Force a Member to Become Primary when Primary fails in mobgodb
If you have 2 members in a set, and one of them is unhealthy then you cannot elect a primary.  There must be >50% (a strict majority) to elect a primary, with 1/2 nodes up you cannot satisfy that condition, you can only ever get 50% exactly, which is not sufficient.  Your only option is to restore the other node or to add another node (perhaps an arbiter). However, adding a node will require a forced reconfiguration because you do not currently have a primary. Fortunately the procedure is well documented here.