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From Greg Zoller <gzol...@gmail.com>
Subject Re: Deep polymorphic type support
Date Fri, 08 Sep 2017 20:09:15 GMT
Thanks, Dan.  I'm not 100% clear how Union would work for Scala marker
traits.  I could see how I'd use them for Message[Alarm], where Alarm is a
first-class object.  I've got 2 layers of marker traits in the middle tho:
 Message[PayloadKind], where PayloadKind is a trait having no fields, Event
is a trait having no fields that extends PayloadKind, and finally Alarm
(concrete class) extends Event.

I guess for pure Avro you could flatten things to just Message[Alarm], but
this weakens the domain model.

Can you think of a way to use Union types to represent marker traits?  My
understanding is Unions can't immediately contain other Unions (i.e. like
Event -> PayloadKind).

Greg

On Fri, Sep 8, 2017 at 1:56 PM, Dan Schmitt <dan.schmitt@gmail.com> wrote:

> Sounds like a job for a Union for payload.
>
> And if the polymorphic payload is avro serialized you don't
> have to know the schema ahead of time, you just need a
> way to merge the new payload schema into the union as
> you get it (so you can push the schema responsibility to
> the client code giving you the event.)
>
> I've been futzing with something similar on the C++ avro side
> and the provided API needs some tweaks to make it go (I'm
> looking at moving the fileReader/Wrtier up to work with
> streams, then you can do memory array with the array stream
> adapter; and then you need a way to get to addField for
> the UnionSchema (which on the C++ side only has default
> constructor, no way to access a Union somewhere in a schema
> and have addField add the schema from the new data
> without knowing a lot about the internals, so you either have
> to save it when first creating the schema, and can't do it from
> an existing schema, or need more code.)
>
>      Dan S.
>
>
>
>
>
>
>
> On Fri, Sep 8, 2017 at 2:35 PM, Greg Zoller <gzoller@gmail.com> wrote:
> > Hello,
> >
> > I'm trying to learn if Avro currently supports deep polymorphic types
> like
> > outlined here:
> > https://stackoverflow.com/questions/46120578/how-might-
> i-represent-a-scala-trait-in-avro
> >
> > If not, could the standard be extended?
> >
> > The use case is this... I want to send an AlarmMessage, serialized as a
> > Message[PayloadKind].  The receiving side would have enough type data to
> > reconstruct the AlarmMessage (Alarm is of type Event, which is of type
> > PayloadKind).  In other words I'd "listen" for Message[PayloadKind] and
> be
> > handed an AlarmMessage.
> >
> > Of course we'd need to have schema entries for any kind of Event (or
> > PayloadKind child for that matter), but there'd also need to be a way to
> > represent these intermediate marker traits.
> >
> > Thoughts?
> > Greg
> >
> > (A trivial json example of what I mean can be found here:
> > https://github.com/gzoller/ScalaJack/blob/master/doc/classesAndTraits.md
> )
>

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