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From "Jayson Minard (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-5964) Change TypeSerializers to allow construction of immutable types
Date Sat, 04 Mar 2017 13:05:45 GMT
Jayson Minard created FLINK-5964:

             Summary: Change TypeSerializers to allow construction of immutable types
                 Key: FLINK-5964
                 URL: https://issues.apache.org/jira/browse/FLINK-5964
             Project: Flink
          Issue Type: Improvement
          Components: Core
    Affects Versions: 2.0.0
            Reporter: Jayson Minard
            Priority: Minor

If your programming language has a lot of Immutable types (and with no default constructor)
Flink forces you to create new versions as read/write POJO otherwise the types are rejected
by the system.  In Kotlin for example, given a class and property values we can determine
which constructor to call and invoke it using knowledge of default values, nullable types
and which properties can be set in construction or after construction.

Flink provides no opportunity to use this model because Serializers are littered with calls
to `createInstance` that are not passed any values so have no opportunity to fully inflate
the object on construction.  

This means that when you use Flink you throw away maybe hundreds of state objects (my worst
case) and have to create Flink-only variations which becomes grunt work that adds no value.

Currently TypeExtractor isn't extendable, and all of the special cases are hard coded.  It
should be configured the order of checking for type information so that pluggable types can
be added into the chain of analysis.  For example before `analyzePojo` is called I could special
case a Kotlin class returning a different TypeInformation instance.  But I don't think that
solves the whole problem since other TypeSerializers make assumptions and call `createInstance`
on other TypeSerializers without knowing how they would want to do the construction (in the
Kotlin case it would be "tell me to construct my instance and give me the list of named fields
and serializers to get their values and let me decide what to do).

What is the best idea for this change?  With guidance, I can work on the PR.

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