ZOOKEEPER-5014: Cache resolved IP addresses as fallback for DNS server failures

[hoyo1744]

Summary

Add DNS fallback feature to improve client resilience during DNS server outages.

JIRA

https://issues.apache.org/jira/browse/ZOOKEEPER-5014

Changes

• Add zookeeper.client.dnsFallback.enabled option (default: false)
• Cache resolved IP addresses in StaticHostProvider
• Use cached IP when DNS resolution fails (if enabled)

Testing

• Added unit tests for DNS fallback scenarios

[anmolnar]

I reject this patch. We've removed any sort of DNS caching from ZooKeeper previously. The DNS infrastructure already has in-built caching at many levels: DNS servers, operating systems, etc. all have caching enabled. DNS also supports multiple primary and secondary servers for high availability. Anything we add to ZooKeeper would be just confusing.

[hoyo1744]

Thank you for reviewing my PR and for the clear explanation.

I understand your reasoning. DNS infrastructure already provides caching at multiple levels (DNS servers, OS, etc.) and supports high availability through primary/secondary servers. Adding another caching layer in ZooKeeper would add unnecessary complexity.

I learned a lot from this contribution process - analyzing the ZooKeeper codebase, understanding the design decisions, and going through the Apache contribution workflow.

Thank you for your time and feedback!