Amazon ElastiCache for Redis
ElastiCache for Redis User Guide (API Version 2015-02-02)

Online Resharding and Shard Rebalancing for Redis (cluster mode enabled)

By using online resharding and shard rebalancing with Amazon ElastiCache for Redis version 3.2.10, you can scale your ElastiCache for Redis (cluster mode enabled) dynamically with no downtime. This approach means that your cluster can continue to serve requests even while scaling or rebalancing is in process.

You can do the following:

  • Scale out – Increase read and write capacity by adding shards (node groups) to your Redis (cluster mode enabled) cluster (replication group).

    If you add one or more shards to your replication group, the number of nodes in each new shard is the same as the number of nodes in the smallest of the existing shards.

  • Scale in – Reduce read and write capacity, and thereby costs, by removing shards from your Redis (cluster mode enabled) cluster.

  • Rebalance – Move the keyspaces among the shards in your ElastiCache for Redis (cluster mode enabled) cluster so they are as equally distributed among the shards as possible.

You can't do the following:

  • Scale up/down: Change your node type. To do this, you must use the offline process.

  • Upgrade your engine: Change your engine version to a newer version. To do this, you must use the offline process.

  • Configure shards independently:

    • You can't specify the number of nodes in each shard independently. To do this, you must use the offline process.

    • You can't specify the keyspace for shards independently. To do this, you must use the offline process.

Currently, the following limitations apply to ElastiCache for Redis online resharding and rebalancing:

  • These processes require Redis engine version 3.2.10 or newer. For information on upgrading your engine version, see Upgrading Engine Versions.

  • There are limitations with slots or keyspaces and large items:

    If any of the keys in a shard contain a large item, that key isn't migrated to a new shard when scaling out or rebalancing. This functionality can result in unbalanced shards.

    If any of the keys in a shard contain a large item (items greater than 256 MB after serialization), that shard isn't deleted when scaling in. This functionality can result in some shards not being deleted.

  • When scaling out, the number of nodes in any new shards equals the number of nodes in the smallest existing shard.

  • When scaling out, any tags that are common to all existing shards are copied to the new shards.

For more information, see Best Practices: Online Cluster Resizing.

You can horizontally scale or rebalance your ElastiCache for Redis (cluster mode enabled) clusters using the AWS Management Console, the AWS CLI, and the ElastiCache API.

Adding Shards with Online Resharding

You can add shards to your Redis (cluster mode enabled) cluster using the AWS Management Console, AWS CLI, or ElastiCache API. When you add shards to a Redis (cluster mode enabled) cluster, any tags on the existing shards are copied over to the new shards.

Adding Shards (Console)

You can use the AWS Management Console to add one or more shards to your Redis (cluster mode enabled) cluster. The following procedure describes the process.

To add shards to your Redis (cluster mode enabled) cluster

  1. Open the ElastiCache console at https://console.aws.amazon.com/elasticache/.

  2. From the navigation pane, choose Redis.

  3. Locate and choose the name, not the box to the left of the cluster's name, of the Redis (cluster mode enabled) cluster that you want to add shards to.

    Tip

    Redis (cluster mode enabled) clusters have a value of 1 or greater in the Shards column.

  4. Choose Add shard.

    1. For Number of shards to be added, choose the number of shards you want added to this cluster.

    2. For Availability zone(s), choose either No preference or Specify availability zones.

    3. If you chose Specify availability zones, for each node in each shard, select the node's Availability Zone from the list of Availability Zones.

    4. Choose Add.

Adding Shards (AWS CLI)

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by adding shards using the AWS CLI.

Use the following parameters with modify-replication-group-shard-configuration.

Parameters

  • --apply-immediately – Required. Specifies the shard reconfiguration operation is to be started immediately.

  • --replication-group-id – Required. Specifies which replication group (cluster) the shard reconfiguration operation is to be performed on.

  • --node-group-count – Required. Specifies the number of shards (node groups) to exist when the operation is completed. When adding shards, the value of --node-group-count must be greater than the current number of shards.

    Optionally, you can specify the Availability Zone for each node in the replication group using --resharding-configuration.

  • --resharding-configuration – Optional. A list of preferred Availability Zones for each node in each shard in the replication group. Use this parameter only if the value of --node-group-count is greater than the current number of shards. If this parameter is omitted when adding shards, Amazon ElastiCache selects the Availability Zones for the new nodes.

The following example reconfigures the keyspaces over four shards in the Redis (cluster mode enabled) cluster my-cluster. The example also specifies the Availability Zone for each node in each shard. The operation begins immediately.

