James Clampffer created HDFS-9616:
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             Summary: libhdfs++ Add runtime hooks to allow a client application 
to add low level monitoring and tests.
                 Key: HDFS-9616
                 URL: https://issues.apache.org/jira/browse/HDFS-9616
             Project: Hadoop HDFS
          Issue Type: Sub-task
            Reporter: James Clampffer
            Assignee: James Clampffer


It would be nice to have a set of callable objects and corresponding event 
hooks in useful places that can be set by a client application at runtime.  

This is intended to provide a scalable mechanism for implementing counters 
(#retries, #namenode requests) or application specific testing e.g. simulate a 
dropped connection when the test system running the client application requests.

Current implementation plan is a struct full of callbacks (std::functions) 
owned by the FileSystemImpl.  A callback could be set (or left as a no-op) and 
when the code hits the corresponding event it will be invoked with a reference 
to the object (for context) and each method argument by reference.  The 
callback returns a bool: true to continue execution or false to bail out of the 
calling method.



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