Docs: Describe the basic principles of Eiffel events Solves: Jira GER-2365 Change-Id: Ibc6c57d6509d30f5d9c923e2efb799828d970ba4

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+# Eiffel events basics
+
+Eiffel events describes a DAG. The link is the meta.id of the downstream event.
+This means, like with git, that you cannot create a node (event) before all
+previous, parent, events are created.
+
+## Eiffel events produced by the plugin
+
+|Event|Gerrit Counterpart|Description|
+|-----|------------------|-----------|
+|[SourceChangeCreated (SCC)](https://github.com/eiffel-community/eiffel/blob/master/eiffel-vocabulary/EiffelSourceChangeCreatedEvent.md)|Patchset Created|Declares that a change request to sources has been created.|
+|[SourceChangeSubmitted (SCS)](https://github.com/eiffel-community/eiffel/blob/master/eiffel-vocabulary/EiffelSourceChangeSubmittedEvent.md)|Ref-Update (branch)|Declares that a commit has been integrated into a source branch|
+|[ArtifactCreated (ArtC)](https://github.com/eiffel-community/eiffel/blob/master/eiffel-vocabulary/EiffelArtifactCreatedEvent.md)|Ref-Update (tag)|Used together with CD to represent at tag (see example below)|
+|[CompositionDefined (CD)](https://github.com/eiffel-community/eiffel/blob/master/eiffel-vocabulary/EiffelCompositionDefinedEvent.md)|Ref-Update (tag)|Used together with ArtC to represent at tag (see example below)|
+
+### Tag representation
+
+Eiffel currently [edition Arica](https://github.com/eiffel-community/eiffel/tree/edition-arica) doesn't have an event to represent a tag. Instead a combination of
+ArtC and CD events are used to represent the tag and reference the SCS.
+
+![Tag events](./images/tag-created-events.png)
+
+## EiffelSourceChange event quirks
+
+Unlike git commits EiffelSourceChange events (SCC, SCS) contains a reference to
+the branch.
+
+This has the unfortunate effect that creating a branch in git is simply to create
+a reference when in Eiffel you'll need to create 2 * NumberOfCommits events to
+represent the branch creation.
+
+Another unfortunate side-effect is that even though git branches and git tags
+are orthogonal and live in separate spaces you cannot (currently) represent the
+creation of a git tag without referencing a SCS that in turn references a branch.
+
+Since the current source-code events are modeled around Gerrit workflow it's a
+requirement that each event that models an integration (SCS) must have a
+corresponding SCC event. This has the unfortunate effect that in order to model
+a direct push (integration from upstream or a newly imported project) you will
+need to fake a change upload (SCC) for each commit even though no such change
+was created.
+
+There are ongoing efforts in the Eiffel community to remedy these shortcomings.
+
+## Consistency vs performance
+
+For complete consistency we wouldn't be able to create a child event until the
+parent event is created and published to the Eiffel infrastructure.
+For performance reasons this isn't plausible, we need to keep the newest part
+of the DAG in memory in the plugin waiting to be published.
+
+Therefore an Eiffel event can have three states within the scope of the plugin:
+* Non existent.
+* Created (living in memory waiting to be published).
+* Published.
+
+The plugin takes all three states into consideration when determining which
+events needs to be created.
+
+## Identifying missing events
+
+In these examples creation is triggered for commit C5 which has a commit
+history like:
+
+![Commit graph](./images/commit-graph.png)
+
+### SCC
+
+SCCs should mirror the commit graph where the parent(s) links are replaced with
+PREVIOUS_VERSION link(s).
+
+![SCC events](./images/scc-events.png)
+
+To find commits that aren't yet represented by a SCC event we walk the commit
+graph until we find a commit where the corresponding events are already created
+for all parent commits (C4 in this case).
+
+From the image we can see that SCC1 is published to Eiffel whereas SCC2 and
+SCC3 are created but only live in memory in the event-hub in the plugin.
+
+### SCS
+
+SCSs are slightly more complicated since it, apart from links to parent events,
+has a link to the SCC of the change that was submitted.
+
+We create SCCs for the commit first (see above).
+After which we identify the commits that aren't yet represented by a SCS and
+create SCS events for them too.
+
+![SCS events](./images/scs-events.png)

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