1、遇到conf文件的http模块。http不是在Nginx的mian函数中启动，而是解析conf文件时遇到http才会去解析并启动。

2、请求到来时，调用http server。

Nginx的http是以NGX_CORE_MODULE模块的方式加载到Nginx中，因此它定义成一个ngx_module_t（/src/http/ngx_http.c）：

static ngx_command_t  ngx_http_commands[] = {

    { ngx_string("http"),
      NGX_MAIN_CONF|NGX_CONF_BLOCK|NGX_CONF_NOARGS,
      ngx_http_block,
      0,
      0,
      NULL },

      ngx_null_command
};


static ngx_core_module_t  ngx_http_module_ctx = {
    ngx_string("http"),
    NULL,
    NULL
};

ngx_module_t  ngx_http_module = {
    NGX_MODULE_V1,
    &ngx_http_module_ctx,                  /* module context */
    ngx_http_commands,                     /* module directives */
    NGX_CORE_MODULE,                       /* module type */
    NULL,                                  /* init master */
    NULL,                                  /* init module */
    NULL,                                  /* init process */
    NULL,                                  /* init thread */
    NULL,                                  /* exit thread */
    NULL,                                  /* exit process */
    NULL,                                  /* exit master */
    NGX_MODULE_V1_PADDING
};

#define NGX_MODULE_V1                                                         
            NGX_MODULE_UNSET_INDEX, NGX_MODULE_UNSET_INDEX,                           
            NULL, 0, 0, nginx_version, NGX_MODULE_SIGNATURE
#define NGX_MODULE_V1_PADDING  0, 0, 0, 0, 0, 0, 0, 0

当解析conf文件时，如果遇到“http”就会获取ngx_http_commands，执行ngx_http_block函数。

static ngx_command_t  ngx_http_commands[] = {

    { ngx_string("http"),
      NGX_MAIN_CONF|NGX_CONF_BLOCK|NGX_CONF_NOARGS,
      ngx_http_block,
      0,
      0,
      NULL },

      ngx_null_command
};

ngx_http_block相当于http的主函数，里面做了很多的初始化工作。其中在最后会调用一个ngx_http_optimize_servers的函数优化端口、地址和服务器名称列表。

static char *
ngx_http_block(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    char                        *rv;
    ngx_uint_t                   mi, m, s;
    ngx_conf_t                   pcf;
    ngx_http_module_t           *module;
    ngx_http_conf_ctx_t         *ctx;
    ngx_http_core_loc_conf_t    *clcf;
    ngx_http_core_srv_conf_t   **cscfp;
    ngx_http_core_main_conf_t   *cmcf;

    if (*(ngx_http_conf_ctx_t **) conf) {
        return "is duplicate";
    }

    /* the main http context */

    ctx = ngx_pcalloc(cf->pool, sizeof(ngx_http_conf_ctx_t));
    if (ctx == NULL) {
        return NGX_CONF_ERROR;
    }

    *(ngx_http_conf_ctx_t **) conf = ctx;


    /* count the number of the http modules and set up their indices */

    ngx_http_max_module = ngx_count_modules(cf->cycle, NGX_HTTP_MODULE);


    /* the http main_conf context, it is the same in the all http contexts */

    ctx->main_conf = ngx_pcalloc(cf->pool,
                                 sizeof(void *) * ngx_http_max_module);
    if (ctx->main_conf == NULL) {
        return NGX_CONF_ERROR;
    }


    /*     * the http null srv_conf context, it is used to merge     * the server{}s' srv_conf's     */

    ctx->srv_conf = ngx_pcalloc(cf->pool, sizeof(void *) * ngx_http_max_module);
    if (ctx->srv_conf == NULL) {
        return NGX_CONF_ERROR;
    }


    /*     * the http null loc_conf context, it is used to merge     * the server{}s' loc_conf's     */

    ctx->loc_conf = ngx_pcalloc(cf->pool, sizeof(void *) * ngx_http_max_module);
    if (ctx->loc_conf == NULL) {
        return NGX_CONF_ERROR;
    }


