FastCGI multiplexing?

Manlio Perillo manlio_perillo at
Mon Jan 14 19:54:53 MSK 2008

Igor Clark ha scritto:
> On 14 Jan 2008, at 13:46, Manlio Perillo wrote:
>> Igor Clark ha scritto:
>>> Hi Igor and everyone,
>>> I'm toying with some ideas for a multithreaded FastCGI application, 
>>> intended to work with nginx.
>>> Seems that FCGI request/connection multiplexing would be an extremely 
>>> valuable feature.
>>> I can't find any reference to this on the wiki, in the mailing list 
>>> or on Google - I assume this is because nginx, like Apache, doesn't 
>>> support FastCGI multiplexing. Is this correct? If so, are there any 
>>> plans to implement it at some point in the future?
>> As far as I know the multiplexing support in FastCGI is broken "by 
>> design".
>> Read this response one of the authors of Twisted gave me about FastCGI 
>> (among other questions):
> Thanks Manlio, that's very interesting. Lack of flow control in the 
> protocol is obviously an issue for multiplexing; now that it's been 
> pointed out, it seems bizarre that it should have been missed out. One 
> wonders if the intention was for the application to send an HTTP 503 
> over the FCGI connection in the event of overloading? 

Maybe they just though that overflow is not possible, who knows.
TCP, as an example, has some form of flow control (but usually FastCGI 
uses an UDP connection)

> I guess this would 
> require a web server module to back off from overloaded application 
> instances based on their HTTP status code - which seems like trying to 
> patch up the shortcomings of the transport in the application.
> It's a shame; it seemed that removing all the TCP overhead between the 
> web server and the application server would be a good thing, but perhaps 
> FCGI just isn't the way. 

But with FCGI you can just execute one request at a time, using a 
persistent connection.

The problems, with nginx, are:
1) the upstream module does not supports persistent connections.
    A new connection is created for every request.
    In fact the http proxy supports HTTP 1.0 with the backend.
2) nginx does not supports some forms of connections queue with upstream
    This means that if nginx is handling 500 connections, then it will
    make 500 concurrent connections to the upstream server, and likely
    the upstream server (usually "a toy") will not be able to handle this

Fixing this will require a redesign of the upstream module.

> I'm still just researching the area at the 
> moment though so any further thoughts or experiences would be very welcome.
> Is there any plan to implement HTTP/1.1 & keepalive connections in 
> nginx's conversations with upstream servers? Can't see anything in the 
> wiki or feature request list.

Igor Sysoev has expressed his intentions to add support for persistent 
upstream connections, try searching the mailing list archives.

> Cheers,
> Igor
> -- 
> Igor Clark // POKE // 10 Redchurch Street // E2 7DD // +44 (0)20 7749 
> 5355 //

Manlio Perillo

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