Smoothly converting Sharded Cluster to Replica Set

From: nnt <tung@xxxxxxxxxx>
To: mongodb-user <mongodb-user@xxxxxxxxxxxxxxxx>
Date: Mon, 18 Apr 2016 22:35:31 -0700 (PDT)
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MongoDB documentation shows how to convert a sharded cluster to a replica 
set: https://docs.mongodb.org/manual/tutorial/convert-sharded-cluster-to-replica-set/

The problem with this process is that it requires downtime to the 
applications. I am think of this way for a smooth transition from sharded 
cluster to replica set:

   1. With applications still working with the sharded cluster, remove all 
   other shards 
   <https://docs.mongodb.org/manual/tutorial/remove-shards-from-cluster/
   except for one shard that can hold all data of other shards (you can add a 
   new big shard before doing this)
   2. Point applications to this sole shard
   3. Decommission the remaining unused sharded cluster infrastructure

Could this be so solution to be applied in production for zero downtime 
transition? I do not really understand why the doc is showing the process 
which introduces application downtime and the complexity of dumping and 
restoring data (see this section 
<https://docs.mongodb.org/manual/tutorial/convert-sharded-cluster-to-replica-set/#convert-a-sharded-cluster-into-a-replica-set>
).

Thank you.

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