docs: ADR for tagging service.

docs/decisions/0004-content-tagging.rst

@@ -0,0 +1,45 @@
+4. Content Tagging
+==================
+
+Context
+-------
+
+Tagging content is central to enable content re-use, facilitate the implementation of flexible content structures different from the current implementation and allow adaptive learning in the Open edX platform.
+
+Content tagging should be classified as "kernel" component following `OEP-57's <https://docs.openedx.org/projects/openedx-proposals/en/latest/processes/oep-0057-proc-core-product.html#kernel>`_ guidelines, as a baked-in feature of the platform.
+
+Decision
+--------
+
+Implement the new tagging service as a pluggable django app, and installed alongside learning-core as a dependency in the edx-platform.
+
+Tagging data models will follow the guidelines for this repository, and focus on extensibility and flexibility. 
+
+Since some use cases for content tagging are not considered "kernel" (like providing data for a marketing site), a generic mechanism to differentiate those uses cases will be built, and proper Python and REST APIs will be provided, to different taxonomies/tags for the same content. 
+
+
+Rejected Alternatives
+---------------------
+
+Implementing tagging as Discovery service plugin
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Discovery is already used as the source of truth place for course/program/block metadata in the previous implementations, so keeping it here means keeping the platform metadata management "consistent" (specially since it needs to work with all kinds of contents - from atomic learning units all the way up to courses).
+
+Concerns:
+
+#. The course discovery repository is not well documented and has code that is only used by edX/2U.
+#. Some implementations are tied to specific closed-source services (example: `openedx/taxonomy-connector <https://github.com/openedx/taxonomy-connector>`_ uses a 3rd party closed-source service to tag video blocks automatically).
+#. The data flow from the discovery service is confusing, and evolved as the needs for that repo changed. `Reference <https://discuss.openedx.org/t/when-to-make-a-new-backend-service/8267/5>`_.
+#. Many people prefer/need to run the platform without the discovery service.
+
+Implement tagging as a new IDA
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A new IDA dedicated for tagging would represent a break from the current codebases and force implementation using REST APIs.
+This option adds extra complexity without a good compelling reason for it.
+
+Concerns:
+
+#. One more app to host: maintenance cost increases over time.
+#. Extra response time between services when handling synchronous operations related to tagging.