Evolving a Schema Safely

GraphQL has its own version of the REST versioning problem from earlier in this course - a schema needs to evolve, but existing clients can't all be forced to update at the moment a change ships. Unlike REST's URL-based versioning, GraphQL's convention leans toward evolving one continuously-versioned schema rather than maintaining `/v1` and `/v2` schemas side by side.

`@deprecated` marks a field as discouraged while keeping it fully functional - existing clients querying `fullName` keep working exactly as before, while schema-aware tooling (IDEs, documentation generators, linters) surfaces the deprecation warning to anyone writing new queries against it. The `reason` argument should point to the replacement and, where practical, a concrete removal date, giving developers both the "why" and the "by when."

• -Additive (safe): adding a new field, adding a new type, adding a new optional argument to an existing field, adding a new value to an enum (with caveats - see below).
• -Breaking: removing a field, renaming a field, changing a field from nullable to non-null, changing a field's return type, removing an argument.

Adding a new enum value deserves its own caveat: it is additive from the schema's perspective, but it can still break a client whose code has an exhaustive switch statement over the enum's previously-known values and no default case - the schema change itself isn't breaking, but it can still break real client code, which is worth knowing rather than assuming "additive" always means "risk-free" in every practical sense.

A deprecated field can't be safely removed just because it was marked deprecated a while ago - the actual safety signal is whether any client is still genuinely querying it. Tracking field-level usage analytics (which deprecated fields are still being queried, by how many distinct clients, how often) turns "has it been long enough" into a concrete, verifiable question: a deprecated field with zero real traffic over a meaningful window is safe to remove; one still receiving regular traffic is not, regardless of how much time has passed since deprecation.