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Guide: Extract core migration

“Extract core” means moving the deterministic domain kernel of an existing service into X07 while keeping the effectful shell (HTTP, DB, queues) thin and replaceable.

Canonical steps​

1) Identify the core boundary​

Pick a slice that is:

  • high-value
  • stable enough to specify
  • small enough to test thoroughly

Write down:

  • inputs
  • outputs
  • invariants
  • error classes

2) Implement the kernel first​

Rules for the kernel:

  • deterministic
  • no I/O
  • no retries
  • explicit error values

Add unit tests that cover:

  • happy paths
  • invalid inputs
  • conflict cases
  • idempotency key derivation (when applicable)

3) Wrap it with a minimal shell​

Shell responsibilities:

  • parsing and validation (transport-specific)
  • correlation (request_id, optional trace_id)
  • effect execution + explicit retry boundaries
  • persistence + idempotency

Keep the shell dumb: the kernel decides; the shell executes.

4) Partial migration stop points​

Safe stop points (you can ship each):

  • kernel-only: used for offline validation
  • shadow mode: kernel computes decision, legacy shell executes
  • dual write/read: kernel backed by new storage but legacy remains source of truth
  • full cutover: shell calls kernel and executes effects via new path

5) Prove replay safety​

Before increasing concurrency or enabling automated rollbacks:

  • simulate duplicates (requests and messages)
  • prove the idempotency boundary
  • ensure correlation fields are present on failures

Example​

See: docs/examples/extract-core/.

  • docs/guides/kernel-shell-production.md
  • docs/guides/scaling-retry-idempotency.md
  • docs/x07import/porting-by-example.md

Expert notes​

For strict boundaries, add an architecture manifest and enforce it with x07 arch check.

If you need to carry a legacy protocol across the boundary, isolate it in the shell and translate into a kernel-native data model early.