clean-architecture-backend-template/docs/adr/ADR-MONGO-ADV-001-capability-promotion.md

ADR-MONGO-ADV-001 — Advanced capability promotion

• Status: Accepted
• Date: 2026-08-13
• Design source: design §2 (D-15), §3.2–§3.3; Advanced expansion plan Task 15

Context

Sharding, time series, CSFLE, Queryable Encryption, search, vector search and multi-tenancy each work in a demo within an afternoon. What they do not do is behave the same way in production, and the differences are not discovered by functional tests:

• Sharding changes which queries are efficient. A query that misses the shard key becomes scatter-gather, which passes every test on a one-shard cluster.
• Encryption's failure modes are KMS failure modes — wrong key, revoked permission, mid-rotation — none of which occur against a local key provider.
• Search and vector search can be functionally correct and useless: the index returns results, and the results are not relevant. Recall is not visible in a pass/fail assertion.
• Database-per-tenant works until the tenant count crosses what the connection and file-handle budget supports, which is an operational property, not a code property.

The failure mode this ADR prevents is a capability marked "done" on the strength of a green test that never touched the environment where it will run.

Decision

Every Advanced and Experimental capability is an opt-in module behind its own flag, and promotion requires evidence, not confidence.

Enablement

MongoAdvancedCapabilityFlags gates construction of every Advanced entry point. A disabled capability does not produce a runtime warning — the type refuses to be constructed, naming the property that enables it (MongoAdvancedCapabilityFlags.propertyFor(capability)). Being on the classpath is not being enabled, and stableNeverDependsOnAdvanced (ArchUnit) keeps the Stable surface free of them.

Promotion evidence

MongoAdvancedPromotionGate.verify(evidence) requires every category:

Category	Means
stable-platform	The Stable release gate passed on the same revision.
actual-topology	The capability ran on the real topology — a real sharded cluster, the real KMS, the actual target deployment. Atlas Local is a pull-request convenience and explicitly not release evidence (MongoAtlasCapabilityContractSuite.Environment.ATLAS_LOCAL).
security	Privileges reviewed; the capability's admin role is separate from the application role.
migration	A documented path in and, where the capability is irreversible, an explicit statement that there is no path back.
failure	Negative cases fail closed: wrong key, missing permission, rotation, non-ready index, unrouted query.
runbook	A runbook exists for the capability's characteristic incident.

Additional per-capability requirements

• Search / vector search: relevance and performance evidence, not functional success alone. MongoVectorSearchBenchmarkGate requires recall alongside latency and index size; a gate that measures only latency certifies a fast wrong answer.
• Database-per-tenant and reshard orchestration remain Experimental until operational scale evidence exists. Both are correct in the small and unbounded in the large.
• Reshard requires an explicit ReshardApproval — a named approver and a stated window. It rewrites the collection.

Promotion does not change the dependency boundary

A capability promoted to Stable remains an opt-in module unless a later starter ADR changes the dependency boundary. Promotion is a statement about evidence, not an invitation to add a transitive dependency to every service.

Consequences

Positive. No capability reaches production on the strength of a container-only test. The evidence list is the same for every capability, so promotion is reviewable rather than negotiated.

Negative. Promotion requires access to real infrastructure — a sharded cluster, a real KMS, the target deployment. That is the cost of the guarantee: the alternative is finding out in production, where encryption and sharding are both expensive to reverse.

Verification

scripts/verify-mongodb-advanced.sh enforced this ADR until it was removed on 2026-08-15. The promotion evidence categories this ADR requires are therefore no longer checked by any automated gate; they are a review obligation until one is rebuilt. See docs/mongodb/repository-adaptation.md §5 for the Gradle lanes the script wrapped.