'Modernizing Legacy Code in Production ' legacycode legacy
I spent over a decade as a consultant working for dozens of companies in many fields and pursuits. The diversity of each codebase is tremendous. This article will try to define general rules for modernizing legacy code that would hopefully apply to all. But it comes from the angle of a Java developer.
We shouldn’t take a decision to overhaul existing code that’s in production lightly. You need to create a plan, evaluate the risks and have a way to back out. It’s very common to expose internal calls in the legacy system to enable smooth migration. For E.g. we can provide fallback capabilities by checking against the legacy system. An old product I worked on had a custom in-house authentication. To keep compatibility during migration, we used a dedicated web service. If user authentication failed on the current server, the system checked against the old server to provide a “seamless” experience.
This is probably the biggest problem/challenge you can face. Sometimes an external contractor with domain expertise can help here. If this is the case, it’s worth every penny! Discovering the features in the system and the ones that are “actually used” is problematic. Especially when we want to focus on minutiae. We want every small detail. This isn’t always possible but if you can use observability tools to indicate what’s used it would help very much. Migrating something that isn’t used is frustrating and we’d want to avoid it, if possible.
We need feature extraction for compliance. We need to go over every feature and behavior of the legacy system and write a generic test that verifies this behavior. This is important both to verify our understanding of the code and that the documentation is indeed correct. I also suggest using a completely separate “thin” adapter for the UI differences. The tests themselves must be identical when running against the legacy and the current codebase.
What we don’t know is whether the new system can handle the peak load througput we require. We need to extract these details and create stress tests for the critical portions of the system. We need to verify performance, ideally compare it to the legacy to make sure we aren’t going back in terms of performance.What should we target first and how should we prioritize this work?Many older systems embed the authorization modules as part of a monolith process.
Simple – this is probably one of the easiest approaches to migration and you can use existing “real world” data to test the new system before going liveData pollution – the new system might insert problematic data and break the legacy system, making reverting impossible.
This approach can work well if there’s still a team that manages and updates the legacy code. If one doesn’t exist, you might have a serious problem with this approach.This can work for a shared database deployment. We can deploy the new product to a separate server, with both products using the same database as mentioned above. There are many challenges with this approach, but I picked it often, as it’s probably the simplest one to start with.
The enormous benefit is that we can migrate the database while keeping compatibility and without migrating all the data in one fell swoop.
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