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wsgi, ajax, and multiple requests

Frank Millman frank at chagford.com
Thu Feb 10 02:30:10 EST 2011 
Frank Millman wrote:

> Hi all
>
> I have a question about wsgi.
>
> As far as I can tell from the spec and from the reference implementation, 
> wsgi requires that each request from the client is responded to by one or 
> more headers, followed by the response body. It is then ready to handle 
> the next request.
>
> 99% of the time this is fine, but I have a situation where I need to vary 
> this, and I cannot see how to do it. Here is the scenario.
>
> I am dabbling with writing an ajax-style app. On occasion, I want to send 
> more than one message from the client to the server. It is important that 
> the server processes the messages in the same order that they are 
> generated. I have found that the Safari browser does not always send them 
> in the correct sequence.
>
> I found an article on the subject -
>
> http://www.sitepen.com/blog/2009/02/27/deterministic-clientserver-interaction/
>
> It explains that you can never guarantee that the messages will arrive in 
> the same order, with *any* browser. Maybe I was lucky with the other ones. 
> It goes on to say that, if the order is important, you must include a 
> header with a sequential counter, and the server must check the sequence. 
> If received out of sequence, it must queue the one received, wait for the 
> correct one, and then process them both in the correct order.
>
> So the question is, how can I achieve this with wsgi? If I detect that I 
> have received a request out of sequence, I can queue the request, and send 
> an empty response. When I receive the correct request, I can process it, 
> call start_response(), and return the response body, but I cannot figure 
> out how to get back to process the queued request.
>

As both client and server are under my control, I have realised that there 
is a simple solution.

On the client side, if there is more than one message to be sent, I pack 
them into an array, and send them in one request. The server receives the 
request, unpacks the array, and processes each message in turn.

It should work reliably, and I save a round-trip into the bargain.

Thanks for the replies - they all help in getting the little grey cells 
working.

Frank
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