Files are static, requests are dynamic

The transition from desktop or web applications to mobile/cloud systems is more than the re-organization of programs. It is a change from data sources: desktop and web applications often store data in files, and mobile/cloud systems store data via web services.

Files are static things. They perform no actions by themselves. A program can read the contents of a file and take action on those contents, but it must consume the contents as they exist. The file may contain just the data that a program needs, or it may contain more, or less. For example, a file containing a Microsoft Word document actually contains the text of the document, revisions to the text, information about fonts and formatting, and meta-information about the author.

A program reading the contents of that file must read all of that information; it has no choice. If the task is to extract the text -- and only the text -- the program must read the entire file, revisions and fonts and meta-information included. If we want only the meta-information, the program must read the entire file, text and revisions... you get the idea.

(The more recent DOCX format does isolate the different sets of information, and makes reading a subset of the file easier. The older DOC format required reading and interpreting the entire file to obtain any part of the file.)

Web services, used by mobile/cloud systems, are not static but dynamic. (At least they have the possibility of being dynamic. You can build web services that mimic files, but you probably want the dynamic versions.)

A web service can be dynamic because there is another program processing the request and creating the response. A web service to read a document can do more than simply return the bytes in the document file. It can perform some processing on your behalf. It can accept instructions, such as "give me the text" or "only the meta-information, please". It can do these things on our behalf. We can delegate them to the web service, and our job becomes easier.

Astute readers will observe that my arrangement of dynamic web services does not reduce the work involved, it merely shifts work to different parts of the system. (The web service must still read the entire document, pick out the bits of interest, and send those to us.) That is true. Yet it is also true that once in place, the web service provide an interface to reading (and writing) documents, and we may then choose to change the implementation of that storage.

With document web services in place, our applications are completely ignorant of the storage format and the web services may change that format to suit their needs. A new version of web services may store documents not in files but in databases, or JSON files, or any method appropriate. I'm pretty sure that Google Docs uses this approach, and I suspect Microsoft's Office 365, if not using it now, will use it soon.

Moving from desktop and web to mobile/cloud lets us do many things. It lets us do many things that we do today but differently. Look at the possibilities, and look at the savings in effort and cost.