Inheritance

extends on a Kernel.Definition declares that the kind fulfills an abstract interface declared by another module. This is the extension point for pluggable subsystems: one module defines the contract, other modules provide implementations.

# modules/workflow-temporal/module.yaml
kind: Kernel.Definition
metadata: { name: Backend, module: WorkflowTemporal }
capability: Provider
extends: Workflow.Backend

extends is distinct from capability. capability assigns a lifecycle role. extends declares which abstract interface this definition implements. The two are orthogonal and may be combined.

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Declaring an Abstract Interface (Kernel.Abstract)

A module declares an abstract interface with kind: Kernel.Abstract. This registers a named slot that other modules can target via extends.

# modules/workflow/module.yaml
kind: Kernel.Abstract
metadata: { name: Backend, module: Workflow }
capability: Provider

Kernel.Abstract accepts the same fields as Kernel.Definition except controllers — it has no implementation. It defines the contract (capability, schema, inputs/outputs) that all implementations must satisfy.

The fully-qualified name of the abstract kind (Workflow.Backend) becomes the value used in extends by implementors.

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Providing an Implementation

A definition extends an abstract interface by setting extends to the abstract kind's qualified name:

kind: Kernel.Definition
metadata: { name: Backend, module: WorkflowTemporal }
capability: Provider
extends: Workflow.Backend
schema:
  type: object
  properties:
    address: { type: string }
    namespace: { type: string }
controllers:
  - pkg:npm/@telorun/workflow-temporal@>=0.1.0?local_path=./nodejs#backend

Kernel behavior: The kernel verifies that the implementing definition's capability is compatible with the abstract's declared capability. The abstract's schema is merged as a base — the implementation may extend it with additional properties.

Analyzer behavior: Any resource whose schema references the abstract kind via x-telo-ref will accept instances of any definition that extends it. This is how Workflow.Graph accepts any Workflow.Backend implementation without knowing about specific implementors.

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Reference from Schema

Abstract kinds are referenced in schemas using x-telo-ref:

# In Workflow.Graph's schema
properties:
  backend:
    x-telo-ref: "std/workflow#Backend"

The analyzer resolves this to Workflow.Backend and accepts any resource instance whose kind definition has extends: Workflow.Backend.

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Summary

Field	Purpose	Scope
capability	Assigns a lifecycle role	Kernel built-ins only
extends	Declares which abstract interface the kind implements	Cross-module, non-kernel
Kernel.Abstract	Declares a pluggable abstract interface	Within a module