Convert a Shard Cluster to a Replicated Shard Cluster

Question

I've been working with mongo for a few weeks and and building my environment in a dev. I started with a single node, then moved to a shard cluster, and now want to move to a replicated shard cluster. From what I read a Replicated Shard Cluster is the best of the best, scalability, durability, performance increase, etc.

I've read most of the (very good) tutorials in their help. It seems their lessons advise going from single node, to replica set, to sharded replica set, which, unfortunately is the opposite way I did it. I can't seem to find anything to upgrade a sharded cluster to a replicated shard cluster.

Here are 5 hosts that I have:

• APPSERVER
• CONFIGSERVER
• SHARD1
• SHARD2
• SHARD3

I started each of the shard servers with:

mongod --shardsvr 

Then I started the config server with:

mongod --configsvr 

Then I started the mongos process on the APPSERVER with:

mongos --configdb CONFIGSERVER

Then in mongos, I added the shards, enabled sharding on my database, and defined a shardkey for a collection:

sh.addShard("SHARD1:27018");//again for 2 and 3
sh.enableSharding("beacon");
sh.shardCollection("beacon.alpha2", {"ip":1});

I want each of the shards replicated on each of the other two. (right?) Do I need to bring down the mongod processes on the shards and restart them with different CL parameters? What commands do I need to issue in my mongos shell to get it to replicate? Should I export all my data, take everything down, restart and reimport? Again, I see a lot of tutorials on how to create a replica set, but I don't really see anything on how to do a replica set given a sharded system to start with.

Thanks!

Answer 1

For each shard, you will need to restart the current member and start both it and two new members (or 1 new member and an arbiter) with the --replset command line option. You could add more members than that, but 3 is the lowest workable set. Then from inside what will become the new primary (your current SHARD1 for example) you could do the following:

rs.add("newmember1:port")
rs.add("newmember2:port")
rs.initiate();

You would then need to check and make sure that the sh.status() has been updated to reflect the new members of the replica set. In 2.2 this has become slightly easier as it should be automatic, for prior versions it was necessary to manually save the shard information in the config database, which is reflected in the documentation under sharded cluster. If it has been automatically added you will see the replica set list in the sh.status() output, similar to the following:

{  "_id" : "shard1",  "host" : "shard1/SHARD1:port,newmember1:port,newmember2:port" }

If this does not happen automatically you will need to follow the procedure outlined in the documentation link above, namely from inside mongos:

db.getSiblingDB("config").shards.save({_id:"<name>", host:"<rsName>/member1,member2,..."})

Following the above example it would look like:

db.getSiblingDB("config").shards.save({_id:"SHARD1", host:"shard1/SHARD1:port,newmember1:port,newmember2:port"})

You would need to do this procedure for each shard, and you should do them one at a time so that you have all of SHARD1 as a replica set before moving on to SHARD2. You will also need to be aware that each replica set will become read-only while the initial election takes place, so at the very least you should schedule this in a downtime or maintenance window. Ideally test first in a staging environment.