Use Case

As a driver user
I want a serverSelector function option on MongoClient that narrows server selection to a specific member of a replica set or a specific mongos
So that I can target one member while the driver keeps discovering the rest of the deployment (unlike directConnection, which forces a Single topology and skips discovery). Matches PyMongo's server_selector.

User Experience

new MongoClient(uri, {
  serverSelector(servers) {
    const member = servers.find(s => s.address === 'db-2.example.com:27017');
    return member ? [member] : [];
  }
});

Not a URI option, since the value is a function. Omitting the option leaves selection unchanged.

Dependencies

N/A

Risks/Unknowns

• N/A

Acceptance Criteria

Implementation Requirements

• Selection order follows PyMongo: read preference, tags, max staleness and overload deprioritization first; then serverSelector (before localThresholdMS); then the latency window applies to the returned list.
• The function is not called when no server is eligible, for Single (directConnection) or load-balanced topologies, or for server-pinned operations (getMore, killCursors).
• Writes still only see writable servers.

Testing Requirements

Unit and integration tests (replica set).

Documentation Requirements

N/A

Follow Up Requirements

Confirm with DBX devs whether this should be a Node/PyMongo-specific option or aligned across drivers.

Implementation PR: https://github.com/mongodb/node-mongodb-native/pull/5078
Related (closed): NODE-1491