Key Monitoring and Transactions with Redis in C++

#include <iostream>
#include <hiredis/hiredis.h>
#include <string>

void updateBalance(redisContext* context, int user_id, int increment) {
    while (true) {
        redisReply* reply;

        // WATCH command
        reply = (redisReply*)redisCommand(context, "WATCH balance:%d", user_id);
        freeReplyObject(reply);

        // Get current balance
        reply = (redisReply*)redisCommand(context, "GET balance:%d", user_id);
        int balance = (reply->type == REDIS_REPLY_NIL) ? 0 : std::stoi(reply->str);
        freeReplyObject(reply);

        // MULTI command to start a transaction
        reply = (redisReply*)redisCommand(context, "MULTI");
        freeReplyObject(reply);

        // Set new balance
        reply = (redisReply*)redisCommand(context, "SET balance:%d %d", user_id, balance + increment);
        freeReplyObject(reply);

        // EXEC command to execute transaction
        reply = (redisReply*)redisCommand(context, "EXEC");

        if (reply->type == REDIS_REPLY_ARRAY && reply->elements == 1) {
            freeReplyObject(reply);
            break; // Transaction succeeded
        }

        std::cerr << "Retrying transaction due to an external modification." << std::endl;
        freeReplyObject(reply);
    }
}

int main() {
    // Connect to the Redis server
    redisContext* context = redisConnect("127.0.0.1", 6379);
    if (context == nullptr || context->err) {
        if (context) {
            std::cerr << "Connection error: " << context->errstr << std::endl;
        } else {
            std::cerr << "Connection error: can't allocate Redis context" << std::endl;
        }
        return 1;
    }

    // Set initial balance for user 1
    redisReply* reply = (redisReply*)redisCommand(context, "SET balance:1 100");
    freeReplyObject(reply);

    // Update balance for user 1
    updateBalance(context, 1, 50);

    // Retrieve updated balance
    reply = (redisReply*)redisCommand(context, "GET balance:1");
    if (reply->type == REDIS_REPLY_STRING) {
        std::cout << "Updated balance for user 1: " << reply->str << std::endl;
    } else {
        std::cerr << "Failed to retrieve the updated balance." << std::endl;
    }
    freeReplyObject(reply);

    // Free the context
    redisFree(context);

    return 0;
}