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Tag Aware Sharding

For sharded clusters, MongoDB makes it possible to associate specific ranges of a shard key with a specific shard or subset of shards. This association dictates the policy of the cluster balancer process as it balances the chunks around the cluster. This capability enables the following deployment patterns:

  • isolating a specific subset of data on specific set of shards.
  • controlling the balancing policy so that in a geographically distributed cluster the most relevant portions of the data set reside on the shards with greatest proximity to the application servers.

This document describes the behavior, operation, and use of tag aware sharding in MongoDB deployments.

Note

Shard key range tags are entirely distinct from replica set member tags.

Behavior and Operations

Tags in a sharded cluster are pieces of metadata that dictate the policy and behavior of the cluster balancer. Using tags, you may associate individual shards in a cluster with one or more tags. Then, you can assign this tag string to a range of shard key values for a sharded collection. When migrating a chunk, the balancer will select a destination shard based on the configured tag ranges.

The balancer migrates chunks in tagged ranges to shards with those tags, if tagged shards are not balanced. [1]

Note

Because a single chunk may span different tagged shard key ranges, the balancer may migrate chunks to tagged shards that contain values that exceed the upper bound of the selected tag range.

Example

Given a sharded collection with two configured tag ranges, such that:

  • Shard key values between 100 and 200 have tags to direct corresponding chunks to shards tagged NYC.
  • Shard Key values between 200 and 300 have tags to direct corresponding chunks to shards tagged SFO.

In this cluster, the balancer will migrate a chunk with shard key values ranging between 150 and 220 to a shard tagged NYC, since 150 is closer to 200 than 300.

After configuring tags on shards and ranges of the shard key, the cluster may take some time to reach the proper distribution of data, depending on the division of chunks (i.e. splits) and the current distribution of data in the cluster. Once configured, the balancer will respect tag ranges during future balancing rounds.

[1]To migrate chunks in a tagged environment, the balancer selects a target shard with a tag range that has an upper bound that is greater than the migrating chunk’s lower bound. If a shard with a matching tagged range exists, the balancer will migrate the chunk to that shard.

Administer Shard Tags

Associate tags with a particular shard using the sh.addShardTag() method when connected to a mongos instance. A single shard may have multiple tags, and multiple shards may also have the same tag.

Example

The following example adds the tag NYC to two shards, and the tags SFO and NRT to a third shard:

sh.addShardTag("shard0000", "NYC")
sh.addShardTag("shard0001", "NYC")
sh.addShardTag("shard0002", "SFO")
sh.addShardTag("shard0002", "NRT")

You may remove tags from a particular shard using the sh.removeShardTag() method when connected to a mongos instance, as in the following example, which removes the NRT tag from a shard:

sh.removeShardTag("shard0002", "NRT")

Tag a Shard Key Range

To assign a tag to a range of shard keys use the sh.addTagRange() method when connected to a mongos instance. Any given shard key range may only have one assigned tag. You cannot overlap defined ranges, or tag the same range more than once.

Example

Given a collection named users in the records database, sharded by the zipcode field. The following operations assign:

  • two ranges of zip codes in Manhattan and Brooklyn the NYC tag
  • one range of zip codes in San Francisco the SFO tag
sh.addTagRange("records.users", { zipcode: "10001" }, { zipcode: "10281" }, "NYC")
sh.addTagRange("records.users", { zipcode: "11201" }, { zipcode: "11240" }, "NYC")
sh.addTagRange("records.users", { zipcode: "94102" }, { zipcode: "94135" }, "SFO")

Note

Shard rages are always inclusive of the lower value and exclusive of the upper boundary.

Remove a Tag From a Shard Key Range

The mongod does not provide a helper for removing a tag range. You may delete tag assignment from a shard key range by removing the corresponding document from the tags collection of the config database.

Each document in the tags holds the namespace of the sharded collection and a minimum shard key value.

Example

The following example removes the NYC tag assignment for the range of zip codes within Manhattan:

use config
db.tags.remove({ _id: { ns: "records.users", min: { zipcode: "10001" }}, tag: "NYC" })

View Existing Shard Tags

The output from sh.status() lists tags associated with a shard, if any, for each shard. A shard’s tags exist in the shard’s document in the shards collection of the config database. To return all shards with a specific tag, use a sequence of operations that resemble the following, which will return only those shards tagged with NYC:

use config
db.shards.find({ tags: "NYC" })

You can find tag ranges for all namespaces in the tags collection of the config database. The output of sh.status() displays all tag ranges. To return all shard key ranges tagged with NYC, use the following sequence of operations:

use config
db.tags.find({ tags: "NYC" })