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Tiered Hardware for Varying SLA or SLO

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MongoDB Tag Aware Sharding allows administrators to define ranges of the shard key and tag them to one or more shards. Using this feature, data can be routed to one or more shards based on the tag. This gives administrators control over data distribution in a sharded cluster.

This tutorial uses Tag Aware Sharding to route documents based on creation date either to shards tagged for supporting recent documents, or those tagged 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 shard tags to satisfy data access SLAs or SLOs.

Diagram of sharded cluster architecture for tiered SLA

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:

copy 
{
  "_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 a tag range on the creation date.

Architecture

The sharded cluster deployment currently consists of three shards.

Diagram of sharded cluster architecture for tiered SLA

Tags

The application requires tagging each shard in the cluster based on its hardware tier. Each hardware tier represents a specific hardware configuration designed to satisfy a given SLA or SLO.

Diagram of sharded cluster architecture for tiered SLA
Fast Tier (“recent”)

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

The tag 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 tag 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 tagging them based on their hardware tier allows for the cluster to scale horizontally.

When defining tag ranges based on time spans, weigh the benefits of infrequent updates to the tag 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 tag-aware sharding, if an inserted or updated document matches a configured tag range, it can only be written to a shard with the related tag.

MongoDB can write documents that do not match a configured tag range 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 tag range. 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:

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

If the requested document falls within the tag range assigned to the recent storage tier, MongoDB would route this query to the tagged shards, ensuring a faster read compared to a cluster-wide broadcast read operation

Balancer

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

Adding or removing tags can result in chunk migrations. Depending on the size of your data set and the chunks a tag affects, these migrations may impact cluster performance. Run your balancer in 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 tags and tag ranges. You cannot create tags 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 tags.

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

copy 
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

Tag each shard

Tag shard0000 with the recent tag.

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

Tag shard0001 with the recent tag.

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

Tag shard0002 with the archive tag.

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

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

3

Define ranges for each tag

Define range for recent photos and associate it to the recent tag 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 tag.
copy 
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 tag 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 tag.
copy 
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

copy 
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 the tag ranges and tags.

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

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

Updating Tag 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 tags.

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

copy 
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 tag ranges

Remove the old recent shard tag 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 tag.
copy 
sh.removeTagRange(
  "photoshare.data",
  { "creation_date" : ISODate("2016-01-01") },
  { "creation_date" : MaxKey },
  "recent"
)

Remove the old archive shard tag 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 tag.
copy 
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 shard tag range for each tag

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

copy 
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 tag ranges and tags.

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

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