下面是函数 ngx_http_upstream_create 的实现：

ngx_int_t
ngx_http_upstream_create(ngx_http_request_t *r)
{
    ngx_http_upstream_t  *u;

    u = r->upstream;
     /*
     * 若已经创建过ngx_http_upstream_t 且定义了cleanup成员，
     * 则调用cleanup清理方法将原始结构体清除；
     */
    if (u && u->cleanup) {
        r->main->count++;
        ngx_http_upstream_cleanup(r);
    }

    u = ngx_pcalloc(r->pool, sizeof(ngx_http_upstream_t));
    if (u == NULL) {
        return NGX_ERROR;
    }

    r->upstream = u;

    u->peer.log = r->connection->log;
    u->peer.log_error = NGX_ERROR_ERR;

#if (NGX_HTTP_CACHE)
    r->cache = NULL;
#endif

    u->headers_in.content_length_n = -1;
    u->headers_in.last_modified_time = -1;

    return NGX_OK;
}

void
ngx_http_upstream_init(ngx_http_request_t *r)
{
    ngx_connection_t     *c;
   /* 获取当前请求所对应的连接 */
    c = r->connection;
    //打印日志
    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
                   "http init upstream, client timer: %d", c->read->timer_set);
//如果是http2 就采用ngx_http_upstream_init_request 去初始化，这里暂时不进行讲解
#if (NGX_HTTP_V2)
    if (r->stream) {
        ngx_http_upstream_init_request(r);
        return;
    }
#endif
    /*
     * 检查当前连接上读事件的timer_set标志位是否为1，若该标志位为1，
     * 表示读事件在定时器机制中，则需要把它从定时器机制中移除；
     * 因为在启动upstream机制后，就不需要对客户端的读操作进行超时管理；
     */
    if (c->read->timer_set) {
        ngx_del_timer(c->read);
    }

    if (ngx_event_flags & NGX_USE_CLEAR_EVENT) {

        if (!c->write->active) {
            if (ngx_add_event(c->write, NGX_WRITE_EVENT, NGX_CLEAR_EVENT)
                == NGX_ERROR)
            {
                ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
                return;
            }
        }
    }

    ngx_http_upstream_init_request(r);
}

static void
ngx_http_upstream_init_request(ngx_http_request_t *r)
{
    ngx_str_t                      *host;
    ngx_uint_t                      i;
    ngx_resolver_ctx_t             *ctx, temp;
    ngx_http_cleanup_t             *cln;
    ngx_http_upstream_t            *u;
    ngx_http_core_loc_conf_t       *clcf;
    ngx_http_upstream_srv_conf_t   *uscf, **uscfp;
    ngx_http_upstream_main_conf_t  *umcf;

    if (r->aio) {
        return;
    }

    u = r->upstream;

#if (NGX_HTTP_CACHE)

    if (u->conf->cache) {
        ngx_int_t  rc;

        rc = ngx_http_upstream_cache(r, u);

        if (rc == NGX_BUSY) {
            r->write_event_handler = ngx_http_upstream_init_request;
            return;
        }

        r->write_event_handler = ngx_http_request_empty_handler;

        if (rc == NGX_ERROR) {
            ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

        if (rc == NGX_OK) {
            rc = ngx_http_upstream_cache_send(r, u);

            if (rc == NGX_DONE) {
                return;
            }

            if (rc == NGX_HTTP_UPSTREAM_INVALID_HEADER) {
                rc = NGX_DECLINED;
                r->cached = 0;
            }

            if (ngx_http_upstream_cache_background_update(r, u) != NGX_OK) {
                rc = NGX_ERROR;
            }
        }

        if (rc != NGX_DECLINED) {
            ngx_http_finalize_request(r, rc);
            return;
        }
    }

