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Tiered Hardware for Varying SLA or SLO

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

Changed in version 4.0.3: 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 uses Zones to route documents based on creation date either to shards zoned for supporting recent documents, or those zoned for supporting archived documents.

The following are some example use cases for segmenting data based on Service Level Agreement (SLA) or Service Level Objective (SLO):

• An application requires providing low-latency access to recently inserted / updated documents
• An application requires prioritizing low-latency access to a range or subset of documents
• An application that benefits from ensuring specific ranges or subsets of data are stored on servers with hardware that suits the SLA’s for accessing that data

The following diagram illustrates a sharded cluster that uses hardware based zones to satisfy data access SLAs or SLOs.

Scenario

A photo sharing application requires fast access to photos uploaded within the last 6 months. The application stores the location of each photo along with its metadata in the photoshare database under the data collection.

The following documents represent photos uploaded by a single user:

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{
  "_id" : 10003010,
  "creation_date" : ISODate("2012-12-19T06:01:17.171Z"),
  "userid" : 123,
  "photo_location" : "example.net/storage/usr/photo_1.jpg"
}
{
  "_id" : 10003011,
  "creation_date" : ISODate("2013-12-19T06:01:17.171Z"),
  "userid" : 123,
  "photo_location" : "example.net/storage/usr/photo_2.jpg"
}
{
  "_id" : 10003012,
  "creation_date" : ISODate("2016-01-19T06:01:17.171Z"),
  "userid" : 123,
  "photo_location" : "example.net/storage/usr/photo_3.jpg"
}

Note that only the document with _id : 10003012 was uploaded within the past year (as of June 2016).

Shard Key

The photo collection uses the { creation_date : 1 } index as the shard key.

The creation_date field in each document allows for creating zones on the creation date.

Architecture

The sharded cluster deployment currently consists of three shards.

Zones

The application requires adding each shard to a zone based on its hardware tier. Each hardware tier represents a specific hardware configuration designed to satisfy a given SLA or SLO.

Fast Tier (“recent”)

These are the fastest performing machines, with large amounts of RAM, fast SSD disks, and powerful CPUs.

The zone requires a range with:

• a lower bound of { creation_date : ISODate(YYYY-mm-dd)}, where the Year, Month, and Date specified by YYYY-mm-dd is within the last 6 months.
• an upper bound of { creation_date : MaxKey }.

Archival Tier (“archive”)

These machines use less RAM, slower disks, and more basic CPUs. However, they have a greater amount of storage per server.

The zone requires a range with:

• a lower bound of { creation_date : MinKey }.
• an upper bound of { creation_date : ISODate(YYYY-mm-dd)}, where the Year, Month, and Date match the values used for the recent tier’s lower bound.

Note

The MinKey and MaxKey values are reserved special values for comparisons.

As performance needs increase, adding additional shards and associating them to the appropriate zone based on their hardware tier allows for the cluster to scale horizontally.

When defining zone ranges based on time spans, weigh the benefits of infrequent updates to the zone ranges against the amount of data that must be migrated on an update. For example, setting a limit of 1 year for data to be considered ‘recent’ likely covers more data than setting a limit of 1 month. While there are more migrations required when rotating on a 1 month scale, the amount of documents that must be migrated is lower than rotating on a 1 year scale.

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 the shard key.

For example, MongoDB can attempt a targeted read operation on the following query because it includes creation_date in the query document:

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photoDB = db.getSiblingDB("photoshare")
photoDB.data.find( { "creation_date" : ISODate("2015-01-01") } )

If the requested document falls within the recent zone range, MongoDB would route this query to the shards inside that zone, ensuring a faster read compared to a cluster-wide broadcast read operation

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, authenticate as a user with at least the clusterManager role on the admin database.

Procedure

You must be connected to a mongos to create zones or zone ranges. You cannot create zone or zone ranges by connecting directly to a shard.

1

Disable the Balancer

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

Use sh.disableBalancing(), specifying the namespace of the collection, to stop the balancer

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sh.disableBalancing("photoshare.data")

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 recent zone.

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

Add shard0001 to the recent zone.

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

Add shard0002 to the archive zone.

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

You can review the zone assigned to any given shard by running sh.status().

3

Define ranges for each zone

Define range for recent photos and associate it to the recent 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 zone.

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sh.addTagRange(
  "photoshare.data",
  { "creation_date" : ISODate("2016-01-01") },
  { "creation_date" : MaxKey },
  "recent"
)

Define range for older photos and associate it to the archive 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 zone.

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sh.addTagRange(
  "photoshare.data",
  { "creation_date" : MinKey },
  { "creation_date" : ISODate("2016-01-01") },
  "archive"
)

MinKey and MaxKey are reserved special values for comparisons.

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

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sh.enableBalancing("photoshare.data")

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 configured zones.

Once balancing finishes, the shards in the recent zone should only contain documents with creation_date greater than or equal to ISODate("2016-01-01"), while shards in the archive zone should only contain documents with creation_date less than ISODate("2016-01-01").

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

Updating Zone Ranges

To update the shard ranges, perform the following operations as a part of a cron job or other scheduled procedure:

1

Disable the Balancer

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

Use sh.disableBalancing(), specifying the namespace of the collection, to stop the balancer

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sh.disableBalancing("photoshare.data")

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

2

Remove the old shard zone ranges

Remove the old recent zone range using the sh.removeTagRange() 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 zone.

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sh.removeTagRange(
  "photoshare.data",
  { "creation_date" : ISODate("2016-01-01") },
  { "creation_date" : MaxKey },
  "recent"
)

Remove the old archive zone range using the sh.removeTagRange() 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 zone.

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sh.removeTagRange(
  "photoshare.data",
  { "creation_date" : MinKey },
  { "creation_date" : ISODate("2016-01-01") },
  "archive"
)

MinKey and MaxKey are reserved special values for comparisons.

3

Add the new zone range for each zone

Define range for recent photos and associate it to the recent 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 zone.

copy

sh.addTagRange(
  "photoshare.data",
  { "creation_date" : ISODate("2016-06-01") },
  { "creation_date" : MaxKey },
  "recent"
)

Define range for older photos and associate it to the archive 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 zone.

copy

sh.addTagRange(
  "photoshare.data",
  { "creation_date" : MinKey },
  { "creation_date" : ISODate("2016-06-01") },
  "archive"
)

MinKey and MaxKey are reserved special values for comparisons.

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

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sh.enableBalancing("photoshare.data")

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

5

Review the changes

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

Before balancing, the shards in the recent zone only contained documents with creation_date greater than or equal to ISODate("2016-01-01"), while shards in the archive zone only contained documents with creation_date less than ISODate("2016-01-01").

Once balancing finishes, the shards in the recent zone should only contain documents with creation_date greater than or equal to ISODate("2016-06-01"), while shards in the archive zone should only contain documents with creation_date less than ISODate("2016-06-01").

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

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