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
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:
{ "_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 byYYYY-mm-ddis 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 therecenttier's lower bound.
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:
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.
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
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.
Add each shard to the appropriate zone
Add shard0000 to the recent zone.
sh.addShardTag("shard0000", "recent")
Add shard0001 to the recent zone.
sh.addShardTag("shard0001", "recent")
Add shard0002 to the archive zone.
sh.addShardTag("shard0002", "archive")
You can review the zone assigned to any given shard by running
sh.status().
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.
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.
sh.addTagRange( "photoshare.data", { "creation_date" : MinKey }, { "creation_date" : ISODate("2016-01-01") }, "archive" )
MinKey and MaxKey are reserved special values for
comparisons.
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("photoshare.data")
Use sh.isBalancerRunning() to check if the balancer process
is currently running.
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:
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
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.
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.
sh.removeTagRange( "photoshare.data", { "creation_date" : ISODate("2016-01-01") }, { "creation_date" : MaxKey } )
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.
sh.removeTagRange( "photoshare.data", { "creation_date" : MinKey }, { "creation_date" : ISODate("2016-01-01") } )
MinKey and MaxKey are reserved special values for
comparisons.
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.
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.
sh.addTagRange( "photoshare.data", { "creation_date" : MinKey }, { "creation_date" : ISODate("2016-06-01") }, "archive" )
MinKey and MaxKey are reserved special values for
comparisons.
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("photoshare.data")
Use sh.isBalancerRunning() to check if the balancer process
is currently running.
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().