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Original Description

Currently we have the documents to upgrade from SCCC to CSRS. However, we don't have a full document on downgrading from CSRS to SCCC.

Some steps are documented in the downgrade document (downgrade from 3.2 to 3.0) - Prepare CSRS Config Servers for downgrade. However this is not a complete list. It would be nice if we can document the full steps. The steps look like:

1. Disable the Balancer..
2. Check the minOpTimeUpdaters value..
3. Prepare CSRS Config Servers for downgrade..
   • If there is any requirement to rollback to the original servers/MMAPv1, after all the nodes in the cluster are stopped in Step c and before Step d to start the SCCC members, dump the config database from a CSRS member (running on standalone mode) and restore to the SCCC members with --drop option.
4. Clear per-shard sharding recovery information..
   • Note that if the cluster is running with authentication enabled, it's needed to grant the proper privilege to the administrator user, in order to remove the minOpTimeRecovery document from the admin.system.version collection. Please replace USER with the proper administrator user.

     use admin;
     
     db.createRole({
       role: "downgrade_csrs",
       privileges: [
          { resource: { db: "admin", collection: "system.version"}, actions: [ "remove" ] },
       ],
       roles: [  ]
     });
     
     db.grantRolesToUser("USER", ["downgrade_csrs"]);
     

5. Restart all the shard mongod instances.
6. Start the mongos pointing to SCCC
7. (Optional) The local database can be drop from the SCCC members if exists.

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Description

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