Codec: Fix EventId, Require timestamp

[bartelink]

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[bartelink]

@nordfjord This exposes the ctors for EventData and TimelineEvent (which I'll migrate Equinox to use in order to make as many field mappings as possible explicit)

Added characterization tests for the unwritten 'obvious' spec of EventData/TimelineEvent.Create

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Regarding the suggestions you had for changing mapCausation result type...

Right now each of the Codec.Create overloads progressively removes options from the table.

I was considering removing the Guid, but not sure that'd serve your usage needs directly anyway, while also being a breaking change)?

The basic thinking for the mpaCausation signature is that any of a) the Event b) the Meta c) Context might be used to derive a deterministic value for the EventId guid. Having that be present in the signature is intended to convey those (potential) relationships.

Now would definitely be a good time to add an overload (and/or tweak a signature, if that's ever to be done) if we can come up with a useful one (and, ideally, get it seconded by someone else that's given this some thought and/or used it in a real system)

[nordfjord]

These changes look good to me!

Definitely appreciate the change to make the timestamp required in the bottom codec constructor.

Regarding the suggestions you had for changing mapCausation result type...

For the record (the suggestion is on the discord, but might be good to have it here), my suggested overload was:

static member Create<'Contract, 'Context when Contract :> TypeShape.UnionContract.IUnionContract>(
  getCausation,
  ?options,
  ?rejectNullaryCases
) =
  let up = snd
  let down event = event, None, None
  let mapCausation (ctx,_) =
    let causation, correlation = getCausation ctx
    ctx, (Guid.NewGuid()), causation, correlation
  FsCodec.Core.Codec.Create(mkEncoder options, up, down, mapCausation, rejectNullaryCases)

My preference would be not needing getCausation at all but rather to chuck the context into meta by default.

This is based on the context I'm in a the moment where 3 things hold true

1. My decisions are made in a context
2. My store doesn't have special handling of correlation and causationId's
3. I don't have event specific metadata except in very rare cases

When the above hold true then by default I create a codec like this:

let codec =
  FsCodec.SystemTextJson.Codec.Create<Event,Event,Context,Context>(
    snd,
    (fun ev -> ev,None,None),
    (fun (ctx, _) -> ctx,Guid.NewGuid(),"","")
  )

Which is very close to what the proposed overload does. Hope this helps 🙏

[bartelink]

Thanks for sharing the full context here - hopefully things will eventually converge...

When the above hold true then by default I create a codec like this ...

You're not wrong about it being ugly - I'd consider making it more explicit by actually using the lower level overload https:/jet/FsCodec/blob/master/src/FsCodec.SystemTextJson/Codec.fs#L47 like so:

 FsCodec.SystemTextJson.Codec.Create<Event,Event,Context,Context>(
    (fun (_raw, e) -> e),
    (fun (ctx, e) -> e, ValueSome ctx, Guid.NewGuid(), null, null, DateTimeOffset.UtcNow
  )