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From "Tzu-Li (Gordon) Tai (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (FLINK-6363) Document precedence rules of Kryo serializer registrations
Date Sun, 23 Apr 2017 10:30:04 GMT

     [ https://issues.apache.org/jira/browse/FLINK-6363?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Tzu-Li (Gordon) Tai updated FLINK-6363:
---------------------------------------
    Description: 
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 default serializer by instance has a higher precedence
than defining by class. Since the existence of this precedence is not due to Kryo's behaviour,
but due to the applied ordering in {{KryoSerializer}}, users that are familiar with Kryo will
be surprised by the unexpected results.

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}

  was:
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 default serializer by instance has a higher precedence
than defining by class. Since the existence of this precedence is not due to Kryo's behaviour,
but due to the applied ordering in {{KryoSerializer}}, even users that are familiar with Kryo
will be surprised with unexpected results.

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}


> 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 default serializer by instance has
a higher precedence than defining by class. Since the existence of this precedence is not
due to Kryo's behaviour, but due to the applied ordering in {{KryoSerializer}}, users that
are familiar with Kryo will be surprised by the unexpected results.
> 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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