Database schemas are generally isomorphic with RPC data structures - usually a subset one way or the other. If you had a service which handed out Zip Codes and

Database schemas are generally isomorphic with RPC data structures - usually a subset one way or the other. If you had a service which handed out Zip Codes and the structure of Zip Codes changed (maybe you used numbers and now you realize you need to use strings, which I infer from your UK example) you'd have an isomorphic problem: updating 12 consumers and 1 producer for the new type in the RPC message. Having an RPC to migrate means you can put code on the read path to support backward compatibility while migrating. But you can do the same thing with the database - support both fields until all readers have ported to the new field - and since big database migrations are expensive, in practice large systems do migrations that way anyhow. Introduce the new field, start writing to it, back-fill it, port readers across individually (there may be many read locations even within a single service), drop the old field. If a system evolved from a monolith, one way to partition services is to provide views to limit set of visible columns. That's another point at which you can put shims for compatibility while porting schemas.