-1\r\n");
    } else if (strcmp(cmd, "DEL") == 0 && n >= 2) {
        int deleted = ht_del_str(db, key);
        snprintf(response, resp_cap, ":%d\r\n", deleted);
    } else {
        snprintf(response, resp_cap, "-ERR unknown command\r\n");
    }
}

int main(void) {
    HashTable *db = ht_new(1024);

    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    int opt = 1;
    setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof opt);
    struct sockaddr_in addr = { .sin_family = AF_INET, .sin_port = htons(6380),
                                 .sin_addr.s_addr = INADDR_ANY };
    bind(server_fd, (struct sockaddr *)&addr, sizeof addr);
    listen(server_fd, 16);
    printf("mini-kv listening on :6380\n");

    int clients[MAX_CLIENTS] = {0};

    while (1) {
        fd_set readfds;
        FD_ZERO(&readfds);
        FD_SET(server_fd, &readfds);
        int max_fd = server_fd;
        for (int i = 0; i < MAX_CLIENTS; i++)
            if (clients[i] > 0) { FD_SET(clients[i], &readfds); if (clients[i] > max_fd) max_fd = clients[i]; }

        select(max_fd + 1, &readfds, NULL, NULL, NULL);

        if (FD_ISSET(server_fd, &readfds)) {
            int fd = accept(server_fd, NULL, NULL);
            for (int i = 0; i < MAX_CLIENTS; i++) if (clients[i] == 0) { clients[i] = fd; break; }
        }

        for (int i = 0; i < MAX_CLIENTS; i++) {
            if (clients[i] > 0 && FD_ISSET(clients[i], &readfds)) {
                char line[BUF_SIZE] = {0};
                ssize_t n = read(clients[i], line, sizeof line - 1);
                if (n <= 0) { close(clients[i]); clients[i] = 0; continue; }

                char response[BUF_SIZE];
                handle_command(db, line, response, sizeof response);
                write(clients[i], response, strlen(response));
            }
        }
    }
}

./kvserver &
printf 'SET name kumaran\r\n' | nc -q1 localhost 6380   # +OK
printf 'GET name\r\n'         | nc -q1 localhost 6380   # $kumaran
printf 'DEL name\r\n'         | nc -q1 localhost 6380   # :1
printf 'GET name\r\n'         | nc -q1 localhost 6380   # $-1

Where to take it next, each a direct application of an earlier Part: persistence via an append-only log you replay on startup (a simplified version of Redis's AOF); a background save via fork() (Section 26) so the child writes a point-in-time snapshot while the parent keeps serving requests, unaffected by the child's memory writes thanks to copy-on-write; multiple databases via multiple hash tables; and expiring keys via a min-heap of (expire_time, key) pairs checked once per event-loop iteration. This is, concept for concept, how Redis itself grew from a single-file prototype into the production system Section 34 walked through.