#endif

    u->store = u->conf->store;

    if (!u->store && !r->post_action && !u->conf->ignore_client_abort) {
        r->read_event_handler = ngx_http_upstream_rd_check_broken_connection;
        r->write_event_handler = ngx_http_upstream_wr_check_broken_connection;
    }

    if (r->request_body) {
        u->request_bufs = r->request_body->bufs;
    }

    if (u->create_request(r) != NGX_OK) {
        ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    if (ngx_http_upstream_set_local(r, u, u->conf->local) != NGX_OK) {
        ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);

    u->output.alignment = clcf->directio_alignment;
    u->output.pool = r->pool;
    u->output.bufs.num = 1;
    u->output.bufs.size = clcf->client_body_buffer_size;

    if (u->output.output_filter == NULL) {
        u->output.output_filter = ngx_chain_writer;
        u->output.filter_ctx = &u->writer;
    }

    u->writer.pool = r->pool;

    if (r->upstream_states == NULL) {

        r->upstream_states = ngx_array_create(r->pool, 1,
                                            sizeof(ngx_http_upstream_state_t));
        if (r->upstream_states == NULL) {
            ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

    } else {

        u->state = ngx_array_push(r->upstream_states);
        if (u->state == NULL) {
            ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

        ngx_memzero(u->state, sizeof(ngx_http_upstream_state_t));
    }

    cln = ngx_http_cleanup_add(r, 0);
    if (cln == NULL) {
        ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    cln->handler = ngx_http_upstream_cleanup;
    cln->data = r;
    u->cleanup = &cln->handler;

    if (u->resolved == NULL) {

        uscf = u->conf->upstream;

    } else {

#if (NGX_HTTP_SSL)
        u->ssl_name = u->resolved->host;
#endif

        host = &u->resolved->host;

        umcf = ngx_http_get_module_main_conf(r, ngx_http_upstream_module);

        uscfp = umcf->upstreams.elts;

        for (i = 0; i < umcf->upstreams.nelts; i++) {

            uscf = uscfp[i];

            if (uscf->host.len == host->len
                && ((uscf->port == 0 && u->resolved->no_port)
                     || uscf->port == u->resolved->port)
                && ngx_strncasecmp(uscf->host.data, host->data, host->len) == 0)
            {
                goto found;
            }
        }

        if (u->resolved->sockaddr) {

            if (u->resolved->port == 0
                && u->resolved->sockaddr->sa_family != AF_UNIX)
            {
                ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                              "no port in upstream \"%V\"", host);
                ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
                return;
            }

            if (ngx_http_upstream_create_round_robin_peer(r, u->resolved)
                != NGX_OK)
            {
                ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
                return;
            }

            ngx_http_upstream_connect(r, u);

            return;
        }

        if (u->resolved->port == 0) {
            ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                          "no port in upstream \"%V\"", host);
            ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

        temp.name = *host;

        ctx = ngx_resolve_start(clcf->resolver, &temp);
        if (ctx == NULL) {
            ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

        if (ctx == NGX_NO_RESOLVER) {
            ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                          "no resolver defined to resolve %V", host);

            ngx_http_upstream_finalize_request(r, u, NGX_HTTP_BAD_GATEWAY);
            return;
        }

        ctx->name = *host;
        ctx->handler = ngx_http_upstream_resolve_handler;
        ctx->data = r;
        ctx->timeout = clcf->resolver_timeout;

        u->resolved->ctx = ctx;

        if (ngx_resolve_name(ctx) != NGX_OK) {
            u->resolved->ctx = NULL;
            ngx_http_upstream_finalize_request(r, u,
                                               NGX_HTTP_INTERNAL_SERVER_ERROR);
            return;
        }

        return;
    }

found:

    if (uscf == NULL) {
        ngx_log_error(NGX_LOG_ALERT, r->connection->log, 0,
                      "no upstream configuration");
        ngx_http_upstream_finalize_request(r, u,
                                           NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    u->upstream = uscf;

#if (NGX_HTTP_SSL)
    u->ssl_name = uscf->host;
#endif

    if (uscf->peer.init(r, uscf) != NGX_OK) {
        ngx_http_upstream_finalize_request(r, u,
                                           NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    u->peer.start_time = ngx_current_msec;

    if (u->conf->next_upstream_tries
        && u->peer.tries > u->conf->next_upstream_tries)
    {
        u->peer.tries = u->conf->next_upstream_tries;
    }

    ngx_http_upstream_connect(r, u);
}