Longping Jie created HBASE-30376:
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Summary: Tolerate transient DNS server failures with an opt-in
client-side DNS resolution cache (serve-stale fallback) for RPC connections
Key: HBASE-30376
URL: https://issues.apache.org/jira/browse/HBASE-30376
Project: HBase
Issue Type: Improvement
Components: Client
Affects Versions: 2.6.7
Reporter: Longping Jie
Assignee: Longping Jie
**Problem**
The client resolves RegionServer/Master hostnames via `java.net.InetAddress`
whenever a new RPC connection is established, and twice more in the Kerberos
security preamble (a second forward lookup plus a PTR lookup for principal
`_HOST`). When the DNS server is intermittently unstable, new connections fail
with `UnknownHostException` until DNS recovers (retry window is only tens of
seconds), the blocking lookup stalls the Netty event loop and all connections
sharing it, and Kerberos connections fail on PTR lookups even when TCP connect
succeeded.
**Fix**
Add an opt-in cache in hbase-client with serve-stale semantics: live DNS always
first; on resolution failure fall back to the last successful mapping,
age-bounded (default 10 min) and kept fresh by a daemon background refresher
(degraded hosts re-probed every 2s); when no usable cache exists, throw the
original `UnknownHostException` unchanged. Two hook points only
(`RpcConnection#getRemoteInetAddress`, `RpcConnection#getServerPrincipals`), so
Kerberos and non-Kerberos behave the same. No server-side or wire-protocol
changes, no new dependencies, JDK 8/17 compatible. Gated by
`hbase.ipc.client.dns.fallback-cache.enabled` (default `false` = original code
path).
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