The package org.apache.hadoop.hdfs.protocol.datatransfer.PipelineAck class is used in client module classes (e.g. DataStreamer$ResponseProcessor in DFSInputStream and DFSOutputStream). This jira tracks the effort of moving this class to hadoop-hdfs-client module.
We should keep the static attribute OOB_TIMEOUT and helper method getOOBTimeout in the hadoop-hdfs module as they're not used (by now) in hadoop-hdfs-client module. Meanwhile, we don't create the HdfsConfiguration statically if we can pass the correct conf object.
The checkstyle warnings can be addressed in HDFS-8979.