Hello!

On Wed, Jun 16, 2010 at 02:29:43AM -0700, Mark Harrison wrote:

> I'd like to convert an apache module I've written to work
> under nginx. Can somebody loan me a clue for getting started?

It's probably good idea to start from here:

http://nginx.org/en/links.html
http://wiki.nginx.org/Nginx3rdPartyModules
http://wiki.nginx.org/NginxResources#Developer_resources

Note well: in most cases it's good idea to rewrite module from
scratch instead of trying to adopt something written for Apache.

> Background
> ----------
>
> The logic is pretty simple:
>
> 1. based on the input URI, determine what file will be
> sent. this will possibly involved a (cached) db lookup.
>
> 2a. set these headers:
>
> Content-disposition:
> ETag:
> X-mod_asset-version: (my custom header)
>
> 2b. set mime type.
>
> 3. allow nginx to send the bytes of the file. Hopefully
> it will handle these headers:
>
> Last-Modified:
> Content-Length:
> Accept-Ranges:
> Content-Range:
> Content-Length:

Problematic part is "db lookup". In nginx you have to do it in
non-blocking way, and this isn't something readily available in
database client libraries. You may want to deligate this part to
some backend or already available modules like ngx_postgres
(http://wiki.nginx.org/Nginx3rdPartyModules#Postgres_Module).

> Questions
> ---------
>
> For 2b, this is the proper way?
>
> r->headers_out.content_type.len = sizeof("image/gif") - 1;
> r->headers_out.content_type.data = (u_char *) "image/gif";

No.

For 0.7.* use:

r->headers_out.content_type_len = sizeof("image/gif") - 1;
r->headers_out.content_type.len = sizeof("image/gif") - 1;
r->headers_out.content_type.data = (u_char *) "image/gif";

For 0.8.37+ use ngx_str_set() macro to set string, i.e.

r->headers_out.content_type_len = sizeof("image/gif") - 1;
ngx_str_set(&r->headers_out.content_type, "image/gif");

> Is there a special way to malloc the .data so that it
> will be freed automatically when the request is complete?

In the above case you don't need to allocate anything as it uses
constants strings. For dynamic allocations use
ngx_pool_alloc(r->pool).

> For 3, should I be using X-Accel-Redirect?
>
> add_header("X-Accel-Redirect: /protected/iso.img");
>
> If so, then I can simply set my headers and pass along to the next
> phase?

No. X-Accel-Redirect is special header understood by upstream
module (proxy, fastcgi) in backend responses. In your own module
you have to use ngx_http_internal_redirect() function for this.

> Any simple example modules I can use as a starting point?

For simple content generation see ngx_http_empty_gif_module.c,
ngx_http_static_module.c, ngx_http_flv_module.c.

For simple examples of internal redirect usage see
ngx_http_index_module.c, ngx_http_random_index_module.c.

Maxim Dounin

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