    /*     * create the main_conf's, the null srv_conf's, and the null loc_conf's     * of the all http modules     */

    for (m = 0; cf->cycle->modules[m]; m++) {
        if (cf->cycle->modules[m]->type != NGX_HTTP_MODULE) {
            continue;
        }

        module = cf->cycle->modules[m]->ctx;
        mi = cf->cycle->modules[m]->ctx_index;

        if (module->create_main_conf) {
            ctx->main_conf[mi] = module->create_main_conf(cf);
            if (ctx->main_conf[mi] == NULL) {
                return NGX_CONF_ERROR;
            }
        }

        if (module->create_srv_conf) {
            ctx->srv_conf[mi] = module->create_srv_conf(cf);
            if (ctx->srv_conf[mi] == NULL) {
                return NGX_CONF_ERROR;
            }
        }

        if (module->create_loc_conf) {
            ctx->loc_conf[mi] = module->create_loc_conf(cf);
            if (ctx->loc_conf[mi] == NULL) {
                return NGX_CONF_ERROR;
            }
        }
    }

    pcf = *cf;
    cf->ctx = ctx;

    for (m = 0; cf->cycle->modules[m]; m++) {
        if (cf->cycle->modules[m]->type != NGX_HTTP_MODULE) {
            continue;
        }

        module = cf->cycle->modules[m]->ctx;

        if (module->preconfiguration) {
            if (module->preconfiguration(cf) != NGX_OK) {
                return NGX_CONF_ERROR;
            }
        }
    }

    /* parse inside the http{} block */

    cf->module_type = NGX_HTTP_MODULE;
    cf->cmd_type = NGX_HTTP_MAIN_CONF;
    rv = ngx_conf_parse(cf, NULL);

    if (rv != NGX_CONF_OK) {
        goto failed;
    }

    /*     * init http{} main_conf's, merge the server{}s' srv_conf's     * and its location{}s' loc_conf's     */

    cmcf = ctx->main_conf[ngx_http_core_module.ctx_index];
    cscfp = cmcf->servers.elts;

    for (m = 0; cf->cycle->modules[m]; m++) {
        if (cf->cycle->modules[m]->type != NGX_HTTP_MODULE) {
            continue;
        }

        module = cf->cycle->modules[m]->ctx;
        mi = cf->cycle->modules[m]->ctx_index;

        /* init http{} main_conf's */

        if (module->init_main_conf) {
            rv = module->init_main_conf(cf, ctx->main_conf[mi]);
            if (rv != NGX_CONF_OK) {
                goto failed;
            }
        }

        rv = ngx_http_merge_servers(cf, cmcf, module, mi);
        if (rv != NGX_CONF_OK) {
            goto failed;
        }
    }


    /* create location trees */

    for (s = 0; s < cmcf->servers.nelts; s++) {

        clcf = cscfp[s]->ctx->loc_conf[ngx_http_core_module.ctx_index];

        if (ngx_http_init_locations(cf, cscfp[s], clcf) != NGX_OK) {
            return NGX_CONF_ERROR;
        }

        if (ngx_http_init_static_location_trees(cf, clcf) != NGX_OK) {
            return NGX_CONF_ERROR;
        }
    }


    if (ngx_http_init_phases(cf, cmcf) != NGX_OK) {
        return NGX_CONF_ERROR;
    }

    if (ngx_http_init_headers_in_hash(cf, cmcf) != NGX_OK) {
        return NGX_CONF_ERROR;
    }


    for (m = 0; cf->cycle->modules[m]; m++) {
        if (cf->cycle->modules[m]->type != NGX_HTTP_MODULE) {
            continue;
        }

        module = cf->cycle->modules[m]->ctx;

        if (module->postconfiguration) {
            if (module->postconfiguration(cf) != NGX_OK) {
                return NGX_CONF_ERROR;
            }
        }
    }

    if (ngx_http_variables_init_vars(cf) != NGX_OK) {
        return NGX_CONF_ERROR;
    }

    /*     * http{}'s cf->ctx was needed while the configuration merging     * and in postconfiguration process     */

    *cf = pcf;


    if (ngx_http_init_phase_handlers(cf, cmcf) != NGX_OK) {
        return NGX_CONF_ERROR;
    }


