docs: add mongo 4.2 upgrade ADR

documentation/decisions/0001-mongo-4.2-upgrade.rst

@@ -0,0 +1,23 @@
+Mongo 4.2 Upgrade
+--------------------------------
+Status
+=====
+Accepted
+
+Context
+=======
+
+The Open edX community relese Maple will be released on December 9th 2021. Mongo version 4.0 security support will be ended on 30 Apr 2022 ( https://endoflife.date/mongodb ) and Mongo 4.0 End-of-Life (EOL) is before we move for a new Open edX release after Maple. 
+Maple should be running with supported version of Mongo during its lifecycle.
+
+Decision
+========
+
+Initial plan was to upgrade Mongo to 4.4.As, we are running mongo community edition in all our environments and mongo cloud manager is breaking the support of incremental backup starting from Mongo 4.2 with FCV (4.2). To run backup and restore for MongoDB 4.2 or later with FCV 4.2 or later Mongo should be running with MongoDB enterprise edition and must run with MongoDB agent on every node of cluster. Because of these requirements and limitations we have decided to upgrade MongoDB to 4.2 with FCV 4.0.
+
+Requirements
+===========
+
+* To upgrade an existing MongoDB deployment to 4.2, earlier version should run Mongo 4.0-series.
+* MongoDB driver should compatible for MongoDB 4.2. 
+  Check python driver (pymongo) compatibility for edx-platform (https://docs.mongodb.com/drivers/pymongo/#compatibility) and ruby driver (mongoid) for cs_comment_service here (https://docs.mongodb.com/mongoid/master/reference/compatibility/#mongodb-server-compatibility)