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1. Is there rolling upgrade feature?
2. How it implemented?
3. How it tested?
4. When it tested: Each PR? Weekly? Only on release? 
5. Network message format.
6. Serdes implementation 

Apache Cassandra

1. Rolling upgrade implemented. Guide -
   1. https://www.datastax.com/learn/whats-new-for-cassandra-4/migrating-cassandra-4x
   2. https://docs.datastax.com/en/luna-cassandra/guides/upgrade/overview.html
2. Server-client compatibility works similar to Ignite Thin Client protocol.
3. 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.
4. On release or by request.
5. POJO
6. Internal serdes.??? 

Apache Kafka

1. Rolling upgrade implemented. Guide - https://kafka.apache.org/documentation/#upgrade
2. Message formats checked on PR reivew. 
   At a first glance, there are no special source code checks to ensure compatibility in day by day coding.
   But, all machinery to code compatible implemented in code generation framework - https://github.com/apache/kafka/blob/trunk/clients/src/main/resources/common/message/AlterPartitionResponse.json
3. Compatibility checked with the special test framework ducktest - https://github.com/apache/kafka/blob/trunk/tests/kafkatest/tests/core/kraft_upgrade_test.py
4. On release , or by request.
   Kafka doesn't provide public resources to run ducktests. Run done by contributors or by confluent employers on private hosts. 
5. POJO
6. Internal autogenerated serdes

Yugabyte

1. Rolling upgrade implemented. Doc - https://docs.yugabyte.com/preview/manage/upgrade-deployment/
2. It seems compatibility checked on review (see commit message section "Upgrade/Rollback safety") - https://github.com/yugabyte/yugabyte-db/commit/e9ab17dea0d3b4f1673531c07404a290f9fbd8f2
3. ???
4. ???
5. simple data structures
6. protobuf

YDB

1. Rolling upgrade implemented.
2. Message formats checked on PR rivew - grpc+protobuf helps to maintain compatibility.
3. Compatibility checked with the special test framework - https://github.com/ydb-platform/ydb/blob/main/ydb/tests/functional/restarts/
4. On request. 
5. simple data structures
6. protobuf

Cockroach DB

Alternative designs

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