实战：负载均衡器 (Load Balancer)

struct backend {
    const char *ip;
    int port;
    int is_up;
};

struct backend pool[] = {
    {"10.0.0.1", 80, 1},
    {"10.0.0.2", 80, 1}
};

struct backend *select_backend() {
    static int idx = 0;
    for (int i = 0; i < 2; ++i) {
        int cur = (idx + i) % 2;
        if (pool[cur].is_up) {
            idx = cur + 1;
            return &pool[cur];
        }
    }
    return NULL;
}

void check_cb(evutil_socket_t fd, short events, void *arg) {
    struct backend *b = arg;
    // 尝试 connect
    // 如果成功 -> b->is_up = 1
    // 如果失败 -> b->is_up = 0
}

// 启动定时器
struct event *ev = event_new(base, -1, EV_PERSIST, check_cb, backend);
struct timeval tv = {2, 0};
event_add(ev, &tv);