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ADR 0004 — Control Plane Does Not Execute Migrations

Status

Accepted

Context

The Control Plane is a coordination service that must remain available and stateless for job management. Coupling migration execution to the Control Plane would make it a stateful, heavyweight process that cannot scale independently.

Decision

The Control Plane coordinates jobs but never executes migration logic. It:

  • Receives job submissions
  • Assigns jobs to available agents via lease
  • Tracks job state (Queued, Running, Completed, Failed)
  • Stores and exposes telemetry, progress, and diagnostics
  • Manages agent leases and heartbeats

Migration execution is performed exclusively by the Migration Agent and TFS Migration Agent.

Alternatives Considered

Execute migrations in the Control Plane: Simpler deployment, but the Control Plane becomes a monolith, cannot scale execution independently, and becomes harder to operate.

Consequences

  • The Control Plane may be deployed as a lightweight service.
  • Agents are stateless workers that can be scaled independently.
  • The Control Plane must not write package artefacts.
  • The CLI and TUI communicate with the Control Plane only — they have no direct connection to agents.

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