I've got a few big web based multi-tenant products now, and very soon I can see that there will be a lot of customizations that are tenant specific.

An extra field here or there, maybe an extra page or some extra logic in the middle of a workflow - that sort of thing.

Some of these customizations can be rolled into the core product, and that's great. Some of them are highly specific and would get in everyone else's way.

I have a few ideas in mind for managing this, but none of them seem to scale well. The obvious solution is to introduce a ton of client-level settings, allowing various 'features' to be enabled on per-client basis. The downside with that, of course, is massive complexity and clutter. You could introduce a truly huge number of settings, and over time various types of logic (presentation, business) could get way out of hand. Then there's the problem of client-specific fields, which begs for something cleaner than just adding a bunch of nullable fields to the existing tables.

So what are people doing to manage this? Force.com seems to be the master of extensibility; obviously they've created a platform from the ground up that is super extensible. You can add on to almost anything with their web-based UI. FogBugz did something similiar where they created a robust plugin model that, come to think of it, might have actually been inspired by Force. I know they spent a lot of time and money on it and if I'm not mistaken the intention was to actually use it internally for future product development.

Sounds like the kind of thing I could be tempted to build but probably shouldn't. :)

Is a massive investment in pluggable architecture the only way to go? How are you managing these problems, and what kind of results are you seeing?

EDIT: It does look as though FogBugz handled the problem by building a fairly robust platform and then using that to put together their screens. To extend it you create a DLL containing classes that implement interfaces like ISearchScreenGridColumn, and that becomes a module. I'm sure it was tremendously expensive to build considering that they have a large of devs and they worked on it for months, plus their surface area is perhaps 5% of the size of my application.

Right now I am seriously wondering if Force.com is the right way to handle this. And I am a hard core ASP.Net guy, so this is a strange position to find myself in.