# ADR-0005: Derived Rail Composition

Date: 2026-07-26
Status: Accepted

## Context

Knative provides distinct workload activation and revision semantics but runs
on Kubernetes. Treating it as an unrelated peer rail would duplicate generic
workload lifecycle and substrate assumptions.

## Decision

Railiance distinguishes base and derived rails.

`rail-kubernetes` is the default base rail for platform workloads.
`rail-knative` is a derived rail that inherits a versioned
`rail-kubernetes` common contract and overrides only Knative-specific
activation, revision, autoscaling, traffic, cold-start, and rollback semantics.

The detailed contract is
[`docs/rail-composition-contract.md`](../rail-composition-contract.md).

## Consequences

- New platform workloads continue to use `rail-kubernetes` unless a specialized
  rail is justified.
- Derived rails declare base-rail compatibility rather than copying lifecycle
  contracts.
- Fabric and conformance tooling must understand rail dependency and readiness.
- Knative installation stays with the S2 substrate owner.