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Zones

Segmenting Data by Application or Customer

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  • Scenario
  • Procedure

In sharded clusters, you can create zones of sharded data based on the shard key. You can associate each zone with one or more shards in the cluster. A shard can associate with any number of zones. In a balanced cluster, MongoDB migrates chunks covered by a zone only to those shards associated with the zone.

Tip

By defining the zones and the zone ranges before sharding an empty or a non-existing collection, the shard collection operation creates chunks for the defined zone ranges as well as any additional chunks to cover the entire range of the shard key values and performs an initial chunk distribution based on the zone ranges. This initial creation and distribution of chunks allows for faster setup of zoned sharding. After the initial distribution, the balancer manages the chunk distribution going forward.

See Pre-Define Zones and Zone Ranges for an Empty or Non-Existing Collection for an example.

This tutorial shows you how to segment data using Zones.

Consider the following scenarios where segmenting data by application or customer may be necessary:

  • A database serving multiple applications

  • A database serving multiple customers

  • A database that requires isolating ranges or subsets of application or customer data

  • A database that requires resource allocation for ranges or subsets of application or customer data

This diagram illustrates a sharded cluster using zones to segment data based on application or customer. This allows for data to be isolated to specific shards. Additionally, each shard can have specific hardware allocated to fit the performance requirement of the data stored on that shard.

Overview of zones used for supporting data segmentation

Scenario

An application tracks the score of a user along with a client field, storing scores in the gamify database under the users collection. Each possible value of client requires its own zone to allow for data segmentation. It also allows the administrator to optimize the hardware for each shard associated to a client for performance and cost.

The following documents represent a partial view of two users:

{
  "_id" : ObjectId("56f08c447fe58b2e96f595fa"),
  "client" : "robot",
  "userid" : 123,
  "high_score" : 181,
  ...,
}
{
  "_id" : ObjectId("56f08c447fe58b2e96f595fb"),
  "client" : "fruitos",
  "userid" : 456,
  "high_score" : 210,
  ...,
}

Shard Key

The users collection uses the { client : 1, userid : 1 } compound index as the shard key.

The client field in each document allows creating a zone for each distinct client value.

The userid field provides a high cardinality and low frequency component to the shard key relative to country.

See Choosing a Shard Key for more general instructions on selecting a shard key.

Architecture

The application requires adding shard to a zone associated to a specific client.

The sharded cluster deployment currently consists of four shards.

Diagram of Data Segmentation Architecture using zones

Zones

For this application, there are two client zones.

Diagram of zones used for supporting data segmentation
Robot client ("robot")
This zone represents all documents where client : robot.
FruitOS client ("fruitos")
This zone represents all documents where client : fruitos.

Write Operations

With zones, if an inserted or updated document matches a configured zone, it can only be written to a shard inside that zone.

MongoDB can write documents that do not match a configured zone to any shard in the cluster.

Note

The behavior described above requires the cluster to be in a steady state with no chunks violating a configured zone. See the following section on the balancer for more information.

Read Operations

MongoDB can route queries to a specific shard if the query includes at least the client field.

For example, MongoDB can attempt a targeted read operation on the following query:

chatDB = db.getSiblingDB("gamify")
chatDB.users.find( { "client" : "robot" , "userid" : "123" } )

Queries without the client field perform broadcast operations.

Balancer

The balancer migrates chunks to the appropriate shard respecting any configured zones. Until the migration, shards may contain chunks that violate configured zones. Once balancing completes, shards should only contain chunks whose ranges do not violate its assigned zones.

Adding or removing zones or zone ranges can result in chunk migrations. Depending on the size of your data set and the number of chunks a zone or zone range affects, these migrations may impact cluster performance. Consider running your balancer during specific scheduled windows. See Schedule the Balancing Window for a tutorial on how to set a scheduling window.

Security

For sharded clusters running with Role-Based Access Control in Self-Managed Deployments, authenticate as a user with at least the clusterManager role on the admin database.

Procedure

You must be connected to a mongos associated to the target sharded cluster to proceed. You cannot create zones or zone ranges by connecting directly to a shard.

1

Disable the Balancer

The balancer must be disabled on the entire sharded cluster to ensure no migrations take place while configuring the new zones.

Use sh.stopBalancer() to stop the balancer for the cluster.

sh.stopBalancer()

Use sh.isBalancerRunning() to check if the balancer process is currently running. Wait until any current balancing rounds have completed before proceeding.

2

Add each shard to the appropriate zone

Add shard0000 to the robot zone.

sh.addShardTag("shard0000", "robot")

Add shard0001 to the robot zone.

sh.addShardTag("shard0001", "robot")

Add shard0002 to the fruitos zone.

sh.addShardTag("shard0002", "fruitos")

Add shard0003 to the fruitos zone.

sh.addShardTag("shard0003", "fruitos")

Run sh.status() to review the zone configured for the sharded cluster.

3

Define ranges for each zone

Define range for the robot client and associate it to the robot zone using the sh.addTagRange() method.

This method requires:

  • The full namespace of the target collection

  • The inclusive lower bound of the range

  • The exclusive upper bound of the range

  • The name of the zone

sh.addTagRange( 
  "gamify.users",
  { "client" : "robot", "userid" : MinKey },
  { "client" : "robot", "userid" : MaxKey }, 
  "robot"
)

Define range for the fruitos client and associate it to the fruitos zone using the sh.addTagRange() method.

This method requires:

  • The full namespace of the target collection

  • The inclusive lower bound of the range

  • The exclusive upper bound of the range

  • The name of the zone

sh.addTagRange( 
  "gamify.users",
  { "client" : "fruitos", "userid" : MinKey },
  { "client" : "fruitos", "userid" : MaxKey }, 
  "fruitos"
)

The MinKey and MaxKey values are reserved special values for comparisons. MinKey always compares as lower than every other possible value, while MaxKey always compares as higher than every other possible value. The configured ranges captures every user for each client.

4

Enable the Balancer

Re-enable the balancer to rebalance the cluster.

Use sh.enableBalancing(), specifying the namespace of the collection, to start the balancer.

sh.enableBalancing("chat.message")

Use sh.isBalancerRunning() to check if the balancer process is currently running.

5

Review the changes

The next time the balancer runs, it splits and migrates chunks across the shards respecting the configured zones.

Once balancing finishes, the shards in the robot zone only contain documents with client : robot, while shards in the fruitos zone only contain documents with client : fruitos.

You can confirm the chunk distribution by running sh.status().

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