MongoDB is a document-oriented database with a similar distribution design to Redis. In a replica set, there exists a single writable primary node which accepts writes, and asynchronously replicates those writes as an oplog to N secondaries. However, there are a few key differences.
First, Mongo builds in its leader election and replicated state machine. There’s no separate system which tries to observe a replica set in order to make decisions about what it should do. The replica set decides among itself which node should be primary, when to step down, how to replicate, etc. This is operationally simpler and eliminates whole classes of topology problems.