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Add event 'ordinal' field #191

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Add event 'ordinal' field #191

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8 changes: 8 additions & 0 deletions spec.md
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Expand Up @@ -318,6 +318,14 @@ that contains both context and data).
* Constraints:
* REQUIRED
* MUST be a non-empty string

### ordinal
* Type: `String`
* Description: Value expressing the relative order of the event. This enables
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@duglin duglin May 24, 2018

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Would we need to say what its relative to? Also, are there any uniqueness constraints? Could a source just put "1" for all events and still be compliant?

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@BenBeattieHood BenBeattieHood May 24, 2018

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@duglin - good call, I should've included that it must be increasing contiguously

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@jroper jroper May 25, 2018

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I think requiring it to increase contiguously would be too limiting, not all sources of events could guarantee that - particularly if you had something that was filtering events, to output a contiguous sequence, it would need to keep track of all ordinal sets in order to renumber the ordinal field and have to implement atomic increments of the field etc. In contrast, if it's monotonically increasing, then such a filter could be stateless.

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@BenBeattieHood BenBeattieHood May 25, 2018 •
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If you're filtering events, then you need to be projecting to a new stream, something like:
event -> consumer -> command (incl validation) -> new_event
because your validation and reprojection changes. Filtering and writing directly into a new stream without validation (unless an explicit pass-thru) would break the stream's truth.
Contiguous events are simple to do if you're validating events - events can't validate without a validating state, and a validating state represents a projection at an ordinal (..ie so you already need to have the ordinal).

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@BenBeattieHood BenBeattieHood May 25, 2018

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Another way of thinking about it is that one can filter commands, but not events.

interpretation of data supercedence.
* Constraints:
* REQUIRED
* MUST be a non-empty lexicographically-orderable string

### source
* Type: `URI`
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