问题描述

zuul1.x使用过程中，偶尔会出现failed to respond的异常信息，对应的异常为httpclient 的 NoHttpResponseException。

解决方案

zuul作为client方连接nginx代理的服务，对应的设置为：

zuul.host.max-per-route-connections=600
zuul.host.socket-timeout-millis=10000
zuul.host.connect-timeout-millis=10000
zuul.host.time-to-live=600000

而nginx设置的keepalive_timeout=180s，所以当空闲达到180s时，服务端已经断开了连接。这时候httpclient在拿到pool里的connection时就有可能出现NoHttpResponseException。

应该将zuul.host.time-to-live设置为 <= nginx的keepalive_timeout。
另外zuul.host.max-per-route-connections应该根据实际情况分配，不宜过大。

但是经过测试，只是偶尔会出现，httpclient内部也在使用各种机制保证connection在断开之后能继续重连。

Most likely persistent connections that are kept alive by the connection manager become stale. That is, the target server shuts down the connection on its end without HttpClient being able to react to that event, while the connection is being idle, thus rendering the connection half-closed or 'stale'. Usually this is not a problem. HttpClient employs several techniques to verify connection validity upon its lease from the pool. Even if the stale connection check is disabled and a stale connection is used to transmit a request message the request execution usually fails in the write operation with SocketException and gets automatically retried. However under some circumstances the write operation can terminate without an exception and the subsequent read operation returns -1 (end of stream). In this case HttpClient has no other choice but to assume the request succeeded but the server failed to respond most likely due to an unexpected error on the server side. The simplest way to remedy the situation is to evict expired connections and connections that have been idle longer than, say, 1 minute from the pool after a period of inactivity.