    /* optimize the lists of ports, addresses and server names */

    if (ngx_http_optimize_servers(cf, cmcf, cmcf->ports) != NGX_OK) {
        return NGX_CONF_ERROR;
    }

    return NGX_CONF_OK;

failed:

    *cf = pcf;

    return rv;
}

ngx_http_optimize_servers里传入对应的端口启动http服务器。

static ngx_int_t ngx_http_optimize_servers(ngx_conf_t *cf, ngx_http_core_main_conf_t *cmcf, ngx_array_t *ports)
{
    ngx_uint_t             p, a;
    ngx_http_conf_port_t  *port;
    ngx_http_conf_addr_t  *addr;

    if (ports == NULL) {
        return NGX_OK;
    }

    port = ports->elts;
    for (p = 0; p < ports->nelts; p++) {

        ngx_sort(port[p].addrs.elts, (size_t) port[p].addrs.nelts,
                 sizeof(ngx_http_conf_addr_t), ngx_http_cmp_conf_addrs);

        /*         * check whether all name-based servers have the same         * configuration as a default server for given address:port         */

        addr = port[p].addrs.elts;
        for (a = 0; a < port[p].addrs.nelts; a++) {

            if (addr[a].servers.nelts > 1
#if (NGX_PCRE)                || addr[a].default_server->captures
#endif               )
            {
                if (ngx_http_server_names(cf, cmcf, &addr[a]) != NGX_OK) {
                    return NGX_ERROR;
                }
            }
        }

        if (ngx_http_init_listening(cf, &port[p]) != NGX_OK) {
            return NGX_ERROR;
        }
    }

    return NGX_OK;
}

在ngx_http_optimize_servers里调用ngx_http_init_listening函数监听。ngx_http_init_listening里面有一个while循环，根据端口调用ngx_http_add_listening进行监听。

static ngx_int_t
ngx_http_init_listening(ngx_conf_t *cf, ngx_http_conf_port_t *port)
{
    ngx_uint_t                 i, last, bind_wildcard;
    ngx_listening_t           *ls;
    ngx_http_port_t           *hport;
    ngx_http_conf_addr_t      *addr;

    addr = port->addrs.elts;
    last = port->addrs.nelts;

    /*     * If there is a binding to an "*:port" then we need to bind() to     * the "*:port" only and ignore other implicit bindings.  The bindings     * have been already sorted: explicit bindings are on the start, then     * implicit bindings go, and wildcard binding is in the end.     */

    if (addr[last - 1].opt.wildcard) {
        addr[last - 1].opt.bind = 1;
        bind_wildcard = 1;

    } else {
        bind_wildcard = 0;
    }

    i = 0;

    while (i < last) {

        if (bind_wildcard && !addr[i].opt.bind) {
            i++;
            continue;
        }

        ls = ngx_http_add_listening(cf, &addr[i]);
        if (ls == NULL) {
            return NGX_ERROR;
        }

        hport = ngx_pcalloc(cf->pool, sizeof(ngx_http_port_t));
        if (hport == NULL) {
            return NGX_ERROR;
        }

        ls->servers = hport;

        hport->naddrs = i + 1;

        switch (ls->sockaddr->sa_family) {

#if (NGX_HAVE_INET6)        case AF_INET6:
            if (ngx_http_add_addrs6(cf, hport, addr) != NGX_OK) {
                return NGX_ERROR;
            }
            break;
#endif        default: /* AF_INET */
            if (ngx_http_add_addrs(cf, hport, addr) != NGX_OK) {
                return NGX_ERROR;
            }
            break;
        }

        if (ngx_clone_listening(cf, ls) != NGX_OK) {
            return NGX_ERROR;
        }

        addr++;
        last--;
    }

    return NGX_OK;
}

ngx_http_add_listening里面调用ngx_create_listening传入IP地址创建listen fd进行监听。

static ngx_listening_t *
ngx_http_add_listening(ngx_conf_t *cf, ngx_http_conf_addr_t *addr)
{
    ngx_listening_t           *ls;
    ngx_http_core_loc_conf_t  *clcf;
    ngx_http_core_srv_conf_t  *cscf;

    ls = ngx_create_listening(cf, &addr->opt.sockaddr.sockaddr,
                              addr->opt.socklen);
    if (ls == NULL) {
        return NULL;
    }

    ls->addr_ntop = 1;

    ls->handler = ngx_http_init_connection;

    cscf = addr->default_server;
    ls->pool_size = cscf->connection_pool_size;
    ls->post_accept_timeout = cscf->client_header_timeout;

    clcf = cscf->ctx->loc_conf[ngx_http_core_module.ctx_index];

    ls->logp = clcf->error_log;
    ls->log.data = &ls->addr_text;
    ls->log.handler = ngx_accept_log_error;

