Implemented the Backend Abstraction Layer (BAL) for the Advanced Context
Management System, following the specification in docs/specification.md
Section ACMS > Backend Abstraction Layer and ADR-014.
Key additions:
- TextBackend protocol with search(query, scope, max_results) returning
list[TextResult], and TextResult frozen dataclass (uko_uri, content,
score, metadata fields)
- VectorBackend protocol with similarity_search(embedding, scope, top_k)
returning list[VectorResult], and VectorResult frozen dataclass
- GraphBackend protocol with sparql_query(query, scope),
get_triples(subject), and traverse(start, depth) methods returning
GraphResult frozen dataclass (triples, metadata fields)
- In-memory stub backends (InMemoryTextBackend, InMemoryVectorBackend,
InMemoryGraphBackend) that validate arguments and return empty results,
serving as development placeholders and test doubles
- DI container registration as configurable Singletons with provider
selection via override_providers()
- Behave BDD feature (35 scenarios / 83 steps) covering protocol
compliance, argument validation, result immutability, and DI resolution
- Robot Framework smoke tests (6 tests) for integration verification
- ASV benchmarks for stub query overhead and instantiation time
- Reference documentation at docs/reference/acms_backends.md
Design decisions:
- Used @runtime_checkable Protocol for structural subtyping, consistent
with existing ResourceHandler pattern
- Used frozen dataclasses (not Pydantic) for result types to minimize
overhead in the hot path of context assembly
- scope parameter typed as frozenset[str] for immutability and hashability
- Stubs registered as default Singletons; production backends swap via DI
ISSUES CLOSED: #498
