Migration strategies in large codebases

Code migrations are a fact of life for large codebases. New technologies pop up, platforms see improvements, and programming languages get new features you might want to adopt. Not performing these migrations and keeping up with the times is simply not an option in most cases. While some migrations are easy 1 for 1 replacements that can/should be scripted, that’s not true for the more “semantic” migrations—the new code accomplishes the same but in an entirely different manner.

We’ve done a number of these semantic migrations in Lyft’s mobile codebases and while we’ve learned something from every one of them (and applied the learnings the next time), they only get harder as time goes on.

What makes migrations so difficult? This generally comes down to how development works in large codebases: there is no 1 mobile team, but rather a lot of vertical teams (e.g. payments, new user experience, etc.) that all build their own features, using shared foundational code and architectural patterns. The coordination of dozens of teams on the progress of a migration is what makes this difficult:

So how do we get these migrations done anyway? There is no silver bullet, but we have started using a number of strategies to make these migrations less painful—and almost all of them have to do with improving the communication and expectations everyone involved has.

Migration tracker

The biggest leap forward for us was the rollout of an in-house web dashboard that shows progress on all ongoing migrations in our iOS and Android codebases. It shows all the important information anyone could want to know: the start date, (projected) completion date, links to documentation and support channels, relative priority (more on that below), etc.

But the real win is automatic progress tracking. Every migration is defined as the presence of a certain pattern in the codebase we want to get rid of. A cron job records all instances of that pattern on the main branch once a day, so we know exactly how much code has been migrated and how much is left.

Furthermore, our codebase is heavily modularized, and through CODEOWNERS every module is required to define what team owns it. This data is shared with the Migration Tracker, so it can give a detailed breakdown by module and by team of how much unmigrated code is left. This is extremely helpful in understanding which teams might need extra time, help, or nudging in getting everything done.


The “deadline” of a migration needs to be carefully chosen. Not giving people enough time makes them not do the work at all, giving them too much time means people will procrastinate and needlessly extend the migration time. If it looks like teams are not going to make the deadline (as projected by the Migration Tracker), we ask them when they can get it done. We usually accept whatever answer the team gives, but hold them accountable to that since they themselves picked that timeframe.


Teams that are far behind in code modernization might have to migrate their code in multiple ways. We give each migration a priority (P0/P1/P2/P3) to give teams a sense of where to focus their tech debt energy first.

A P0 migration is a “must-do-or-else” (externally mandated changes, tight deadlines, extremely high impact on overall trajectory of the codebase or product, etc.), a P1 is high impact but not as urgent, P2 is a nice to have, and P3 is almost more of an FYI.

This helps everyone understand where they should focus their energy, instead of seeing a potentially long list of items that are all seemingly equally important migrations.

Incremental value

Where possible (which is most cases), we try to bring incremental value as a migration is being performed. For example, if we set out to replace all uses of class A with class B because class B performs better, the places where class B is used should see that performance increase before the rest of the code has been migrated. This has a few benefits:

Only invest in the new

The initial value (like performance improvements in the example above) teams get from migrating their own code might not be enough to convince them to do it. But as more and more improvements are made to “class B” (e.g. API ergonomics, integrated analytics, compatibility with other systems) that “class A” doesn’t have, the value proposition for the migration changes with time. We no longer update A, which means there's some amount of bit rot and it becomes more cumbersome to use.

In extreme cases we could even remove features of class A that would necessitate a migration, but we haven’t done that so far and would like to avoid it to avoid losing goodwill with teams.

Develop great partnerships

Getting adoption of a new pattern or tool is difficult even if it doesn’t involve a new migration, because there is no precedent. Early adopters will hit all the rough edges and have often incomplete documentation and no sample code. The developer experience isn’t great (yet), so we make ourselves very available to their needs. We take their feedback seriously and address it promptly, sit down with them to help solve their issues, and generally spend a lot of time with them working out the kinks. We do this 1) to build more confidence that what we built is production-ready and 2) to recognize/reward people that want to try new things by reducing as much of the growing pains as possible.

Of course, good developer experience is important for all teams and not just early adopters, but piloting teams can help us get the ball rolling with other teams as well. Positive reviews from users of Great New Thing is different than hearing it from the developers of Great New Thing.

Avoid new uses

To avoid creating a moving target, as soon as we start a new migration we lock down the uses of that pattern to only the current ones and no longer accept new uses of this pattern. Depending on the situation we have a few different ways of accomplishing this:

These measures are just to generate awareness that a pattern is outdated, it doesn’t always communicate why it’s being deprecated and what’s replacing it. But if people come to us asking for an exception we can explain it or link to docs, an opportunity we wouldn’t have had if we didn’t do any of the things above.

Support, support, support

And that brings me to the most critical and time-consuming piece: support. This means helping people with questions, plugging gaps new patterns have compared to the old ones, checking in with people on progress, celebrating the completion of major (or all!) migrations with everyone that helped contribute, assisting teams that are having difficulty prioritizing the work, and anything else that gets the work done.

Migrations are a very specific type of codebase maintenance due to the large footprint they often have in a lot of the codebase. It’s an all-hands on deck situation and requires proper coordination and communication, sometimes for years on end. It’s hard to get them right and we’re still fine-tuning our strategies but applying the learnings I described here have made us become better at them. I can only hope one day refactoring tools become smart enough to automate the work for us.