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

• Is there rolling upgrade feature?
• How it implemented?
• How it tested?
• When it tested: Each PR? Weekly? Only on release? 

Apache Cassandra

Has rolling upgrade feature.
Server-client compatibility works similar to Ignite 
Compatibility checked with the special test framework - https://github.com/apache/cassandra-dtest/blob/trunk/upgrade_tests/README.md
At a first glance, there are no special source code checks to ensure compatibility in day by day coding. 

Apache Kafka

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Yugabyte

Cockroach DB

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Alternative designs

• subsystem API versions (like in REST API)
• runtime component(IgniteProcessor, IgniteManager) upgrade with the dynamic class loading.
• ability to run tests with random node versions
• patterns for implementing commons cases: new version of algorithmes, testing against new versions.