#if (NGX_WIN32)    {
    ngx_iocp_conf_t  *iocpcf = NULL;

    if (ngx_get_conf(cf->cycle->conf_ctx, ngx_events_module)) {
        iocpcf = ngx_event_get_conf(cf->cycle->conf_ctx, ngx_iocp_module);
    }
    if (iocpcf && iocpcf->acceptex_read) {
        ls->post_accept_buffer_size = cscf->client_header_buffer_size;
    }
    }
#endif
    ls->backlog = addr->opt.backlog;
    ls->rcvbuf = addr->opt.rcvbuf;
    ls->sndbuf = addr->opt.sndbuf;

    ls->keepalive = addr->opt.so_keepalive;
#if (NGX_HAVE_KEEPALIVE_TUNABLE)    ls->keepidle = addr->opt.tcp_keepidle;
    ls->keepintvl = addr->opt.tcp_keepintvl;
    ls->keepcnt = addr->opt.tcp_keepcnt;
#endif
#if (NGX_HAVE_DEFERRED_ACCEPT && defined SO_ACCEPTFILTER)    ls->accept_filter = addr->opt.accept_filter;
#endif
#if (NGX_HAVE_DEFERRED_ACCEPT && defined TCP_DEFER_ACCEPT)    ls->deferred_accept = addr->opt.deferred_accept;
#endif
#if (NGX_HAVE_INET6)    ls->ipv6only = addr->opt.ipv6only;
#endif
#if (NGX_HAVE_SETFIB)    ls->setfib = addr->opt.setfib;
#endif
#if (NGX_HAVE_TCP_FASTOPEN)    ls->fastopen = addr->opt.fastopen;
#endif
#if (NGX_HAVE_REUSEPORT)    ls->reuseport = addr->opt.reuseport;
#endif
    return ls;
}

http状态机描述定义（/src/http/ngx_http_core_module.h）：

typedef enum {
    NGX_HTTP_POST_READ_PHASE = 0,

    NGX_HTTP_SERVER_REWRITE_PHASE,

    NGX_HTTP_FIND_CONFIG_PHASE,
    NGX_HTTP_REWRITE_PHASE,
    NGX_HTTP_POST_REWRITE_PHASE,

    NGX_HTTP_PREACCESS_PHASE,

    NGX_HTTP_ACCESS_PHASE,
    NGX_HTTP_POST_ACCESS_PHASE,

    NGX_HTTP_PRECONTENT_PHASE,

    NGX_HTTP_CONTENT_PHASE,

    NGX_HTTP_LOG_PHASE
} ngx_http_phases;

Nginx的HTTP模块不是一开始就启动的，它得先解析你的配置文件，看到

这个模块才会开始加载。  你可以理解成，Nginx启动的时候先打个底，只有看到

了，它才开始搭HTTP服务这个“台子”。Nginx启动后会进入一个“状态机”模式，按顺序执行各种阶段，比如请求读取、配置查找、访问控制、内容生成等等，一步一步处理你的请求。

Nginx的HTTP服务启动，就像搭积木一样，一步一步来，先搭框架，再监听端口，最后按流程处理请求。 

解读Nginx：深入剖析HTTP启动流程

Nginx的HTTP服务并非启动即运行，而是在解析配置文件时遇到http模块才开始初始化。通过ngx_module_t定义模块信息，ngx_http_block执行初始化，包括创建上下文、模块配置。简而言之，Nginx的HTTP服务是按需加载，逐步初始化，最终按流程处理用户请求。

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解读Nginx：深入剖析HTTP启动流程

解读Nginx：深入剖析HTTP启动流程

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