Example - Adding Shards

For Linux, macOS, or Unix:

aws elasticache modify-replication-group-shard-configuration \ --replication-group-id my-cluster \ --node-group-count 4 \ --resharding-configuration \ "PreferredAvailabilityZones=us-east-2a,us-east-2c" \ "PreferredAvailabilityZones=us-east-2b,us-east-2a" \ "PreferredAvailabilityZones=us-east-2c,us-east-2d" \ "PreferredAvailabilityZones=us-east-2d,us-east-2c" \ --apply-immediately

For Windows:

aws elasticache modify-replication-group-shard-configuration ^ --replication-group-id my-cluster ^ --node-group-count 4 ^ --resharding-configuration ^ "PreferredAvailabilityZones=us-east-2a,us-east-2c" ^ "PreferredAvailabilityZones=us-east-2b,us-east-2a" ^ "PreferredAvailabilityZones=us-east-2c,us-east-2d" ^ "PreferredAvailabilityZones=us-east-2d,us-east-2c" ^ --apply-immediately

For more information, see modify-replication-group-shard-configuration in the AWS CLI documentation.

Adding Shards (ElastiCache API)

You can use the ElastiCache API to reconfigure the shards in your Redis (cluster mode enabled) cluster online by using the ModifyReplicationGroupShardConfiguration operation.

Use the following parameters with ModifyReplicationGroupShardConfiguration.

Parameters

  • ApplyImmediately=true – Required. Specifies the shard reconfiguration operation is to be started immediately.

  • ReplicationGroupId – Required. Specifies which replication group (cluster) the shard reconfiguration operation is to be performed on.

  • NodeGroupCount – Required. Specifies the number of shards (node groups) to exist when the operation is completed. When adding shards, the value of NodeGroupCount must be greater than the current number of shards.

    Optionally, you can specify the Availability Zone for each node in the replication group using ReshardingConfiguration.

  • ReshardingConfiguration – Optional. A list of preferred Availability Zones for each node in each shard in the replication group. Use this parameter only if the value of NodeGroupCount is greater than the current number of shards. If this parameter is omitted when adding shards, Amazon ElastiCache selects the Availability Zones for the new nodes.

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by adding shards using the ElastiCache API.

Example - Adding Shards

The following example adds node groups to the Redis (cluster mode enabled) cluster my-cluster, so there are a total of four node groups when the operation completes. The example also specifies the Availability Zone for each node in each shard. The operation begins immediately.

https://elasticache.us-east-2.amazonaws.com/ ?Action=ModifyReplicationGroupShardConfiguration &ApplyImmediately=true &NodeGroupCount=4 &ReplicationGroupId=my-cluster &ReshardingConfiguration.ReshardingConfiguration.1.PreferredAvailabilityZones.AvailabilityZone.1=us-east-2a &ReshardingConfiguration.ReshardingConfiguration.1.PreferredAvailabilityZones.AvailabilityZone.2=us-east-2c &ReshardingConfiguration.ReshardingConfiguration.2.PreferredAvailabilityZones.AvailabilityZone.1=us-east-2b &ReshardingConfiguration.ReshardingConfiguration.2.PreferredAvailabilityZones.AvailabilityZone.2=us-east-2a &ReshardingConfiguration.ReshardingConfiguration.3.PreferredAvailabilityZones.AvailabilityZone.1=us-east-2c &ReshardingConfiguration.ReshardingConfiguration.3.PreferredAvailabilityZones.AvailabilityZone.2=us-east-2d &ReshardingConfiguration.ReshardingConfiguration.4.PreferredAvailabilityZones.AvailabilityZone.1=us-east-2d &ReshardingConfiguration.ReshardingConfiguration.4.PreferredAvailabilityZones.AvailabilityZone.2=us-east-2c &Version=2015-02-02 &SignatureVersion=4 &SignatureMethod=HmacSHA256 &Timestamp=20171002T192317Z &X-Amz-Credential=<credential>

For more information, see ModifyReplicationGroupShardConfiguration in the ElastiCache API Reference.

Removing Shards with Online Resharding

You can remove shards from your Redis (cluster mode enabled) cluster using the AWS Management Console, AWS CLI, or ElastiCache API.

Removing Shards (Console)

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by removing shards using the AWS Management Console.

Before removing node groups (shards) from your replication group, ElastiCache makes sure that all your data fits in the remaining shards. If the data fits, the specified shards are deleted from the replication group as requested. If the data doesn't fit in the remaining node groups, the process is terminated. Terminating the process leaves your replication group with the same node group configuration as before the request was made.

