I think there are enough instances of this pattern to warrant work to provide a uniform means of achieving this.

These are some existing means of linking mongos and mongod logic separately, along with common logic in mongos and mongod

• mongod_main.cpp and mongos_main.cpp
  • Typically for code that only starts up in one or the other, not common code
• Interface with factory construction method like query’s MongoProcessInterface.
• ServiceContext has ConstructorActionRegisterer.
  • Keeps code separate in their own files, don’t need factory classes.

Would be great to have a unified means of doing this. There are already a lot of separate instances.

Some existing command interfaces:

• DBDirectClient
• ShardRegistry
• ConfigShard