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Persistence Tests and Backend Swappability

Concept Position

flowchart TD
  family["Python Programming"] --> program["Python Object-Oriented Programming"]
  program --> module["Module 06: Persistence, Serialization, and Schema Evolution"]
  module --> concept["Persistence Tests and Backend Swappability"]
  concept --> capstone["Capstone pressure point"]
flowchart TD
  problem["Start with the design or failure question"] --> example["Study the worked example and trade-offs"]
  example --> boundary["Name the boundary this page is trying to protect"]
  boundary --> proof["Carry that question into code review or the capstone"]

Read the first diagram as a placement map: this page is one concept inside its parent module, not a detached essay, and the capstone is the pressure test for whether the idea holds. Read the second diagram as the working rhythm for the page: name the problem, study the example, identify the boundary, then carry one review question forward.

Purpose

Verify repository behavior as a contract so backend changes do not silently alter domain semantics.

1. Repository Contracts Need Executable Proof

If you promise that every repository implementation:

  • preserves invariants,
  • detects stale writes,
  • round-trips aggregates faithfully,

then those promises need tests that every backend must pass.

2. Contract Tests Beat One-Off Happy Paths

Write backend-agnostic tests once, then run them against:

  • an in-memory repository
  • a file-backed repository
  • a database-backed repository

That gives you storage flexibility without semantic drift.

3. Test Real Failure Modes

Focus on behaviors that often change across backends:

  • missing aggregate semantics
  • duplicate identity handling
  • version conflicts
  • codec mismatch

Do not stop at “save then load works once.”

4. Swappability Has a Cost

Not every backend deserves full interchangeability. Sometimes backend differences are real and should appear in separate contracts. Swappability is valuable only when the shared contract is honest.

Practical Guidelines

  • Write backend-independent repository contract tests.
  • Include corruption, conflict, and missing-object cases.
  • Share fixtures across implementations to keep expectations aligned.
  • Split contracts when backend capabilities truly differ.

Exercises for Mastery

  1. Write one repository contract test and run it against two implementations.
  2. Add a failure case for a stale save or malformed stored payload.
  3. Decide whether one backend-specific behavior belongs in the shared contract or outside it.