Investigate whether backend Mongo tests should shift from mongodb-memory-server to a local replica set

[rendall]

Summary

Priority 4 currently favors modernizing the existing backend Mongo test stack in place rather than immediately replacing it. That is the lower-risk near-term move, but we should still make an explicit determination about whether the repo ought to shift from mongodb-memory-server to an actual local MongoDB replica set for backend tests.

Current repo practice

The current backend Jest path still depends on the @shelf/jest-mongodb + mongodb-memory-server stack:

• jest.backend.config.ts composes the @shelf/jest-mongodb preset
• jest-mongodb-config.js pins MongoDB to 6.0.14
• .github/workflows/netlify-api-test.yml and scripts/ci-local.sh export MONGOMS_DOWNLOAD_URL to work around platform/archive resolution problems
• backend tests consume preset-provided globals such as __MONGO_URI__ and __MONGO_DB_NAME__

This is workable, but it is also the source of the repo's current Mongo test fragility.

Why this is still worth investigating

MongoDB's own documentation points toward replica-set-backed development/testing when production-like behavior matters:

• MongoDB documents deploying a replica set for testing and development: https://www.mongodb.com/docs/manual/tutorial/deploy-replica-set-for-testing/
• MongoDB documents that transactions require a replica set or sharded cluster: https://www.mongodb.com/docs/manual/data-modeling/enforce-consistency/transactions/

That is not the same as saying the repo must switch immediately. It does mean we should decide intentionally whether our long-term backend test model should remain a modernized mongodb-memory-server path or move to an actual local replica set.

Important nuance

This is not a claim that mongodb-memory-server is obsolete. Current upstream docs say it supports modern MongoDB versions, supports replica sets, and works on current Node lines:

• default MongoDB binary is 8.2.1: https://typegoose.github.io/mongodb-memory-server/docs/api/config-options/
• v10 migration notes / Node support: https://typegoose.github.io/mongodb-memory-server/docs/guides/migration/migrate10/
• project overview: https://typegoose.github.io/mongodb-memory-server/

So the near-term Priority 4 direction can still be: modernize the existing Mongo test stack first.

Requested outcome

Please investigate and make a determination on this question:

• Should this repo continue with a modernized mongodb-memory-server-based backend test stack?
• Or should it shift to an actual local MongoDB replica set for backend tests?

Expected follow-up

A later plan/checklist should:

1. compare the two approaches for this repo specifically
2. evaluate contributor ergonomics, CI/local parity, determinism, and maintenance cost
3. make an explicit recommendation
4. if a shift is preferred, define the migration plan and documentation updates

Related planning artifact: docs/plans/Priority4MongoReplicaSetTestingMiniPlan.md