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;//佇列中伺服器數量

 /* ngx_mutex_t                    *mutex; */

    ngx_uint_t                      total_weight;//所有伺服器總權重

    unsigned                        single:1;//為1表示後端伺服器總共只有一臺，用於優化，此時不需要再做選擇
    unsigned                        weighted:1;//為1表示總的權重值等於伺服器數量

    ngx_str_t                      *name;

    ngx_http_upstream_rr_peers_t   *next;//後備伺服器列表掛載在這個欄位下

    ngx_http_upstream_rr_peer_t     peer[1];
};
 

//函式：初始化伺服器負載均衡表	
//引數：
//us：ngx_http_upstream_main_conf_t結構體中upstreams陣列元素
ngx_int_t
ngx_http_upstream_init_round_robin(ngx_conf_t *cf,
    ngx_http_upstream_srv_conf_t *us)
{
    ngx_url_t                      u;
    ngx_uint_t                     i, j, n, w;
    ngx_http_upstream_server_t    *server;
    ngx_http_upstream_rr_peers_t  *peers, *backup;

    //回撥指標設定
    us->peer.init = ngx_http_upstream_init_round_robin_peer;

    //伺服器陣列指標不為空
    if (us->servers) {
        server = us->servers->elts;

        n = 0;
        w = 0;

	//遍歷所有伺服器
        for (i = 0; i < us->servers->nelts; i++) {
	    //是後備伺服器，跳過
            if (server[i].backup) {
                continue;
            }

	    //伺服器地址數量統計
            n += server[i].naddrs;
	    //總的權重計算
            w += server[i].naddrs * server[i].weight;
        }

        if (n == 0) {
            ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
                          "no servers in upstream \"%V\" in %s:%ui",
                          &us->host, us->file_name, us->line);
            return NGX_ERROR;
        }

	//為非後備伺服器分配空間
        peers = ngx_pcalloc(cf->pool, sizeof(ngx_http_upstream_rr_peers_t)
                              + sizeof(ngx_http_upstream_rr_peer_t) * (n - 1));
        if (peers == NULL) {
            return NGX_ERROR;
        }

	//非後備伺服器列表頭中各屬性設定
        peers->single = (n == 1);
        peers->number = n;
        peers->weighted = (w != n);
        peers->total_weight = w;
        peers->name = &us->host;

        n = 0;

	//後備伺服器列表中各伺服器項設定
        for (i = 0; i < us->servers->nelts; i++) {
            for (j = 0; j < server[i].naddrs; j++) {
                if (server[i].backup) {
                    continue;
                }

                peers->peer[n].sockaddr = server[i].addrs[j].sockaddr;
                peers->peer[n].socklen = server[i].addrs[j].socklen;
                peers->peer[n].name = server[i].addrs[j].name;
                peers->peer[n].max_fails = server[i].max_fails;
                peers->peer[n].fail_timeout = server[i].fail_timeout;
                peers->peer[n].down = server[i].down;
                peers->peer[n].weight = server[i].weight;
                peers->peer[n].effective_weight = server[i].weight;
                peers->peer[n].current_weight = 0;
                n++;
            }
        }

	//非後備伺服器列表掛載的位置
        us->peer.data = peers;

        /* backup servers */
	//後備伺服器
        n = 0;
        w = 0;

        for (i = 0; i < us->servers->nelts; i++) {
            if (!server[i].backup) {
                continue;
            }
	    //後備伺服器地址數量統計
            n += server[i].naddrs;
	    //後備伺服器總權重計算
            w += server[i].naddrs * server[i].weight;
        }

        if (n == 0) {
            return NGX_OK;
        }

	//後備伺服器列表地址空間分配
        backup = ngx_pcalloc(cf->pool, sizeof(ngx_http_upstream_rr_peers_t)
                              + sizeof(ngx_http_upstream_rr_peer_t) * (n - 1));
        if (backup == NULL) {
            return NGX_ERROR;
        }

        peers->single = 0;
	//後備伺服器列表頭中各屬性設定
        backup->single = 0;
        backup->number = n;
        backup->weighted = (w != n);
        backup->total_weight = w;
        backup->name = &us->host;

        n = 0;

	//後備伺服器列表中各伺服器項設定
        for (i = 0; i < us->servers->nelts; i++) {
            for (j = 0; j < server[i].naddrs; j++) {
                if (!server[i].backup) {
                    continue;
                }

                backup->peer[n].sockaddr = server[i].addrs[j].sockaddr;
                backup->peer[n].socklen = server[i].addrs[j].socklen;
                backup->peer[n].name = server[i].addrs[j].name;
                backup->peer[n].weight = server[i].weight;
                backup->peer[n].effective_weight = server[i].weight;
                backup->peer[n].current_weight = 0;
                backup->peer[n].max_fails = server[i].max_fails;
                backup->peer[n].fail_timeout = server[i].fail_timeout;
                backup->peer[n].down = server[i].down;
                n++;
            }
        }

	//後備伺服器掛載
        peers->next = backup;

        return NGX_OK;
    }

    //us引數中伺服器指標為空，例如使用者直接在proxy_pass等指令後配置後端伺服器地址
    /* an upstream implicitly defined by proxy_pass, etc. */

    if (us->port == 0) {
        ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
                      "no port in upstream \"%V\" in %s:%ui",
                      &us->host, us->file_name, us->line);
        return NGX_ERROR;
    }

    ngx_memzero(&u, sizeof(ngx_url_t));

    u.host = us->host;
    u.port = us->port;

    //IP地址解析
    if (ngx_inet_resolve_host(cf->pool, &u) != NGX_OK) {
        if (u.err) {
            ngx_log_error(NGX_LOG_EMERG, cf->log, 0,
                          "%s in upstream \"%V\" in %s:%ui",
                          u.err, &us->host, us->file_name, us->line);
        }

        return NGX_ERROR;
    }

    n = u.naddrs;

    peers = ngx_pcalloc(cf->pool, sizeof(ngx_http_upstream_rr_peers_t)
                              + sizeof(ngx_http_upstream_rr_peer_t) * (n - 1));
    if (peers == NULL) {
        return NGX_ERROR;
    }

    peers->single = (n == 1);
    peers->number = n;
    peers->weighted = 0;
    peers->total_weight = n;
    peers->name = &us->host;

    for (i = 0; i < u.naddrs; i++) {
        peers->peer[i].sockaddr = u.addrs[i].sockaddr;
        peers->peer[i].socklen = u.addrs[i].socklen;
        peers->peer[i].name = u.addrs[i].name;
        peers->peer[i].weight = 1;
        peers->peer[i].effective_weight = 1;
        peers->peer[i].current_weight = 0;
        peers->peer[i].max_fails = 1;
        peers->peer[i].fail_timeout = 10;
    }

    us->peer.data = peers;

    /* implicitly defined upstream has no backup servers */

    return NGX_OK;
}
 

選擇後端伺服器