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1.因为没有配置dubbo请求连接超时时间，导致出现以下场景
因为没有配置dubbo请求连接超时时间，消费者请求到生产者之后，调用了查询语句，但是消费者端报错了。说连接超时异常，当再请求一次，发现又可以了。
com.alibaba.dubbo.rpc.RpcException: Failed to invoke the method getUserList in the service com.tscyq.shop.central.api.service.UserRemoteOpenService. Tried 1 times of the providers [172.16.50.109:20880] (1/1) from the registry 47.93.13.108:2181 on the consumer 172.16.50.109 using the dubbo version 2.5.3. Last error is: Invoke remote method timeout. method: getUserList, provider: dubbo://172.16.50.109:20880/com.tscyq.shop.central.api.service.UserRemoteOpenService?accesslog=true&anyhost=true&application=consumer-shop-admin&check=false&dubbo=2.5.3&interface=com.tscyq.shop.central.api.service.UserRemoteOpenService&methods=getUserList&pid=18980&retries=0&side=consumer×tamp=1529996384883, cause: Waiting server-side response timeout. start time: 2018-06-26 15:03:06.647, end time: 2018-06-26 15:03:07.649, client elapsed: 1 ms, server elapsed: 1001 ms, timeout: 1000 ms, request: Request [id=3, version=2.0.0, twoway=true, event=false, broken=false, data=RpcInvocation [methodName=getUserList, parameterTypes=[], arguments=[], attachments={path=com.tscyq.shop.central.api.service.UserRemoteOpenService, interface=com.tscyq.shop.central.api.service.UserRemoteOpenService, version=0.0.0}]], channel: /172.16.50.109:56368 -> /172.16.50.109:20880问题分析：由于查询耗时，导致请求超时，dubbo在没有配置请求超时时间的前提下，请求默认超时时间是1s，如果生产者和消费者都配置了超时时间，那么系统会以消费者的超时时间为准。第二次又可以了原因是因为mysql默认查询是有缓存的，第二次请求默认读取mysql缓存，没有超过1s就返回了，所以成功。
解决方案：
在对应的接口节点上配置timeout属性即可
<dubbo:reference id="userRemoteOpenService" interface="com.tscyq.shop.central.api.service.UserRemoteOpenService" retries="0" timeout="${dubbo-timeout}"/>