Replica Set Protocol Version

Preservation of Writes

Availability

pv1 is available in MongoDB version 3.2 or later and is the default for all new replica sets created with version 3.2 or later.

Arbiters

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 (no longer supported in MongoDB 4.0+) for replica sets with arbiters:

• MongoDB 3.4.1

• MongoDB 3.4.0

• MongoDB 3.2.11 or earlier

For the other versions of MongoDB that support pv1, pv1 does not increase the likelihood of w:1 rollbacks for replica sets with arbiters.

Priorities

For the following MongoDB versions, pv1 increases the likelihood of w:1 rollbacks compared to pv0 (no longer supported in MongoDB 4.0+) for replica sets with different members[n].priority settings:

• MongoDB 3.4.1

• MongoDB 3.4.0

• MongoDB 3.2.11 or earlier

For the other versions of MongoDB that support pv1, pv1 does not increase the likelihood of w:1 rollbacks for replica sets with different members[n].priority settings.

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, in MongoDB 3.6+ (as well as MongoDB 3.4.2+ and 3.2.12+), 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.

Modify Replica Set Protocol Version

Starting in version 4.0, MongoDB only supports replica set protocol version 1 (pv1).

However, MongoDB 3.2 through MongoDB 3.6 support replica set protocol version 1 and protocol version 0.

Before changing the protocol version for MongoDB 3.2 through MongoDB 3.6, ensure that at least one oplog entry (generated from the current protocol version) has replicated from the primary to all secondaries. To check, on each secondary, check the optimes.lastCommittedOpTime.t field returned from rs.status(). For example, connect mongosh to each secondary and run:

rs.status().optimes.lastCommittedOpTime.t

• If the current replica set protocol version is 0, the t is equal to -1.

• If the current replica set protocol version is 1, the t is greater than -1.

Once you have verified that at least one oplog entry (using the current protocol version) has replicated to all the secondaries, you can change the protocol version.

To change the replica set protocol version, reconfigure (rs.reconfig()) the replica set with the new protocolVersion. For example, to upgrade to pv1, connect mongosh to the current primary and perform the following sequence of operations:

cfg = rs.conf();
cfg.protocolVersion=1;
rs.reconfig(cfg);

You can use catchUpTimeoutMillis to prioritize between faster failovers and preservation of w:1 writes.