Details
Description
Description
FCV downgrade from 4.4 to 4.2 does a reconfig automatically and waits for the new config to replicate on all nodes in the replica set, including non-voting nodes and arbiters.
Description of Linked Ticket
Safe reconfig introduces a new field on the config which will make 4.2 binary to fail to start up. Now that we allow 4.4 binary to use and store a config with the new field. We cannot just remove the extra field on downgrade on each node as in SERVER-45092, since in mixed version replset, a reconfig can propagate the config with the extra field to 4.4 binary with FCV 4.2. That will fail 4.2 binary after downgrade.
Instead, we'll do a reconfig with a higher config version but no term if the last config has a term field with the same configuration and wait for it to replicate to all nodes before downgrading FCV.
Also remove the FCV check in ReplSetHeartbeatArgsV1::addToBSON, we don't need to check FCV to decide whether to include "configTerm" in heartbeats.
Scope of changes
Impact to Other Docs
MVP (Work and Date)
Resources (Scope or Design Docs, Invision, etc.)
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Issue Links
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SERVER-46288 Reconfig in 4.2 style with the current config on FCV downgrade
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