Accessing subrequest output in memory

Eugaia ngx.eugaia at
Sat Oct 16 18:49:47 MSD 2010

  Hi Peter,

Thanks.  What Yichun is referring to there is how echo issues 
subrequests, however I was just testing with the basic 'echo' commands.  
The thing is that no special handling of data is made when it is 'in 
memory'.  This appears to be a *feature* of Nginx - the in-memory 
handling needs to be done by specific modules, and currently the only 
one that seems to do this properly is the upstream module.  The FastCGI 
and WSGI modules explicitly say that they don't work with 
r->subrequest_in_memory set.

I think the best solution appears to be to create an output filter, then 
save the output chain to a module-specific context struct.  This should 
then work properly with all modules, and the in-memory flag can be 
removed so that it will work with FastCGI, WSGI etc too.



On 16/10/2010 10:29, Peter Leonov wrote:
> Hi Marcus,
> On 16.10.2010, at 4:44, Eugaia <ngx.eugaia at> wrote:
>> Hi again,
>> I've found the output that the module I'm trying to get this to work 
>> with (ngx_http_echo_module) sent the content directly to the parent 
>> request.  I'm not sure if this is as a result of a generic feature, 
>> or something that should be fixed in echo - which I'll look into.
> AFAICT, this is by desing. Yichun wrote about it in nearest past:
>> Cheers,
>> Marcus.
>> On 15/10/2010 20:59, Eugaia wrote:
>>> Hi,
>>> I'm trying to get the output of an in-memory subrequest, but I can't 
>>> find where the output is stored.
>>> Basically, I'm doing :
>>> ...
>>> ngx_uint_t flags;
>>> ngx_http_post_subrequest_t *psr;
>>> ngx_str_t location;
>>> ngx_str_t *url_args;
>>> psr->handler = my_callback;
>>> ngx_http_subrequest (r, &location, url_args, &sr, psr, flags);
>>> ...
>>> // waiting for for subrequest to process
>>> my_callback (ngx_http_request_t *sr, void *data, ngx_int_t rc)
>>> {
>>>    ngx_http_request_t *r;
>>>    r = sr->parent;
>>>    ...
>>>    // I want to be able to access the (body) output from the 
>>> subrequest here
>>> }
>>> If the subrequest uses an upstream, I know I can access the output 
>>> using sr->upstream->buffer.  However, if the subrequest does not use 
>>> upstreams, then this won't work.  The output is not available from 
>>> sr->out - r->out is set to NULL at some point during the output 
>>> filtering.  As far as I can make out, what I need to do is find 
>>> where the body output ngx_chain_t * struct is saved after all the 
>>> body filtering has been processed, but I'm struggling to find it.
>>> Can anyone tell me where it is, or where I can find the output?
>>> Note : I don't want to remove the in-memory flag because I don't 
>>> want the output flushed to the client.
>>> Thanks,
>>> Marcus.
> Best regards,
> Peter.
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