[GitHub] [kafka] tvainika commented on pull request #10277: KAFKA-9914: Fix replication cycle detection

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tvainika commented on pull request #10277:
URL: https://github.com/apache/kafka/pull/10277#issuecomment-792942212


   > Seems harmless, but can you share the motivation behind this change? Did you come across some scenario where the existing logic didn't work?
   
   Yes, those are documented in the issue [KAFKA-9914](https://issues.apache.org/jira/browse/KAFKA-9914)
   
   The old code assumes each source or target Kafka cluster is referred with same name over all MM2 instances. If different names are used pointing to same Kafka cluster, 
   
   1. With one MM2 instance where both `kafka` and `backup` are actually same one Kafka cluster, e.g. mm2.properties like
   ```
   clusters=kafka,backup
   kafka.bootstrap.servers=XXX:9092
   backup.bootstrap.servers=XXX:9092
   kafka->backup.enabled=true
   ```
   will start create topic `kafka.X` for each topic `X` in the cluster, eventually leading to topic such as `kafka.kafka.kafka.kafka.X` etc.
   
   Even if one argues this is a misconfiguration, it still allows one to shoot themselves into the foot with infinite recursion.
   
   2. With two MM2 instances in two different data centers having mm2.properties configured using different named aliases to Kafka clusters
   mm2.properties in DC A
   ```
   cluster_A_A=XXX:9092
   cluster_A_B=YYY:9092
   cluster_A_A->cluster_A_B.enabled=true
   ```
   
   In DC B:
   ```
   cluster_B_A=XXX:9092
   cluster_B_B=YYY:9092
   cluster_B_B->cluster_B_A.enabled=true
   ```
   
   for topic `X` in Kafka cluster XXX, the replication flow will create `cluster_A_A.X` in Kafka cluster YYY, and second replication will create topic `cluster_B_B.cluster_A_A.X`, and then the replication flow of DC A's MM will create `cluster_A_A.cluster_B_B.cluster_A_A.X` etc.


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