Summary:

On commands from mongos, mongod should return its lastCommittedOpTime (i.e. the timestamp from the opTime of its last committed write).

Design:

Inside appendReplyMetadata and appendReplyMetadataOnError in src/mongo/db/service_entry_point_mongod.cpp, check if the node's ShardingState is enabled (which means its in a sharded cluster) or that it is a config server, and get the lastCommittedOpTime from the replication coordinator and append it to the metadata BSONObjBuilder.

if (ShardingState::get(opCtx)->enabled() || serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
    auto lastCommittedOpTime =
        repl::ReplicationCoordinator::get(opCtx)->getLastCommittedOpTime();
    metadataBob->append(kLastCommittedOpTimeFieldName, lastCommittedOpTime.getTimestamp());
}

Open Questions:

• Should config servers return their lastCommittedOpTime?
  • Decision: Yes they should

POC: https://mongodbcr.appspot.com/188020001/