Data Stitching

If you collect some data with respect to a specific context, when there are significant changes in that context, the original data no longer fits seamlessly with any newly collected data.

One way to handle this is to change all of the old data, updating it to the new changes. This works, but it could take quite a while in production and can be extremely risky. A change in massive data can take days or even weeks to run. A small mistake could effectively destroy the old data. 

The other way to handle this is at ‘presentation’. You split the computations, doing something slightly different for the older data.

All you need is a precise ‘line’ between them to test for. Then, to keep the code sane, you make both sides polymorphic and hide the test. When some code asks for the final data, if that request crosses the line, it gets handled correctly.

The way you don’t want to do this is by hardcoding the test somewhere high. Doing this once may not cause too much damage, but if the context has changed once already, it is highly likely to change again. A few times is really ugly; more than that is a classic bug sinkhole.

The overall idea is to always preserve what you collect and stitch it together with polymorphism. Then you can double-check it for correctness, so you have the best possible options for getting it right, and you don’t leave around landmines for later efforts.

For anyone worried about performance, presentations are almost always fixed-sized small data sets; the extra costs of the tests and the polymorphism are usually hardly noticeable. If there is some other type of dynamic large-scale bulk interface over the data, then it’s an architectural problem, not a data one. That is, if the performance is really noticeable, then you’re probably doing something else in the software that you shouldn’t be doing.

Data stitching is often avoided, but when done reasonably, it ensures that the data appears correctly and that it is straightforward to fix later if it doesn’t. Depending on the data, it often requires a domain expert to verify, which usually happens very late in the effort, so it's best to expect last-minute changes.