Durability is about whether the data was persisted to durable media before control was returned. Eventual consistency in a replica set has a document that are composed to each and every servers, to the primaries and secondaries before the control was returned to the application. For completing the application before control is return to the application, issue an update command or a write command to the primary and control is not returned to document and if all the process work properly then the durable and constant undergo properly.
MongoDB will take that update and apply it to the primary,replicate it to the secondaries and so on every thing will be consistent.There is a slight chance, however,if a primary fails before before the document has been made durable on the disk and that record will be lost. MongoDB then continues and replicates to the rest of the secondaries, but the control has already been returned to the application.