Today, RouterServer is a role part of all nodes in a sharded cluster. The only possible combinations are:

{ ShardServer, RouterServer } => Shard
{ ShardServer, ConfigServer, RouterServer } => Config / Config-Shard
{ RouterServer } => MongoS

With that, there are a lot of outdated calls in the codebase that do not rely that a node, part of a sharded cluster, has always a RouterServer. In the codebase this is checked as `serverGlobalParams.clusterRole.has(ClusterRole::RouterServer)`.

It is important to point out that checking for `serverGlobalParams.clusterRole.hasExclusively(ClusterRole::RouterServer)` has a different meaning, as we are asking if that node is a MongoS.

Goal

We need to either remove RouterServer checks or change them to check if we are in a sharded cluster.

References

 - Example 1
 - Example 2

Estimation

 - User impact: Improvement.
 - Raw Estimation (man-day): 2