From a modular monolith

If your codebase already uses a “modular monolith” pattern (MediatR-style, or hand-rolled module classes), the lift to CephalonEngine is mostly a renaming exercise + adopting the manifest.

Mapping

Existing concept	CephalonEngine equivalent
IModule (hand-rolled)	IModule from Cephalon.Abstractions.Modules
IRequestHandler<T> (MediatR)	IRestBehavior for HTTP, IMessageHandler<T> for events
IServiceCollection extensions	RegisterServices(IServiceCollection) on the module
IPipelineBehavior<T>	engine-blessed decorators in the behavior pipeline
Module startup	OnRegister / OnStart lifecycle hooks
Module config	Engine:<ModuleName>:* configuration section

The diff that matters

For each existing module:

1. Inherit from RestBehaviorModuleBase (or the matching transport base).
2. Implement Describe() to declare the descriptor.
3. Move existing DI registrations into RegisterServices(...).
4. Convert MediatR handlers into behaviors / message handlers.
5. Adopt the manifest — let the runtime tell you what’s loaded.

Verification

• The composition smoke tests pass with the same module set you had before.
• /engine/manifest lists every module.
• The OpenAPI shape is identical (or improves, because behaviors auto-document better than MapGet chains).

Status

Full step-by-step lands in the next docs push. Until then, From plain ASP.NET Core is the closest analogue.