You can use the AWS Management Console to remove one or more shards from your Redis (cluster mode enabled) cluster. You cannot remove all the shards in a replication group. Instead, you must delete the replication group. For more information, see Deleting a Replication Group. The following procedure describes the process for deleting one or more shards.

To remove shards from your Redis (cluster mode enabled) cluster

  1. Open the ElastiCache console at https://console.aws.amazon.com/elasticache/.

  2. From the navigation pane, choose Redis.

  3. Locate and choose the name, not the box to the left of the cluster's name, of the Redis (cluster mode enabled) cluster you want to remove shards from.

    Tip

    Redis (cluster mode enabled) clusters have a value of 1 or greater in the Shards column.

  4. From the list of shards, choose the box to the left of the name of each shard that you want to delete.

  5. Choose Delete shard.

Removing Shards (AWS CLI)

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by removing shards using the AWS CLI.

Important

Before removing node groups (shards) from your replication group, ElastiCache makes sure that all your data fits in the remaining shards. If the data fits, the specified shards (--node-groups-to-remove) are deleted from the replication group as requested and their keyspaces mapped into the remaining shards. If the data doesn't fit in the remaining node groups, the process is terminated. Terminating the process leaves your replication group with the same node group configuration as before the request was made.

You can use the AWS CLI to remove one or more shards from your Redis (cluster mode enabled) cluster. You cannot remove all the shards in a replication group. Instead, you must delete the replication group. For more information, see Deleting a Replication Group.

Use the following parameters with modify-replication-group-shard-configuration.

Parameters

  • --apply-immediately – Required. Specifies the shard reconfiguration operation is to be started immediately.

  • --replication-group-id – Required. Specifies which replication group (cluster) the shard reconfiguration operation is to be performed on.

  • --node-group-count – Required. Specifies the number of shards (node groups) to exist when the operation is completed. When removing shards, the value of --node-group-count must be less than the current number of shards.

  • --node-groups-to-remove – Required when --node-group-count is less than the current number of node groups (shards). A list of shard (node group) IDs to remove from the replication group.

The following procedure describes the process for deleting one or more shards.

Example - Removing Shards

The following example removes two node groups from the Redis (cluster mode enabled) cluster my-cluster, so there are a total of two node groups when the operation completes. The keyspaces from the removed shards are distributed evenly over the remaining shards.

For Linux, macOS, or Unix:

aws elasticache modify-replication-group-shard-configuration \ --replication-group-id my-cluster \ --node-group-count 2 \ --node-groups-to-remove "0002" "0003" \ --apply-immediately

For Windows:

aws elasticache modify-replication-group-shard-configuration ^ --replication-group-id my-cluster ^ --node-group-count 2 ^ --node-groups-to-remove "0002" "0003" ^ --apply-immediately

For more information, see modify-replication-group-shard-configuration in the AWS CLI documentation.

Removing Shards (ElastiCache API)

You can use the ElastiCache API to reconfigure the shards in your Redis (cluster mode enabled) cluster online by using the ModifyReplicationGroupShardConfiguration operation.

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by removing shards using the ElastiCache API.

Important

Before removing node groups (shards) from your replication group, ElastiCache makes sure that all your data fits in the remaining shards. If the data fits, the specified shards (NodeGroupsToRemove) are deleted from the replication group as requested and their keyspaces mapped into the remaining shards. If the data doesn't fit in the remaining node groups, the process is terminated. Terminating the process leaves your replication group with the same node group configuration as before the request was made.

You can use the ElastiCache API to remove one or more shards from your Redis (cluster mode enabled) cluster. You cannot remove all the shards in a replication group. Instead, you must delete the replication group. For more information, see Deleting a Replication Group.

Use the following parameters with ModifyReplicationGroupShardConfiguration.

Parameters

  • ApplyImmediately=true – Required. Specifies the shard reconfiguration operation is to be started immediately.

  • ReplicationGroupId – Required. Specifies which replication group (cluster) the shard reconfiguration operation is to be performed on.

  • NodeGroupCount – Required. Specifies the number of shards (node groups) to exist when the operation is completed. When removing shards, the value of NodeGroupCount must be less than the current number of shards.

  • NodeGroupsToRemove – Required when --node-group-count is less than the current number of node groups (shards). A list of shard (node group) IDs to remove from the replication group.

The following procedure describes the process for deleting one or more shards.

