Tuesday, December 29, 2009

nginx - Module Documentation - Some notes

I was going through the  nginx module development guide.  While I was going through the example module code, I did not understand how the module registers itself to the nginx infrastructure.  For example,  the upstream module has following structure initializations.

static ngx_command_t  ngx_http_upstream_commands[] = {

    { ngx_string("upstream"),
      NULL },

    { ngx_string("server"),
      NULL },


static ngx_http_module_t  ngx_http_upstream_module_ctx = {
    ngx_http_upstream_add_variables,       /* preconfiguration */
    NULL,                                  /* postconfiguration */

    ngx_http_upstream_create_main_conf,    /* create main configuration */
    ngx_http_upstream_init_main_conf,      /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    NULL,                                  /* create location configuration */
    NULL                                   /* merge location configuration */

ngx_module_t  ngx_http_upstream_module = {
    &ngx_http_upstream_module_ctx,         /* module context */
    ngx_http_upstream_commands,            /* module directives */
    NGX_HTTP_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_http_upstream_commands and ngx_http_upstream_module_ctx are referred in ngx_http_upstream_module.   So far so good.  I expected ngx_http_upstream_module to be referred in some other structure or I expected this to be registered with infrastructure via some API function.  I did not find it being referred anywhere.

Finally I figured out that ./configure shell script with the help of  /auto/modules shell script  is creating a C file called ngx_module.c.    See this shell script bit in /auto/modules file.

cat << END                                    > $NGX_MODULES_C




for mod in $modules
    echo "extern ngx_module_t  $mod;"         >> $NGX_MODULES_C

echo                                          >> $NGX_MODULES_C
echo 'ngx_module_t *ngx_modules[] = {'        >> $NGX_MODULES_C

for mod in $modules
    echo "    &$mod,"                         >> $NGX_MODULES_C

cat << END                                    >> $NGX_MODULES_C


Above shell script is creating C file with array of all module structure variables.  When I executed ./configure, content of the file looked like this. Since this file is big, I have taken few lines out of this file to illustrate the output.

extern ngx_module_t  ngx_core_module;
extern ngx_module_t  ngx_errlog_module;
extern ngx_module_t  ngx_conf_module;
extern ngx_module_t  ngx_events_module;
extern ngx_module_t  ngx_event_core_module;
extern ngx_module_t  ngx_select_module;
extern ngx_module_t  ngx_poll_module;
extern ngx_module_t  ngx_http_module;
extern ngx_module_t  ngx_http_core_module;
extern ngx_module_t  ngx_http_upstream_module;
extern ngx_module_t  ngx_http_static_module;
extern ngx_module_t  ngx_http_autoindex_module;

ngx_module_t *ngx_modules[] = {


Core module seems to be using ngx_modules array to get hold of different function pointers of modules to set the configuration and other operations.

It appears from the source code that there is no dynamic addition of modules.  ./configure script is only method available of module registration.    I feel this is one drawback nginx has with respect to Apache in my view.  Ofcourse, nginx has several advantages such as:
  •  Even driven model:  That is one thread can handle multiple connections simultaneously.
  •  Multiple threads to take advantage of multiple processors.
  •  Usage of 'epoll' mechanism. 
Anyway, I thought I would write this notes for people who also may wonder while going through the documentation  on how module gets registered with nginx infrastructure.  Hope it helps.

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