ENH: Add schema migration support for versioned resources

Author: GibranAlfa

ARCHITECTURE.md

@@ -65,7 +65,10 @@ classDiagram
 
     class UserFMUDirectory
 
-    class CacheManager
+    class CacheManager {
+        +get_revision_content(..., migration_manager)
+        +restore_revision(..., migration_manager)
+    }
     class LockManager {
         +acquire()
         +ensure_can_write()
@@ -75,12 +78,20 @@ classDiagram
     }
 
     class PydanticResourceManager~PydanticResource~ {
+        +migration_manager
         +load(force, store_cache)
         +save(model)
         +get_resource_diff(incoming_resource)
         +get_structured_model_diff(current_model, incoming_model)
     }
 
+    class MigrationManager~MigratableResource~ {
+        +model_class
+        +current_version: int
+        +migrate_resource(data)
+        +requires_backup(data)
+    }
+
     class MutablePydanticResourceManager~MutablePydanticResource~ {
         +get(key, default)
         +set(key, value)
@@ -129,6 +140,8 @@ classDiagram
 
     FMUDirectoryBase *-- LockManager
     FMUDirectoryBase *-- CacheManager
+    PydanticResourceManager o-- MigrationManager
+    CacheManager ..> MigrationManager
     ProjectFMUDirectory *-- ProjectConfigManager
     ProjectFMUDirectory *-- ChangelogManager
     ProjectFMUDirectory *-- MappingsManager
@@ -140,10 +153,44 @@ The main library split is:
 - `FMUDirectoryBase` is the filesystem-centered abstraction. It owns the `.fmu` path, lock manager, cache manager, and generic read/write helpers.
 - `ProjectFMUDirectory` and `UserFMUDirectory` specialize the base class for project-local `.fmu/` directories and `$HOME/.fmu/`.
 - `PydanticResourceManager` is the generic resource engine for loading, saving, diffing, and caching JSON-backed Pydantic models.
+- `MigrationManager` applies registered, forward-only schema migrations and validates the result as the current Pydantic model.
 - `MutablePydanticResourceManager` adds dot-notation `get`, `set`, `update`, `reset`, and merge behavior for editable resources.
 - `ProjectConfigManager`, `UserConfigManager`, `MappingsManager`, and `LogManager` bind specific Pydantic models to managed files inside `.fmu/`.
 - Directory objects compose the correct managers and delegate resource operations to them.
 
+## Schema Migration
+
+`ProjectConfig`, `UserConfig`, and `InternalMappings` are versioned resources. Each
+model declares its current schema version, and each resource manager has a
+`MigrationManager` with the migration registry for that model.
+
+Migration is forward-only. Data without `schema_version` is treated as version 1.
+Each registered function advances the data by one version. The manager rejects a
+newer schema, a missing migration step, an invalid version, or data that does not
+validate as the current model.
+
+The migration boundary depends on the resource operation:
+
+- **Load:** `PydanticResourceManager` decodes the stored JSON and asks
+  `MigrationManager` for a validated current model. Migration happens in memory.
+  Loading does not write the resource, create a cache revision, or add a changelog
+  entry.
+- **Save:** The resource manager checks the write lock first. If the stored data is
+  unversioned or older, it stores the original JSON as a normal cache revision
+  before writing the current model. Normal cache retention applies.
+- **Cache read:** `CacheManager` uses the migration manager to return old revisions
+  as current validated models.
+- **Restore:** `CacheManager` migrates the selected revision before writing it, so
+  the restored resource uses the current schema. Project restore operations use the
+  existing restore changelog entry.
+
+There is no backward migration. After a current-schema resource is saved, an older
+`fmu-settings` release can reject it as newer than supported.
+
+See the
+[schema migration guide](src/fmu/settings/_migrations/README.md)
+for the implementation, test, and coordinated release process.
+
 ## Runtime Flow
 
 The full runtime spans multiple repositories, but `fmu-settings` owns the `.fmu/` directory operations used by the API and CLI.

src/fmu/settings/__init__.py

@@ -21,6 +21,7 @@
     init_fmu_directory,
     init_user_fmu_directory,
 )
+from ._migrations import MigrationError
 from .models._enums import CacheResource
 from .models.mappings import (
     InternalBaseMapping,
@@ -47,6 +48,7 @@
     "InternalWellboreIdentifierMapping",
     "InternalWellboreMappings",
     "InvalidFMUProjectPathError",
+    "MigrationError",
     "ProjectFMUDirectory",
     "REQUIRED_FMU_PROJECT_SUBDIRS",
     "UserFMUDirectory",

src/fmu/settings/_fmu_dir.py

@@ -22,6 +22,9 @@
 from .models.project_config import ProjectConfig
 from .models.user_config import UserConfig
 
+if TYPE_CHECKING:
+    from ._migrations import MigrationManager
+
 logger: Final = null_logger(__name__)
 
 FMUConfigManager: TypeAlias = ProjectConfigManager | UserConfigManager
@@ -470,13 +473,19 @@ def restore_from_cache(
         if manager is self.config:
             previous_max_revisions = self._cache_manager.max_revisions
             restored_config = self.cache.get_revision_content(
-                relative_path, revision_id, model_class=ProjectConfig
+                relative_path,
+                revision_id,
+                model_class=ProjectConfig,
+                migration_manager=self.config.migration_manager,
             )
             self._cache_manager.max_revisions = restored_config.cache_max_revisions
 
             try:
                 self.cache.restore_revision(
-                    relative_path, revision_id, model_class=ProjectConfig
+                    relative_path,
+                    revision_id,
+                    model_class=ProjectConfig,
+                    migration_manager=self.config.migration_manager,
                 )
             except Exception:
                 # Restore the previous runtime retention if config restore fails
@@ -495,7 +504,13 @@ def restore_from_cache(
             return
 
         self.cache.restore_revision(
-            relative_path, revision_id, model_class=manager.model_class
+            relative_path,
+            revision_id,
+            model_class=cast("type[BaseModel]", manager.model_class),
+            migration_manager=cast(
+                "MigrationManager[BaseModel] | None",
+                manager.migration_manager,
+            ),
         )
 
         # Refresh the resource manager's in-memory cache
@@ -531,7 +546,13 @@ def get_cache_content(
             )
 
         return self.cache.get_revision_content(
-            relative_path, revision_id, model_class=manager.model_class
+            relative_path,
+            revision_id,
+            model_class=cast("type[BaseModel]", manager.model_class),
+            migration_manager=cast(
+                "MigrationManager[BaseModel] | None",
+                manager.migration_manager,
+            ),
         )
 
     def _cacheable_resource_managers(