Memcached源码阅读七 cas属性

//为新的item生成cas值
uint64_t get_cas_id(void)
{
    static uint64_t cas_id = 0;
    return ++cas_id;
}

//这段代码是store_item的代码片段，这里是执行cas存储时执行的判断逻辑，
else if (ITEM_get_cas(it) == ITEM_get_cas(old_it))//cas值一致
{
    pthread_mutex_lock(&c->thread->stats.mutex);
    c->thread->stats.slab_stats[old_it->slabs_clsid].cas_hits++;
    pthread_mutex_unlock(&c->thread->stats.mutex);

    item_replace(old_it, it, hv);//执行存储逻辑
    stored = STORED;
}
//cas值不一致，不进行实际的存储
else 
{
    pthread_mutex_lock(&c->thread->stats.mutex);
    c->thread->stats.slab_stats[old_it->slabs_clsid].cas_badval++; 
    //更新统计信息
    pthread_mutex_unlock(&c->thread->stats.mutex);

    if (settings.verbose > 1)
    {
        //打印错误日志
        fprintf(stderr, "CAS:  failure: expected %llu, got %llu\n",
                (unsigned long long) ITEM_get_cas(old_it),
                (unsigned long long) ITEM_get_cas(it));
    }
    stored = EXISTS;
}