问题描述
在Java项目中，使用Redisson作为连接Redis的客户端，间歇性的出现了DNS Monitor throwable 错误。
DNSMonitor throwable="java.net.UnknownHostException: failed to resolve 'xxxxxx.redis.cache.chinacloudapi.cn'
Redisson版本为：
org.redisson:redisson:3.12.0
全部的异常信息为：
level=ERROR logger=org.redisson.connection.DNSMonitor throwable="java.net.UnknownHostException: failed to resolve 'xxxxxx.redis.cache.chinacloudapi.cn' after 2 queries at io.netty.resolver.dns.DnsResolveContext.finishResolve(DnsResolveContext.java:901) at io.netty.resolver.dns.DnsResolveContext.tryToFinishResolve(DnsResolveContext.java:860) at io.netty.resolver.dns.DnsResolveContext.query(DnsResolveContext.java:328) at io.netty.resolver.dns.DnsResolveContext.access$600(DnsResolveContext.java:63) at io.netty.resolver.dns.DnsResolveContext$2.operationComplete(DnsResolveContext.java:377) at io.netty.util.concurrent.DefaultPromise.notifyListener0(DefaultPromise.java:502) at io.netty.util.concurrent.DefaultPromise.notifyListeners0(DefaultPromise.java:495) at io.netty.util.concurrent.DefaultPromise.notifyListenersNow(DefaultPromise.java:474) at io.netty.util.concurrent.DefaultPromise.notifyListeners(DefaultPromise.java:415) at io.netty.util.concurrent.DefaultPromise.setValue0(DefaultPromise.java:540) at io.netty.util.concurrent.DefaultPromise.setFailure0(DefaultPromise.java:533) at io.netty.util.concurrent.DefaultPromise.tryFailure(DefaultPromise.java:114) at io.netty.resolver.dns.DnsQueryContext.setFailure(DnsQueryContext.java:220) at io.netty.resolver.dns.DnsQueryContext.access$300(DnsQueryContext.java:43) at io.netty.resolver.dns.DnsQueryContext$4.run(DnsQueryContext.java:170) at io.netty.util.concurrent.PromiseTask$RunnableAdapter.call(PromiseTask.java:38) at io.netty.util.concurrent.ScheduledFutureTask.run(ScheduledFutureTask.java:127) at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163) at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:405) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:500) at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:906) at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.lang.Thread.run(Thread.java:748) Caused by: io.netty.resolver.dns.DnsNameResolverTimeoutException: [/8.8.4.4:53] query timed out after 5000 milliseconds (no stack trace available) " thread=redisson-netty-4-21 message="Unable to resolve xxxxxx.redis.cache.chinacloudapi.cn" fluentd=517d053c90c2 tag=logback.aliClient.10.218.192.150
问题分析
根据错误提示，在Github上找到 Redisson 和 Netty 都存在相似的问题。最后都建议升级 Redisson 版本到最新，同时也把 Netty 的版本升级到最新。
Regression: DNS resolver failing to find valid DNS record ：https://github.com/redisson/redisson/issues/1625
DNS resolver failing to find valid DNS record ： https://github.com/netty/netty/issues/8261
如果升级比较复杂的话，也可以将redisson的dnsMonitoringInterval参数设置为-1。
参考资料
Regression: DNS resolver failing to find valid DNS record : https://github.com/redisson/redisson/issues/1625
DNS resolver failing to find valid DNS record ： https://github.com/netty/netty/issues/8261
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