I came across a new concept in my travels this week, an invitation with compulsory acceptance.

Is an invitation that is compulsory to accept an invitation at all? Surely most people would see it as nothing more than a dressed up way of demanding attendance? Surely such “garnish” (Ben Elton fan’s will know exactly what I’m talking about) just engenders bad feeling in many recipients?

Meanwhile in the world of systems…….

We often do the equivalent of compulsory invitations in the way we build software - consider for example the whole dependency injection thing.

In many cases we build some object via a constructor that expects to be injected with a whole heap of other things. How do we guarantee all those things will attend and remain present for the lifetime of our object? After all without these things our object cannot perform it’s intended task. About the only way this works is by insisting that all these things exist within the same JVM as the object to be injected - then they all either exist or don’t exist.

Clearly, this doesn’t work so well in distributed systems where achieving the same availability assurance is considerably more difficult. A slightly more subtle issue is that if our systems are written in the above manner, upgrades are also more difficult because we must manage a dependency tree. e.g. We wish to upgrade System A and it has dependants, Systems B and C both of which will need to be taken off-line whilst we upgrade A.

What is required is a more dynamic form of injection, something that can be reinitialised or changed which at least invalidates injection via constructors (well, unless we throw the whole object out and rebuild it from scratch). In fact, we probably require some kind of event-based solution such that systems can get liveness information about the systems they rely upon and take appropriate action (which will likely include dealing with in flight operations).

Technorati Tags: architecture, design, distributed systems, philosophy