# Feature: Selective Collection Sync

## Purpose and Business Outcome

Allow teams to replicate only selected collections so bandwidth and operational overhead stay aligned to business-critical data.

## Scope and Non-Goals

Scope:

- Register collections for replication using `WatchCollection()`.
- Replicate changes for registered collections across peers.

Non-goals:

- Automatic replication of all database collections.
- Schema migration management.

## User and System Workflows

1. Developer registers target collections in the document store.
2. Local writes trigger CDC events.
3. Oplog entries propagate through peer sync.
4. Remote peers apply updates for matching collections.

## Interfaces, APIs, and Events Involved

- `WatchCollection(collectionName, collection, keySelector)`
- CDC trigger pipeline
- Oplog append and apply operations

## Permissions and Data Handling

- Access to source collections is controlled by host application permissions.
- Only approved collections should be registered for sync in sensitive environments.

## Dependencies and Failure Modes

Dependencies:

- Correct collection registration
- Stable peer connectivity
- Persistence availability

Failure modes:

- Missed replication due to unregistered collection
- Delayed propagation during network partition

## Monitoring, Alerts, and Troubleshooting Pointers

- Monitor replication lag and peer confirmation metrics.
- Use [Runbook](../runbook.md) and [Troubleshooting](../troubleshooting.md) for incident response.

## Rollout and Change Considerations

- Introduce new synced collections behind staged rollout.
- Validate downstream consumer compatibility before production enablement.

## Validation and Testability Guidance

- Add integration tests verifying only registered collections replicate.
- Smoke test by writing to registered and non-registered collections and confirming expected behavior.
- Validate no unexpected collection appears in remote peers after deployment.

## Related Security Controls

- [Security](../security.md)
- [Access and Permissions](../access.md)