[jira] [Created] (NIFI-4994) Reload instance class loader at every component restart

["Koji Kawamura (JIRA)" <ji...@apache.org>]

Koji Kawamura created NIFI-4994:
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             Summary: Reload instance class loader at every component restart
                 Key: NIFI-4994
                 URL: https://issues.apache.org/jira/browse/NIFI-4994
             Project: Apache NiFi
          Issue Type: Improvement
          Components: Core Framework
            Reporter: Koji Kawamura
            Assignee: Koji Kawamura


h2. Current Class loader reload mechanism

NiFi framework provides RequiresInstanceClassLoading to annotate components (Processor, Controller Service or Reporting Task) to use per instance class loader. Instance class loaders are reloaded when associated components are restarted, if any component property flagged as 'dynamicallyModifiesClasspath' is updated. Such component property is used to let user specify additional class pass resource URLs. E.g. PutHDFS 'Additional Classpath Resources'.

h2. Limitation of current mechanism

Current mechanism requires component properties to trigger reloading to represent class path resource URLs. That works well in scenarios for users to add/del/mod additional class path resources.

However there is different needs for reloading instance class loader. That is re-initializing static state in dependencies.

For example, ReportLineageToAtlas reporting task uses Atlas client library, and the library has static code blocks to initialize its dependencies, such as Kafka client with parameters passed by NiFi reporting task based on user inputs. Once the initialization is done, there is no way to re-initialize those objects, meaning even if NiFi user changes reporting task configuration, those will not be able to reflected. 
https://github.com/apache/atlas/blob/master/notification/src/main/java/org/apache/atlas/hook/AtlasHook.java#L63

The only possible operations to reset such static state are, creating another instance, configure the instance again, remove the old one and start new one. Or restart NiFi process. This limitation affects UX negatively.

h2. Improvement design

Possible approaches are:

# Enhance RequiresInstanceClassLoading annotation so that custom components can specify whether it needs to be reloaded on every component restart without condition.
# Add new PropertyDescriptor instance field to denote that instance class loader needs to be reloaded if a property is changed. Similar to 'dynamicallyModifiesClasspath' but for configurations other than URI. Then, include those property value to AbstractConfiguredComponent.additionalResourcesFingerprint

No.1 is more naive approach, but require less framework code change. No.2 looks redundant, also could be error-prone, e.g. by forgetting to mark some properties.



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