Example - Removing Shards

The following example removes two node groups from the Redis (cluster mode enabled) cluster my-cluster, so there are a total of two node groups when the operation completes. The keyspaces from the removed shards are distributed evenly over the remaining shards.

https://elasticache.us-east-2.amazonaws.com/ ?Action=ModifyReplicationGroupShardConfiguration &ApplyImmediately=true &NodeGroupCount=2 &ReplicationGroupId=my-cluster &NodeGroupsToRemove.member.1=0002 &NodeGroupsToRemove.member.2=0003 &Version=2015-02-02 &SignatureVersion=4 &SignatureMethod=HmacSHA256 &Timestamp=20171002T192317Z &X-Amz-Credential=<credential>

For more information, see ModifyReplicationGroupShardConfiguration in the ElastiCache API Reference.

Online Shard Rebalancing

You can rebalance shards in your Redis (cluster mode enabled) cluster using the AWS Management Console, AWS CLI, or ElastiCache API.

Online Shard Rebalancing (Console)

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by rebalancing shards using the AWS Management Console.

To rebalance the keyspaces among the shards on your Redis (cluster mode enabled) cluster

  1. Open the ElastiCache console at https://console.aws.amazon.com/elasticache/.

  2. From the navigation pane, choose Redis.

  3. Choose the name, not the box to the left of the name, of the Redis (cluster mode enabled) cluster that you want to rebalance.

    Tip

    Redis (cluster mode enabled) clusters have a value of 1 or greater in the Shards column.

  4. Choose Rebalance.

  5. When prompted, choose Rebalance. You might see a message similar to this one: Slots in the replication group are uniformly distributed. Nothing to do. (Service: AmazonElastiCache; Status Code: 400; Error Code: InvalidReplicationGroupState; Request ID: 2246cebd-9721-11e7-8d5b-e1b0f086c8cf). If you do, choose Cancel.

Online Shard Rebalancing (AWS CLI)

Use the following parameters with modify-replication-group-shard-configuration.

Parameters

  • -apply-immediately – Required. Specifies the shard reconfiguration operation is to be started immediately.

  • --replication-group-id – Required. Specifies which replication group (cluster) the shard reconfiguration operation is to be performed on.

  • --node-group-count – Required. To rebalance the keyspaces across all shards in the cluster, this value must be the same as the current number of shards.

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by rebalancing shards using the AWS CLI.

Example - Rebalancing the Shards in a Cluster

The following example rebalances the slots in the Redis (cluster mode enabled) cluster my-cluster so that the slots are distributed as equally as possible. The value of --node-group-count (4) is the number of shards currently in the cluster.

For Linux, macOS, or Unix:

aws elasticache modify-replication-group-shard-configuration \ --replication-group-id my-cluster \ --node-group-count 4 \ --apply-immediately

For Windows:

aws elasticache modify-replication-group-shard-configuration ^ --replication-group-id my-cluster ^ --node-group-count 4 ^ --apply-immediately

For more information, see modify-replication-group-shard-configuration in the AWS CLI documentation.

Online Shard Rebalancing (ElastiCache API)

You can use the ElastiCache API to reconfigure the shards in your Redis (cluster mode enabled) cluster online by using the ModifyReplicationGroupShardConfiguration operation.

Use the following parameters with ModifyReplicationGroupShardConfiguration.

Parameters

  • ApplyImmediately=true – Required. Specifies the shard reconfiguration operation is to be started immediately.

  • ReplicationGroupId – Required. Specifies which replication group (cluster) the shard reconfiguration operation is to be performed on.

  • NodeGroupCount – Required. To rebalance the keyspaces across all shards in the cluster, this value must be the same as the current number of shards.

The following process describes how to reconfigure the shards in your Redis (cluster mode enabled) cluster by rebalancing the shards using the ElastiCache API.

Example - Rebalancing a Cluster

The following example rebalances the slots in the Redis (cluster mode enabled) cluster my-cluster so that the slots are distributed as equally as possible. The value of NodeGroupCount (4) is the number of shards currently in the cluster.

https://elasticache.us-east-2.amazonaws.com/ ?Action=ModifyReplicationGroupShardConfiguration &ApplyImmediately=true &NodeGroupCount=4 &ReplicationGroupId=my-cluster &Version=2015-02-02 &SignatureVersion=4 &SignatureMethod=HmacSHA256 &Timestamp=20171002T192317Z &X-Amz-Credential=<credential>

For more information, see ModifyReplicationGroupShardConfiguration in the ElastiCache API Reference.