static ngx_int_t
ngx_http_upstream_init_ip_hash(ngx_conf_t *cf, ngx_http_upstream_srv_conf_t *us)
{//调用了加权轮询if (ngx_http_upstream_init_round_robin(cf, us) != NGX_OK) {return NGX_ERROR;}//修改了针对单个请求进行初始化的回调函数us->peer.init = ngx_http_upstream_init_ip_hash_peer;return NGX_OK;
}

选择后端服务器

static ngx_int_t
ngx_http_upstream_init_ip_hash_peer(ngx_http_request_t *r,ngx_http_upstream_srv_conf_t *us)
{struct sockaddr_in                     *sin;//针对IPv6的支持
#if (NGX_HAVE_INET6)struct sockaddr_in6                    *sin6;
#endifngx_http_upstream_ip_hash_peer_data_t  *iphp;iphp = ngx_palloc(r->pool, sizeof(ngx_http_upstream_ip_hash_peer_data_t));if (iphp == NULL) {return NGX_ERROR;}r->upstream->peer.data = &iphp->rrp;//调用了RR算法中的初始化函数if (ngx_http_upstream_init_round_robin_peer(r, us) != NGX_OK) {return NGX_ERROR;}//回调函数设置，具体做选择的回调函数r->upstream->peer.get = ngx_http_upstream_get_ip_hash_peer;switch (r->connection->sockaddr->sa_family) {//保存客户端地址case AF_INET:sin = (struct sockaddr_in *) r->connection->sockaddr;iphp->addr = (u_char *) &sin->sin_addr.s_addr;//转储IPv4只用到了前3个字节，因为在后面的hash计算过程中只用到了3个字节iphp->addrlen = 3;break;#if (NGX_HAVE_INET6)case AF_INET6:sin6 = (struct sockaddr_in6 *) r->connection->sockaddr;iphp->addr = (u_char *) &sin6->sin6_addr.s6_addr;iphp->addrlen = 16;break;
#endifdefault:iphp->addr = ngx_http_upstream_ip_hash_pseudo_addr;iphp->addrlen = 3;}//初始化hash种子iphp->hash = 89;//初始化尝试失败次数iphp->tries = 0;//做RR选择的函数iphp->get_rr_peer = ngx_http_upstream_get_round_robin_peer;return NGX_OK;
}

其中结构体ngx_http_upstream_ip_hash_peer_data_t：

typedef struct {/* the round robin data must be first */ngx_http_upstream_rr_peer_data_t   rrp;//hash种子值ngx_uint_t                         hash;//IP地址u_char                             addrlen;u_char                            *addr;//尝试连接的次数u_char                             tries;ngx_event_get_peer_pt              get_rr_peer;
} ngx_http_upstream_ip_hash_peer_data_t;typedef struct {//指向所有服务器的指针ngx_http_upstream_rr_peers_t   *peers;//当前服务器ngx_uint_t                      current;//指向位图的指针uintptr_t                      *tried;//位图的实际存储位置uintptr_t                       data;
} ngx_http_upstream_rr_peer_data_t;typedef struct ngx_http_upstream_rr_peers_s  ngx_http_upstream_rr_peers_t;struct ngx_http_upstream_rr_peers_s {ngx_uint_t                      number;//所有服务器地址总数/* ngx_mutex_t                    *mutex; */ngx_uint_t                      total_weight;//所有服务总权重unsigned                        single:1;//是否只有一个后端服务unsigned                        weighted:1;//number ！= total_weight ?ngx_str_t                      *name;ngx_http_upstream_rr_peers_t   *next;ngx_http_upstream_rr_peer_t     peer[1];
};

具体做选择的函数是ngx_http_upstream_get_ip_hash_peer：

static ngx_int_t
ngx_http_upstream_get_ip_hash_peer(ngx_peer_connection_t *pc, void *data)
{ngx_http_upstream_ip_hash_peer_data_t  *iphp = data;time_t                        now;ngx_int_t                     w;uintptr_t                     m;ngx_uint_t                    i, n, p, hash;ngx_http_upstream_rr_peer_t  *peer;ngx_log_debug1(NGX_LOG_DEBUG_HTTP, pc->log, 0,"get ip hash peer, try: %ui", pc->tries);/* TODO: cached *///如果失败次数太多，或者只有一个后端服务，那么直接做RR选择if (iphp->tries > 20 || iphp->rrp.peers->single) {return iphp->get_rr_peer(pc, &iphp->rrp);}now = ngx_time();pc->cached = 0;pc->connection = NULL;hash = iphp->hash;for ( ;; ) {//计算IP的hash值for (i = 0; i < iphp->addrlen; i++) {//113质数，可以让哈希结果更散列hash = (hash * 113 + iphp->addr[i]) % 6271;}//根据哈希结果得到被选中的后端服务器if (!iphp->rrp.peers->weighted) {p = hash % iphp->rrp.peers->number;} else {w = hash % iphp->rrp.peers->total_weight;for (i = 0; i < iphp->rrp.peers->number; i++) {w -= iphp->rrp.peers->peer[i].weight;if (w < 0) {break;}}p = i;}//服务器对应在位图中的位置计算n = p / (8 * sizeof(uintptr_t));m = (uintptr_t) 1 << p % (8 * sizeof(uintptr_t));if (!(iphp->rrp.tried[n] & m)) {ngx_log_debug2(NGX_LOG_DEBUG_HTTP, pc->log, 0,"get ip hash peer, hash: %ui %04XA", p, m);//获取服务器peer = &iphp->rrp.peers->peer[p];/* ngx_lock_mutex(iphp->rrp.peers->mutex); *///服务器未挂掉if (!peer->down) {//失败次数已达上限if (peer->max_fails == 0 || peer->fails < peer->max_fails) {break;}if (now - peer->checked > peer->fail_timeout) {peer->checked = now;break;}}//更改位图标记值iphp->rrp.tried[n] |= m;/* ngx_unlock_mutex(iphp->rrp.peers->mutex); *///在连接一个远端服务器时，当前连接异常失败后可以尝试的次数pc->tries--;}//已经尝试的次数超过阈值，采用RR轮询if (++iphp->tries >= 20) {return iphp->get_rr_peer(pc, &iphp->rrp);}}//当前服务索引iphp->rrp.current = p;//服务器地址及名字保存pc->sockaddr = peer->sockaddr;pc->socklen = peer->socklen;pc->name = &peer->name;/* ngx_unlock_mutex(iphp->rrp.peers->mutex); *///位图更新iphp->rrp.tried[n] |= m;//保留种子，使下次get_ip_hash_peer的时候能够选到同一个peer上iphp->hash = hash;return NGX_OK;
}

        //指向服务器列表指针        server = us->servers->elts;n = 0;w = 0;//遍历服务器列表，计算地址总数以及总的权值for (i = 0; i < us->servers->nelts; i++) {if (server[i].backup) {continue;}n += server[i].naddrs;w += server[i].naddrs * server[i].weight;}//weighted计算peers->weighted = (w != n);

typedef struct {ngx_addr_t                      *addrs;//指向存储IP地址的数组的指针，host信息(对应的是 ngx_url_t->addrs )ngx_uint_t                       naddrs;//与第一个参数配合使用，数组元素个数(对应的是 ngx_url_t->naddrs )ngx_uint_t                       weight;//权值ngx_uint_t                       max_fails;time_t                           fail_timeout;unsigned                         down:1;unsigned                         backup:1;
} ngx_http_upstream_server_t;

轮询策略和IP哈希策略对比

参考资料：

《深入剖析Nginx》