[jira] [Created] (FLINK-6363) Document precedence rules of Kryo serializer registrations

["Tzu-Li (Gordon) Tai (JIRA)" <ji...@apache.org>]

Tzu-Li (Gordon) Tai created FLINK-6363:
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             Summary: Document precedence rules of Kryo serializer registrations
                 Key: FLINK-6363
                 URL: https://issues.apache.org/jira/browse/FLINK-6363
             Project: Flink
          Issue Type: Improvement
          Components: Documentation, Type Serialization System
            Reporter: Tzu-Li (Gordon) Tai
            Assignee: Tzu-Li (Gordon) Tai


Currently, there is no documentation / Javadoc mentioning the precedence rules of Kryo registrations via the register methods in {{StreamExecutionEnvironment}} / {{ExecutionEnvironment}}.

It is important for the user to be notified of the precedence because the {{KryoSerializer}} applies the configurations in a specific order that is not visible from the public API.

For example:
{code}
env.addDefaultKryoSerializer(SomeClass.class, SerializerA.class);
env.addDefaultKryoSerializer(SomeClass.class, new SerializerB());
{code}

from this API usage, it may seem as if {{SerializerA}} will be used as the default serializer for {{SomeClass}} (or the other way around, depends really on how the user perceives this).

However, whatever the called order in this example, {{SerializerB}} will always be used because in the case of defining default serializers, due to the ordering that the internal {{KryoSerializer}} applies these configurations, defining serializer instance has a higher precedence than defining with serializer class.

These methods are also subject to the same issue:
{code}
env.registerType(SomeClass.class, SerializerA.class);
env.registerTypeWithKryoSerializer(SomeClass.class, SerializerA.class);
env.registerTypeWithKryoSerializer(SomeClass.class, new SerializerB());
{code}



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