MongoDB: The Definitive Guide, 3rd Edition

Chapter 17. Sharding Administration

As with replica sets, you have a number of options for administering sharded clusters. Manual administration is one option. These days it is becoming increasingly common to use tools such as Ops Manager and Cloud Manager and the Atlas Database-as-a-Service (DBaaS) offering for all cluster administration. In this chapter, we will demonstrate how to administer a sharded cluster manually, including:

• Inspecting the cluster’s state: who its members are, where data is held, and what connections are open

• Adding, removing, and changing members of a cluster

• Administering data movement and manually moving data

Seeing the Current State

There are several helpers available to find out what data is where, what the shards are, and what the cluster is doing.

Getting a Summary with sh.status()

sh.status() gives you an overview of your shards, databases, and sharded collections. If you have a small number of chunks, it will print a breakdown of which chunks are where as well. Otherwise it will simply give the collection’s shard key and report how many chunks each shard has:

> sh.status()
--- Sharding Status --- 
sharding version: {
  "_id" : 1,
  "minCompatibleVersion" : 5,
  "currentVersion" : 6,
  "clusterId" : ObjectId("5bdf51ecf8c192ed922f3160")
}
shards:
  {  "_id" : "shard01",  
     "host" : "shard01/localhost:27018,localhost:27019,localhost:27020",  
     "state" : 1 }
  {  "_id" : "shard02",  
     "host" : "shard02/localhost:27021,localhost:27022,localhost:27023",  
     "state" : 1 }
  {  "_id" : "shard03" ...