[docs] take care of index changes when upgrading from 0.x

doc/upgrading-from-0.x.md

@@ -0,0 +1,70 @@
+# Upgrading Server CE/Pro with a DB that was used since Server CE/Pro version 0.x
+
+Thank you for your continued trust in Overleaf as a long term customer of Server Pro!
+
+> **Note**
+> For reference Server CE/Pro version 1.x was released in 2017.
+>
+> If you are still using 0.x and plan to upgrade, please do so one major version at a time
+(e.g. from 0.6.3 to 1.24, then 2.7.1, then 3.y).
+
+## Potential issues when upgrading to version 3.x
+
+### Duplicate indexes
+
+> MongoError: Error during migrate "20190912145023_create_projectInvites_indexes": Index      with name: expires_1 already exists with different options
+
+Early versions of Server CE/Pro had their indexes defined inline in the model layer of the application.
+The indexes were automatically created/updated as needed by an application framework.
+
+We have since moved to explicit creation/removal of indexes as part of migration scripts.
+Some of these very early indexes were not taken into account when writing these new migrations.
+
+The `expires_1` index in the `projectInvites` collection is one of these old indexes that needs to be recreated with different parameters.
+It is safe to delete it when the application (Server CE/Pro) is not running.
+
+```shell
+# Change the directory to the root of the toolkit checkout. E.g.:
+# cd ~/toolkit
+
+# Stop the application
+bin/docker-compose stop sharelatex
+
+# Check the existing indexes
+echo 'db.projectInvites.getIndexes()' | bin/docker-compose exec -T mongo mongo --quiet sharelatex
+# Expected output:
+#    [
+#            {
+#                    "v" : 2,
+#                    "key" : {
+#                            "_id" : 1
+#                    },
+#                    "name" : "_id_"
+#            },
+#            {
+#                    "v" : 2,
+#                    "key" : {
+#                            "expires" : 1
+#                    },
+#                    "name" : "expires_1",
+#                    "expireAfterSeconds" : 2592000,
+#                    "background" : true
+#            }
+#    ]
+
+# Also check for completed migrations
+echo 'db.migrations.count({"name":"20190912145023_create_projectInvites_indexes"})' | bin/docker-compose exec -T mongo mongo --quiet sharelatex
+# Expected output:
+#    0
+
+# If the output of any of the two above commands does not match, stop here.
+# Please reach out to support@overleaf.com for assistance and provide the output.
+
+
+# If the output matches, continue below.
+# Drop the expires_1 index
+echo 'db.projectInvites.dropIndex("expires_1")' | bin/docker-compose exec -T mongo mongo sharelatex
+
+# Start the application again
+bin/docker-compose start sharelatex
+```