Stutter testing for idempotency verification.

Stutter testing automatically retries commands to verify that the SUT behaves idempotently - that retrying a command produces the same result.

How It Works

When stutter testing is enabled, some commands are executed multiple times:

1. First execution: Events applied to projections normally
2. Retry executions: Events captured but NOT applied to projections
3. Framework compares retry events to first execution
4. Mismatch = idempotency violation

This approach tests SUT idempotency without requiring idempotent projections.

Configuration

PropertyDamage.run(
  model: MyModel,
  adapter: MyAdapter,
  stutter: %{
    probability: 0.1,      # 10% of commands stuttered
    max_repeats: 2,        # Up to 2 retries (3 total executions)
    delay_ms: {0, 100},    # Random delay between retries
    commands: :all,        # Or list of specific command modules
    comparison: :strict    # :strict, {:structural, fields}, {:custom, fn}
  }
)

Command Callbacks

Commands can opt into idempotency testing by implementing:

• idempotent?/0 - Return true if command should be stuttered (default: true)
• idempotency_key/1 - Return the idempotency key for requests
• acceptable_retry_events/0 - Event modules acceptable as retry responses

Comparison Modes

• :strict - Events must be exactly equal
• {:structural, ignore_fields} - Ignore specified fields when comparing
• {:custom, fun} - Custom comparison function