MongoDB only supports replica set protocol version 1 (pv1). pv1 is the default for all new replica sets.
Preservation of Writes
w:1 Writes
With pv1, you can use catchUpTimeoutMillis to prioritize between faster failovers and preservation of w:1 writes.
w: "majority" Writes
pv1 guarantees the preservation of confirmed w: "majority" writes.
Availability
pv1 is available in MongoDB version 3.2 or later and is the default for all new replica sets.
Vetoes
pv1 does not use vetoes. Individual members can vote for or against a candidate in a particular election, but cannot individually veto (abort) an election unilaterally.
Detection of Simultaneous Primaries
In some circumstances, two nodes in a replica set may transiently believe that they are the primary, but at most, one of them will be able to complete writes with { w: "majority" } write concern. The node that can complete { w: "majority" } writes is the current primary, and the other node is a former primary that has not yet recognized its demotion, typically due to a network partition. When this occurs, clients that connect to the former primary may observe stale data despite having requested read preference primary, and new writes to the former primary will eventually roll back.
pv1 uses the concept of term. This allows for a faster detection of simultaneous primaries and for multiple successful elections in a short period of time.
Back to Back Elections
pv1 makes a "best-effort" attempt to have the secondary with the highest priority available call an election. This could lead to back-to-back elections as eligible members with higher priority can call an election.
However, for pv1:
Priority elections have been limited to occur only if the higher priority node is within 10 seconds of the current primary.
Arbiters will vote no in elections if they detect a healthy primary of equal or greater priority to the candidate.
Double Voting
pv1 prevents double voting in one member's call for election. This is achieved through its use of terms.