Event message model

[krzysztof]

This is the description of the top-level event model (not specific to any payload).

Terms:

	Type	Description	Required
event_type	string	Controlled vocabulary: "relation_created", "relation_removed"	Yes
creator	string	"Zenodo", "ADS"	Yes
source	string	"Zenodo.Curation", "Zenodo.System", "ADS.Discovery"	Yes
payload	object	Object specific to the event_type.	Yes
id	string	Global identifier of the event (UUID)	Yes
time	string	describing the time of the event creation. This is the date at which the event was created on the participating systems, e.g. for Zenodo it would be the time at which the record was published on Zenodo, NOT when the event was sent or received	Yes

[krzysztof]

Some suggestions:
creator and source could be merged into one structured field, e.g.: "source": {"creator": "Zenodo", "type": "SystemEvent"}. The necessity of this field is up for discussion. The reason for this field is to describe how given event was created, as to allow for better interpretation by third parties, e.g.: events of type "ADS.Discovery" might come from a heuristic algorithm, while "Zenodo.Curation" might come from the user-updated metadata. Those might have different levels of "trust" or accuracy.