docs/protocol_versioning_matrix.md

🔄 Protocol Versioning & Client Compatibility Matrix Specification

This document defines the semantic versioning scheme, gRPC header handshake protocol, REST/WebSocket schema versioning, and backward compatibility invariants for the Collaborative Fraud Intelligence (CFI) platform.

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📌 1. Protocol Versioning Scheme

Protocol releases strictly adhere to Semantic Versioning (SemVer 2.0.0) (MAJOR.MINOR.PATCH):

• MAJOR (X.0.0): Breaking changes to protobuf wire formats (fl_service.proto), required parameter serialization schemas, or cryptographic primitives (e.g. v1.x to v2.x). Requires client SDK upgrades.
• MINOR (x.Y.0): Backward-compatible feature additions (e.g., new optional telemetry fields, updated drift metrics, additive database columns).
• PATCH (x.y.Z): Backward-compatible bug fixes, internal algorithmic optimizations, and performance enhancements.

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📑 2. Platform Compatibility Matrix

The domain compatibility bounds are enforced by VersionCompatibilityMatrix:

Platform Version	gRPC Wire Protocol	Supported Client SDK Range	Schema Digest (SHA-256)	Lifecycle Status	Deprecation Date
v1.0.0	1.0.0	1.0.0 - 1.99.99	a1b2c3d4e5f60718...	⚠️ Deprecated	2026-10-01
v1.1.0	1.1.0	1.0.0 - 1.99.99	e5f6g7h8i9j01234...	⚠️ Maintenance	2026-12-31
v2.0.0	2.0.0	2.0.0 - 2.99.99	9988776655443322...	✅ Active Production	N/A
v2.1.0	2.1.0	2.0.0 - 2.99.99	3344556677889900...	✅ Active Production	N/A
v3.0.0 (Roadmap)	3.0.0	3.0.0 - 3.99.99	(Planned)	🔬 Planned (PQC / zk-SNARK)	2027-Q2

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🤝 3. gRPC Header Handshake & Context Metadata

Every gRPC streaming request (RegisterClient, Heartbeat, StreamModelParameters) is intercepted by ProtocolVersionInterceptor to validate client protocol metadata:

┌────────────────────────────────────────────────────────────────────────────────────────┐
│                              gRPC CLIENT HANDSHAKE METADATA                            │
├──────────────────────────┬──────────────────────────────────┬──────────────────────────┤
│ METADATA KEY             │ EXAMPLE VALUE                    │ VALIDATION PURPOSE       │
├──────────────────────────┼──────────────────────────────────┼──────────────────────────┤
│ `x-cfi-protocol-version` │ `2.1.0`                          │ SemVer compatibility     │
│ `x-cfi-schema-hash`      │ `e3b0c44298fc1c14...`            │ Feature schema alignment │
│ `x-cfi-tenant-id`        │ `bank_alpha`                     │ ContextVar routing       │
│ `x-cfi-mtls-fingerprint` │ `SHA256:7b908f24...`             │ X.509 cert binding       │
└──────────────────────────┴──────────────────────────────────┴──────────────────────────┘

Protocol Rejection & Negotiation Semantics

The server evaluates (client_version, client_schema_hash) against the active matrix:

• VersionNegotiationStatus.COMPATIBLE: Client version satisfies SemVer major alignment and falls within [min_supported_version, max_supported_version].
• VersionNegotiationStatus.DEGRADED_COMPATIBLE: Version matches, but client feature schema hash differs from the consortium registry (logged as a drift warning).
• VersionNegotiationStatus.INCOMPATIBLE: Aborts request with grpc.StatusCode.FAILED_PRECONDITION (or OUT_OF_RANGE), directing the client node to the consortium upgrade portal.

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🌐 4. HTTP REST & WebSocket Versioning Invariants

1. Path-Based Prefix Routing:
   • Production REST endpoints are namespaced under /api/v1 or /v1 (e.g. /api/v1/score-transaction, /api/v1/predict, /v1/webhooks/subscriptions, /v1/inference/score).
2. RFC 8594 Standard Deprecation & Sunset Headers:
   • APIVersionLifecycleMiddleware in backend/app/main.py attaches standard version lifecycle headers to all HTTP responses:

     X-API-Version: v1
     Deprecation: Sat, 01 Jan 2026 00:00:00 GMT
     Sunset: Sat, 01 Jul 2026 00:00:00 GMT
     

3. Consortium Deprecation Warning Headers:
   • During rolling upgrades and migration windows, legacy endpoints signal target versions:

     x-cfi-deprecation-warning: Version v2.0.0 will be retired on 2026-10-01.
     x-cfi-target-version: v2.1.0
     

4. Additive JSON Contracts:
   • Pydantic models across backend/app/presentation/routers/ enforce additive field updates with default values, preventing serialization crashes in older client libraries.

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🧪 5. Automated Test Verification Matrix

Protocol negotiation, SemVer comparison, and lifecycle header adherence are continuously verified:

Test Suite	File Path	Verified Capabilities	Status
Protocol Versioning	backend/tests/unit/test_protocol_versioning.py	SemVer parsing (1.0.0 < 2.0.0), matrix negotiation, gRPC metadata extraction	3/3 PASSED
OpenAPI Contract Accuracy	backend/tests/unit/test_openapi_contract_accuracy.py	Route schema accuracy, lifecycle headers, FinCEN export endpoints	2/2 PASSED