docs: ADR for pipeline choice

Author: bseverino

docs/decisions/01-pipeline-choice.rst

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+1. Pipeline Choice
+------------------
+
+Status
+------
+
+Accepted
+
+Context
+-------
+
+While migrating the analytics pipeline to a new technology, we'd like a toggle which:
+
+- Lets particular users in edX and maybe selected partner organizations see new data
+  for validation.
+- Is fast for us to flip back and forth to look at new vs. old, ideally without leaving
+  the client dashboard.
+
+We also need to make a decision on how to store the new data. In any case, this should
+involve a duplicate store to compare the old and new data.
+
+Store Options
+-------------
+
+**New tables in original store**
+
+- Least amount of configuration needed.
+- Duplicate tables in the same store may get messy. (IE CourseActivity vs V1CourseActivity?)
+
+**Separate store in the same instance**
+
+- Allows for complete duplication with the same table names.
+- Cleaner separation of data.
+
+API Options
+-----------
+
+**New V1 API**
+
+- Cleaner separation, but there will be duplication of code.
+- Easier for us to maintain the original API for Edge and Open edX. (and deprecate later,
+  if needed)
+- Simple to switch between; we can implement a check on the client dashboard to determine
+  which base URL to use.
+
+**Alter the existing API**
+
+- Less code duplication.
+- The check for which data to use will have to be passed down from the client to the API.
+
+Decision
+--------
+
+To house the new data, we will create a separate database in the same MySQL instance. We
+will access this database using a new V1 API, and implement a simple toggle on the client
+dashboard to switch between the two APIs.
+
+This toggle can be controlled by a query string (IE `?new_data=true`), and converted to
+a Django setting once the new API is fully rolled out.
+
+Consequences
+------------
+
+- We will be maintaining two databases and two APIs until the old API is retired (if at all).
+- We will rely on the client (in our case,
+  `edx-analytics-dashboard <https://github.com/edx/edx-analytics-dashboard>`_) in order to
+  determine which API to use.
+
+References
+----------
+
+- `Insights Replumbing <https://openedx.atlassian.net/wiki/spaces/PT/pages/3139764337/Insights+Replumbing>`_