Mark Payne created NIFI-5496:
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Summary: Remove finalizer from StandardProcessSession
Key: NIFI-5496
URL: https://issues.apache.org/jira/browse/NIFI-5496
Project: Apache NiFi
Issue Type: Improvement
Components: Core Framework
Reporter: Mark Payne
Assignee: Mark Payne
The NiFi API and documentation have always been pretty clear that Processors
are responsible for properly committing/rolling back their Process Sessions.
However, what happens when a Processor fails to do so?
The AbstractProcessor (from which 99+% of all Processors extend) will
automatically call ProcessSession.commit() if the subclass's onTrigger() method
returns successfully and call ProcessSession.rollback() if any Throwable is
thrown from the subclass's onTrigger().
However, those Processors that extend from AbstractSessionFactoryProcessor do
not have this protection. As a fail-safe, we have always had a finalizer in
StandardProcessSession. The finalizer simply calls rollback(). This ensures
that any FlowFiles that were not properly handled (transferred, removed, etc.)
are accounted for and re-queued/discarded.
Java 9 has now made finalizers deprecated, though. Additionally, through doing
some research, I have seen that the finalizer ends up being quite expensive in
the effects that it causes on the system's Garbage Collection. Specifically, GC
ends up reclaiming less and less and less heap until a point at which a lot
more is reclaimed. This results in a JVM that is sometimes "choppy" in its
responsiveness, which can lead to instabilities. (Will attach screenshots
showing this behavior).
Given this, and given that only a handful of Processors need to make use of the
AbstractSessionFactoryProcessor, we should remove the Process Session's
finalizer.
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