Feature Flag Best Practices

Chapter 6. Best Practice #4: Incremental, Backward-Compatible Database Changes

Whenever we make code changes to a production system, we need to take existing database data—and, more generally, any shared persistent state—into account. The database schema in place needs to be compatible with the expectations of any newly deployed code; sometimes that means applying a migration to our database schema, as illustrated in Figure 6-1.

Figure 6-1. Database schema before and after migration

We can orchestrate a code deployment and its corresponding database migration in a few ways.

Code First

We can perform the code deployment first, shown in Figure 6-2, making sure that the new version of our code is backward-compatible with the existing database schema.

Figure 6-2. Code-first approach

Data First

Alternatively, we can perform our database migration first, as shown in Figure 6-3, which means that we must ensure that the new schema is backward-compatible with the existing code.

Figure 6-3. Data-first approach

Big Bang

In simple systems, there’s a third option (Figure 6-4): update data and code simultaneously in a lockstep deployment in which you stop your system, update your data ...