...

• communication: Communication messages consists of two parts:
  
  • Message format: message format itself. Communication API should be reworked to force backward compatible messages.
  • User data: Message can store user data. User data format must be backward compatible.
• discovery:
  • Message format: message format itself. Communication API must be reworked to force backward compatible messages.
• binary marshaller:
  • Currently, binary marshaller code highly coupled with the other Ignite code. We must modularize Binary infrastructure (IEP-119 Binary infrastructure modularization) and provide compatibility guarantees for each part of it.
• features:
  • Framework to enable/disable features for mixed version clusters must be developed.
• affinity: 
  • Affinity function must the same for each online node version.

Additional materials

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

...