THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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Implementing a CombinedKeySerde depends on the specific framework with json. A full key would look like this "{ "a" :{ "key":"a1" }, "b": {"key":"b5" } }" to generate a true prefix one had to generate "{ "a" :{ "key":"a1" }", which is not valid json. This invalid Json will not leave the jvm but it might be more or less tricky to implement a serializer generating it. Maybe we could provide users with a utility method to make sure their serde statisfies our invariants.
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