上一篇logstash-output-jdbc使用中提到“運行bin/logstash -f test.conf時可能提示注冊插件失敗”，通過分析詳細的錯誤日志，發現其赫然寫着“connection is not available,request time out after 10000ms”，原來是mysql連接的問題，插件注冊失敗只是表象。
由於沒有在logstash所在的機器A安裝mysql，而是通過mysql connector 的jar包（logstash conf文件里指定了jar報路徑），所以沒有辦法測試A到數據庫機器B的連接是否成功，嘗試過增加mysql connection_timeout的時間到300s，發現錯誤還是request time out after 10000ms，說明jar里有自己的時間設置。
百度良久，最后在mysql的my.ini文件（默認C:\PragramData\MySQL\MySQLServer 5.7目錄下）中增加skip-name-resolve，重啟mysql服務后，問題解決。原因是
MySQL處理用戶連接時進行如下過程來驗證用戶的合法性（參考http://www.cnblogs.com/timeship/archive/2013/02/28/2937730.html）：
When a new client connects to mysqld, mysqld spawns a new thread to handle the request. This thread first checks whether the host name is in the host name cache. If not, the thread attempts to resolve the host name: The thread takes the IP address and resolves it to a host name (using gethostbyaddr()). It then takes that host name and resolves it back to the IP address (using gethostbyname()) and compares to ensure it is the original IP address.