Migrating from desktop to cloud is no small deal

The popularity of smart phones and tablets has made mobile (and its server-side colleague cloud) the new darling technology. They are the new buzzwords that garner attention -- and probably funding. They get attention for new apps, and some folks have been migrating existing desktop and web apps.

If you are embarking on such a project, I encourage you to look into three technologies: mobile devices, cloud computing, and functional programming.

Functional programming is a specific way of designing programs, much like structured programming and object-oriented programming. But in contrast to those techniques, functional programming guides one to smaller units of code. Structured programming and object-oriented programming allow one to build small units, but do not encourage it. As a result, most (non-homework) code built with structured and object-oriented techniques contains collections of large modules and objects.

Cloud computing is a specific way of designing systems out of smaller components, many of which are part of the cloud infrastructure. Cloud-based applications use data stores, message queues, and a measured amount of code. (Previous architectures such as desktop and web often saw applications made of whole cloth with little use of pre-made components.)

Mobile devices force a split of code between the server (typically a cloud-based server) and the user interface on the device. The device usually performs some processing with most occurring on the server. (A few apps perform all processing on the device, such as in the game "Angry Birds". But they are exceptions.) Splitting the code between the device and the server forces you to build not a single application but two programs that communicate.

The new technology set encourages us to build systems of smaller programs. Mobile/cloud is for collaborating programs, not monolithic ones. Functional programming techniques reward small programs and penalize large ones.

This "impedance mismatch" between mobile/cloud and desktop applications (and to a somewhat lesser extent, web applications) means that porting to mobile/cloud is difficult. The legacy systems on the earlier platforms were built to take advantage of the strengths of the platform, and small, collaborating programs were not efficient.

The mismatch between object-oriented systems and functional programming is even greater. Object-oriented programming is often about objects holding state -- mutable state, so that objects change over time -- and functional programming is about immutable objects.

A few well-designed (from the mobile/cloud perspective) applications for the desktop and web can be migrated to mobile/cloud. Most, I think, will be difficult. Some may be close to impossible.

My advice: Build some new apps for mobile/cloud, to gain experience with the new design paradigm. Try out the functional programming languages (or at least, your favorite object-oriented language is with immutable objects). Once you have that experience, evaluate your legacy apps and select a small number (perhaps two) of "easy" apps for migration. I'm sure that the exercise will give you greater insight into the effort for the other (more difficult) legacy apps.

Then, armed with experience of the new technologies and a good understanding of your code base, should you migrate from desktop to mobile/cloud.