Connection statuses

[Maxim Dounin]

Hello!

On Wed, Feb 18, 2015 at 09:25:01PM -0500, K Young wrote:

> Hello, I've been reading through nginx source to understand the metrics
> <http://nginx.org/en/docs/http/ngx_http_stub_status_module.html> that are
> reported. I've got an idea of what is happening, but since the flow is
> asynchronous, I'm not 100% confident, and would love your feedback.
> 
> Is the following correct?
> 
>    - 'accepts' is incremented when a worker gets a new request for a
>    connection from the OS
>    - 'handled' is incremented when a worker gets a new connection for a
>    request
> 
> 
> And then once a connection is opened:
> 
>    - 'active' is incremented (actually, it is incremented right before
>    'handled', but will be decremented if the worker doesn't handle the
>    connection request).
>    - then the connection briefly goes into a waiting state while the
>    request is waiting to be processed
>    - then the connection goes into a short reading state while request
>    headers are read. Simultaneously, 'request' is incremented every time a new
>    request header begins to be read.
>    - then the connection goes into a writing state while all work is done
>    by nginx and by upstream components
>    - then if the connection will be kept alive, it goes back into waiting
>    state, which is synonymous with 'idle'
>    - finally when the connection is closed, active is decremented
> 
> The things I'm least certain of about the 'waiting' state:
> 
>    - Does active always sum up to waiting+reading+writing?

Generally yes.  There are nuances with SPDY where additional 
connections are emulated for SPDY streams being handled, and 
waiting+reading+writing may be bigger than active.

>    - Does each new connection enter a waiting state just before it goes
>    into the reading state?

No.  If there are data available right after connect (this usually 
happens with accept filters / deferred accept), nginx will not 
mark a newly accepted connection as waiting, and will start 
reading a request directly.

>    - While waiting for upstream responses, is the connection in writing
>    state or waiting state?

Writing.

>    - While waiting for new client requests on an open connection, is the
>    connection in a waiting state?

Yes.

> Is the attached sketch of the above connection states correct? (underneath
> "READ" it says "request++" to indicate that this is where the request
> counter gets incremented).
> [image: Inline image 1]

Mostly, see above.

-- 
Maxim Dounin
http://nginx.org/