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0044-instance-based-mediator-for-testability.md

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id 0044-instance-based-mediator-for-testability
title Instance-Based Mediator for Testability
status Proposed
author
Brighter Team
created 2025-12-23
summary Converts the static OutboxProducerMediator singleton to an instance field in CommandProcessor, enforcing singleton semantics via DI registration to enable parallel test execution with isolated mediator instances per test.
tags
mediator
di
testing
architecture

44. Instance-Based Mediator for Testability

Date: 2025-12-23

Status

Proposed

Context

ADR 0033 established that OutboxProducerMediator should be a static singleton to ensure one outbox per application. However, this creates testability issues:

  1. Tests cannot run in parallel - require [Collection("CommandProcessor")] serialization
  2. Tests require explicit ClearServiceBus() calls in teardown
  3. Static state bleeds between tests causing flaky failures
  4. InMemoryOutbox instances are shared across tests unexpectedly

The singleton requirement is valid, but enforcing it via static fields is the wrong mechanism.

Decision

  1. Convert static to instance-based: Change s_mediator to _mediator instance field in CommandProcessor
  2. Preserve singleton via DI: Register IAmAnOutboxProducerMediator with ServiceLifetime.Singleton
  3. Add service provider overloads: Enable Func<IServiceProvider, T> configuration for AddBrighter, AddProducers, AddConsumers
  4. Integrate Options pattern: Allow PostConfigure<BrighterOptions> for test-time overrides
  5. Deprecate ClearServiceBus: Mark obsolete (still functional for transition)

Consequences

Positive:

  • Tests can run in parallel without [Collection] attributes
  • Each test can have isolated CommandProcessor with own mediator
  • Standard .NET Options pattern enables test overrides
  • InMemoryOutbox properly isolated per test

Negative:

  • Migration effort for existing tests (optional - old pattern still works)
  • Slight API surface increase with new overloads

Backward Compatibility:

  • All existing Action<T> overloads unchanged
  • ClearServiceBus() still works (deprecated)
  • DI registration patterns unchanged