I have a PHP webserver and a .NET mobile application. The .NET application needs data from a database, which is provided (for now) by the php webserver. I'm fairly new to this kind of scenario so I'm not sure what the best practices are. I ran into a couple of problems and I am not certain how to overcome them.

For now, I have the following setup. I run a PHP SOAP server which has a couple of operations which simply retrieve data from the database. For this web service I have created a WSDL file. In Visual Studio I added a web reference to my project using the WSDL file and it generated some classes for it. I simply call something like "MyWebService.GetItems();" and I get an array of items in my .NET application, which come straight from the database. Next I also serialize all these retrieved objects to local (permanent) storage.

I face a couple of challenges which I don't know how to resolve properly.

• The idea is for the mobile client to synchronize the data once (at the start of the day), before working, and then use the local storage throughout the day, and synchronize it back at the end of the day.

• At the moment all data is downloaded through SOAP, and not a subset (only what is needed). How would I know which new information should be sent to the client? Only the server knows what is new, but only the client knows for sure which data it already has.

• I seem to be doing double work now. The data which is transferred with SOAP basically already are serialized objects. But at the moment I first retrieve all objects through SOAP and the .NET framework automatically deserializes it. Then I serialize all data again myself. It would be more efficient to simply download it to storage, and then deserialize it.

• The mobile device does not have a lot of memory. In my test data this is not really a problem, but with potentially tenths of thousands of records I can imagine this will become a problem. Currently when I call the SOAP method, it loads all data into memory (as an array) but as described above perhaps it would be better to have it stored into storage directly, and only retrieve from storage the objects that are needed. But how would I store this? An array would serialize to one big XML (or binary) file from which I cannot choose which objects to load. Would I make (possible tenths of thousands) separate files? Also at the end of the day when I want to send the changes back to the server, how would I know which objects to send... since they're not in memory?

I hope my troubles are clear and I hope you are able to help me figure out how to implement this the best way. :)

Some things are already fixed (like using .NET on the mobile device, using PHP and having a MySQL database server) but other things can most certainly be changed (like using SOAP).