...

• remove DirectByteBufferStreamImpl  V1-V3, keep V4, only.
• remove all items from IgniteFeatures  and corresponding checks.
• remove all code and checks for GridContinuousProcessor#discoProtoVer
• TcpDiscoverySpi#setForceServerMode
• All old version of classes - keep only max from V2, V3, etc. versions. StartRequestV2 that belongs to internal communication.
  Classes releated to thin client, jdbc, odbc interaction must stay.
  • StartRequest must be deleted. Rename StartRequestV2 → StartRequest.
  • CacheMetricsSnapshot must be deleted. Rename CacheMetricsSnapshotV2 → CacheMetricsSnapshot.
• MessageFactory

Additional materials

Many distributed open source systems has rolling upgrade feature, already.
We must study implemented approaches to gain some insights from them.

...