Schema Snapshot Restore: Does it Re‑install Resolvers?

Answer

Restoring a Gatsby schema snapshot only reinstates the serialized GraphQL type definitions; it does not re‑install the resolver functions that were contributed by a removed createSchemaCustomization plugin.

Confirmed facts

• A snapshot stores the result of buildSchema after all plugins have run; executable resolver code is not serialized.
• When the snapshot is restored, Gatsby uses the stored type system to create a new GraphQLSchema instance, attaching default resolvers for any fields that lack an explicit resolver.
• If the plugin that supplied custom resolvers has been uninstalled, those resolvers are not present in the snapshot and therefore are not re‑registered after restore.
• The restore process does not scan or purge resolver definitions from removed plugins; it simply replaces the current schema with the snapshot.

Likely explanation

Because resolvers are ordinary JavaScript functions, they cannot be captured in a JSON‑like snapshot. The snapshot mechanism is a schema cache, not a plugin lifecycle replay. After a restore you will see the original type names and fields, but any custom logic will fall back to Gatsby’s default resolver (returning undefined) or cause errors if the field expects a non‑null value.

Steps for this case

1. Verify that the snapshot you intend to restore was taken before the plugin was added.
2. Run gatsby develop (or gatsby build) with the snapshot enabled (gatsby develop --schema-snapshot).
3. Introspect the restored schema (e.g., via GraphiQL) to confirm that the custom type fields exist.
4. Test a query that depends on the removed plugin’s resolver; if it returns unexpected values or errors, the resolver was not reinstalled.
5. If you need the original resolver logic, regenerate the snapshot after reinstalling the plugin or add the resolver code directly to createResolvers in a remaining plugin.

Missing diagnostic detail

Did you create the snapshot before or after the plugin that adds the custom types was installed? Knowing this determines whether a simple restore is a safe rollback or whether you must regenerate the snapshot after removing the plugin.