docs: Adds ADR 11. Course and block data

Author: pomegranited

docs/decisions/0011_course_block_data.rst

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+11. Course and block data
+#########################
+
+Status
+******
+
+Accepted
+
+Context
+*******
+
+Aspects needs a way to access up-to-date course and block metadata in order to display human-friendly labels and data to
+instructors in a way that makes sense in the context of their course. 
+
+The tracking event data that Aspects processes contains "object IDs", or string representations of UsageKeys that
+identify the piece of content the event relates to. But these UsageKeys are not very legible to humans, and the metadata
+for that content (display title of the block, title of the course it appears in, its position in the course, etc.) is
+part of the context where these events occur, and is not stored in the events themselves.
+
+Example:
+
+*"As a course author, I need to know how many times the videos in my course have been viewed so I can see which videos are
+the most helpful (or the most confusing)."*
+
+Aspects receives tracking events every time someone watches a video, and so the videos watched can be counted purely
+from these tracking events. However, videos which haven't been watched won't appear in the tracking events. To include
+these unwatched videos in our chart, Aspects needs a way to query a course for all the video blocks it contains.
+
+Open edX sends signals (which, confusingly, are also called "events", see OEP-41) when some actions are taken on the
+platform, including publishing a course, or updating a course outline. These signals can be used to tell Aspects when
+its course or block data needs to be updated.
+
+Decision
+********
+
+Use Open edX's "course published" signal to trigger updating course outline and block relationship data stored in
+Clickhouse. This approach is supported by `OEP-50 Hooks extension framework`_, and will be compatible with deployments
+that use the Event Bus to manage signals sent between applications.
+
+Consequences
+************
+
+#. Create an "event sink" which uses course and/or block signals from within the LMS/CMS to trigger course and block
+   data synchronization to Clickhouse.
+#. This "event sink" will run as a plugin on Open edX, and so has access to its models and data, which it can query and
+   package up for insertion in Clickhouse.
+#. Aspects will create custom views into this course and block data to ensure that the most relevant information is
+   easily available to the datasets.
+
+Rejected Alternatives
+*********************
+
+**Use course graph**
+
+Previously Aspects used the `coursegraph`_ application in Open edX as the source of truth for course outline and block
+relationships.
+
+The synchronization script had to be run regularly in order to keep the data up-to-date.
+
+However as the Aspects project has grown, course data isn't the only Open edX data that Aspects needs a copy of. So the
+more general solution of "event sinks" was preferred.
+
+References
+**********
+
+.. OEP-41 Asynchronous Server Event Message Format: https://open-edx-proposals.readthedocs.io/en/latest/architectural-decisions/oep-0041-arch-async-server-event-messaging.html
+.. OEP-50 Hooks extension framework: https://open-edx-proposals.readthedocs.io/en/latest/architectural-decisions/oep-0050-hooks-extension-framework.html
+.. OEP-52 Event Bus Architecture: https://open-edx-proposals.readthedocs.io/en/latest/architectural-decisions/oep-0052-arch-event-bus-architecture.html