Developer Tip: Why Did My Job Fail with Error Message “Class alluxio.hadoop.FileSystem not found”?

From time to time, a question pops up on the user mailing list referencing job failures with the error message “java.lang.ClassNotFoundException: Class alluxio.hadoop.FileSystem not found”. This post explains the reason for the failure and the solution to the issue when it occurs.
This error indicates the Alluxio client is not available at runtime. This causes an exception when the job tries to access the Alluxio filesystem but fails to find the implementation of Alluxio client to connect to the service.