Should I be worried about HTTP overhead when calling local web services?

Question

I'm currently re-developing a fairly large-scale PHP web application. Part of this redevelopment involves moving the bulk of some fairly hefty business logic out of the core of the web app and moving it into a set of SOAP web services.

What's currently concerning me (just slightly) is perceived overhead this brings with it in terms of local HTTP traffic. I should explain that the SOAP web services currently, and for the foreseeable future, will reside on the same physical server, and if and when they move they will remain on the same network. What concerns me is the fact that for each call that was an internal php function call, it is now an http request invoking a similar function call.

Obviously this is something I can measure as we move further along the line, but I was wondering if anyone could offer any advice, or more importantly share any previous experience of taking an application down this route.

Answer 1

Are you doing hundreds or thousands of these calls a second? If not, then you probably don't have to worry.

But profile it. Set up a prototype of the system working across the network with a large number of SOAP calls, and see if it slows down to unacceptable levels.

Answer 2

If the server is running on the same physical box then you can't have any privilege seperation. And increasing capacity by adding tiers to the stack (instead of euivalent nodes) is a recipe for non-availability.

If you're hiding something behind SOAP, the the HTTP overhead is likely to be relatively small in comparison to what the 'something' is doing (e.g. reading from database). However when you add up the cost of constructing the SOAP request, decomposing the soap request the compositing the response, add the overhead for HTTP then one has to wonder why you don't provide a shortcut by calling the PHP code implemented within the server directly in the same thread of execution.

If you were to design an abstract soap interface which mapped directly to and from php types then this could be shortcutted without having any overhead in maintining different APIs.