Question on Empty GIF module

[Maxim Dounin]

Hello!

On Fri, Apr 25, 2008 at 07:54:48AM -0700, Rt Ibmer wrote:

>Is it possible to have the post_action be load balanced across a 
>set of backend servers (like the way proxy_pass works) or will it 
>only work with one specific server?  If that later is the case, 

You may specify any uri for proxy_pass, and it will be served almost 
as regular request (with the exception that results won't be 
served to client).  So you may do everything you want with it - 
including proxy_pass'ing to load-balanced cluster of backends.

>>The only disadvantage of post_action as of current implementation 
>>is that it blocks connection, i.e. following keep-alive request 
>>won't be processed until post_action terminates.
>
>Can you please explain the implication of this in some more 
>detail, as I am not very familiar with the details on how all the 
>connection stuff works under the hood?
>
>For instance, lets say that the server handling the post_action 
>takes 500ms to execute before returning the 204 No Content 
>response. What connection is actually being blocked during this 
>period? The browser that originated the request has been satisfied 
>immediately by empty_gif so I know it doesn't see this delay, but 
>what does?

The problem only manifest itself with keep-alive connections.  If 
browser sends requests, gets response, then sends next request 
within the same connection - this next request processing will be 
delayed.

>Perhaps what you are saying is that no matter how many requests 
>come in at the same time that are destined for post_action, that 
>nginx can only process those one at a time? So that my app doing 
[...]

No.

>At any rate, how difficult of a change is it to make post_action 
>so that it does not block, and are there any near-term plans for 
>this?

I'm not aware of such plans.

>>Docs for post_action doesn't exist (yet).  There are some relevant 
>>russian mailing list posts, but I can't recall anything in 
>>English.
>
>Are there any optional parameters or attributes that can be 
>specified with the post_action I should know about?

No, it takes only one parameter.  As of 0.6.27 this may be named 
location, too.

>>And of course really best way do things is to write logs and just process them as needed.
>
>Depending on how this all works out, it may be worthwhile down 
>the road to rewrite our app to process off the log files. Am I 
>correct to assume that I can use an access_log directive inside 
>the area where empty_gif is used, so that these specific gif 
>requests are logged to their own log file separate from the other 
>types of requests that nginx is handling?

Yes.

Maxim Dounin