Designing an OSS from NFRs backwards

When we’re preparing a design (or capturing requirements) for a new or updated OSS, I suspect most of us design with functional requirements (FRs) in mind. That is, our first line of thinking is on the shiny new features or system behaviours we have to implement.

But what if we were to flip this completely? What if we were to design against Non-Functional Requirements (NFRs) instead? [In case you’re not familiar with NFRs, they’re the requirements that measure the function or performance of a solution rather than features / behaviours]

What if we already have all the really important functionality in our OSS (the 80/20 rule suggests you will), but those functions are just really inefficient to use? What if we can meet the FR of searching a database for a piece of inventory… but our loaded system takes 5 mins to return the results of the query? It doesn’t sound much, but if it’s an important task that you’re doing dozens of times a day, then you’re wasting hours each day. Worse still, if it’s a system task that needs to run hundreds of times a day…

I personally find NFRs to be really hard to design for because we usually won’t know response times until we’ve actually built the functionality and tried different load / fail-over / pattern (eg different query types) models on the available infrastructure. Yes, we can benchmark, but that tends to be a bit speculative.

Unfortunately, if we’ve built a solution that works, but end up with queries that take minutes… when our SLAs might be 5-15 mins, then we’ve possibly failed in our design role.

We can claim that it’s not our fault. We only have finite infrastructure (eg compute, storage, network), each with inherent performance constraints. It is what it is right?…. maybe.

What if we took the perspective of determining our most important features (the 80/20 rule again), setting NFR benchmarks for each and then designing the solution back from there? That is, putting effort into making our most important features super-efficient rather than adding new nice-to-have features (features that will increase load, thus making NFRs harder to hit mind you!)?

In this new world of open-source, we have more “product control” than we’ve probably had before. This gives us more of a chance to start with the non-functionals and work back towards a product. An example might be redesigning our inventory to work with Graph database technology rather than the existing relational databases.

How feasible is this NFR concept? Do you know anyone in OSS who does it this way? Do you have any clever tricks for ensuring your developed features stay within NFR targets?

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