Evaluate your table's auto scaling settings - Amazon DynamoDB
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Evaluate your table's auto scaling settings

This section provides an overview of how to evaluate the auto scaling settings on your DynamoDB tables. Amazon DynamoDB auto scaling is a feature that manages table and global secondary index (GSI) throughput based on your application traffic and your target utilization metric. This ensures your tables or GSIs will have the required capacity required for your application patterns.

The Amazon auto scaling service will monitor your current table utilization and compare it to the target utilization value: TargetValue. It will notify you if it is time to increase or decrease the capacity allocated.

Understanding your auto scaling settings

Defining the correct value for the target utilization, initial step, and final values is an activity that requires involvement from your operations team. This allows you to properly define the values based on historical application usage, which will be used to trigger the Amazon auto scaling policies. The utilization target is the percentage of your total capacity that needs to be hit during a period of time before the auto scaling rules apply.

When you set a high utilization target (a target around 90%) it means your traffic needs to be higher than 90% for a period of time before the auto scaling kicks in. You should not use a high utilization target unless your application is very constant and doesn’t receive spikes in traffic.

When you set a very low utilization (a target less than 50%) it means your application would need to reach 50% of the provisioned capacity before it triggers an auto scaling policy. Unlless your application traffic grows at a very aggressive rate, this usually translates into unused capacity and wasted resources.

How to identify tables with low target utilization (<=50%)

You can use either the Amazon CLI or Amazon Web Services Management Console to monitor and identify the TargetValues for your auto scaling policies in your DynamoDB resources:

Amazon CLI
  1. Return the entire list of resources by running the following command:

    aws application-autoscaling describe-scaling-policies --service-namespace dynamodb

    This command will return the entire list of auto scaling policies that are issued to any DynamoDB resource. If you only want to retrieve the resources from a particular table, you can add the –resource-id parameter. For example:

    aws application-autoscaling describe-scaling-policies --service-namespace dynamodb --resource-id "table/<table-name>”
  2. Return only the auto scaling policies for a particular GSI by running the following command

    aws application-autoscaling describe-scaling-policies --service-namespace dynamodb --resource-id "table/<table-name>/index/<gsi-name>”

    The values we're interested in for the auto scaling policies are highlighted below. We want to ensure that the target value is greater than 50% to avoid over-provisioning. You should obtain a result similar to the following:

    { "ScalingPolicies": [ { "PolicyARN": "arn:aws:autoscaling:<region>:<account-id>:scalingPolicy:<uuid>:resource/dynamodb/table/<table-name>/index/<index-name>:policyName/$<full-gsi-name>-scaling-policy", "PolicyName": $<full-gsi-name>”, "ServiceNamespace": "dynamodb", "ResourceId": "table/<table-name>/index/<index-name>", "ScalableDimension": "dynamodb:index:WriteCapacityUnits", "PolicyType": "TargetTrackingScaling", "TargetTrackingScalingPolicyConfiguration": { "TargetValue": 70.0, "PredefinedMetricSpecification": { "PredefinedMetricType": "DynamoDBWriteCapacityUtilization" } }, "Alarms": [ ... ], "CreationTime": "2022-03-04T16:23:48.641000+10:00" }, { "PolicyARN": "arn:aws:autoscaling:<region>:<account-id>:scalingPolicy:<uuid>:resource/dynamodb/table/<table-name>/index/<index-name>:policyName/$<full-gsi-name>-scaling-policy", "PolicyName":$<full-gsi-name>”, "ServiceNamespace": "dynamodb", "ResourceId": "table/<table-name>/index/<index-name>", "ScalableDimension": "dynamodb:index:ReadCapacityUnits", "PolicyType": "TargetTrackingScaling", "TargetTrackingScalingPolicyConfiguration": { "TargetValue": 70.0, "PredefinedMetricSpecification": { "PredefinedMetricType": "DynamoDBReadCapacityUtilization" } }, "Alarms": [ ... ], "CreationTime": "2022-03-04T16:23:47.820000+10:00" } ] }
Amazon Web Services Management Console
  1. Log into the Amazon Web Services Management Console and navigate to the CloudWatch service page at Getting Started with the Amazon Web Services Management Console. Select the appropriate Amazon region if necessary.

  2. On the left navigation bar, select Tables. On the Tables page, select the table's Name.

  3. On the Table Details page under the Additional Settings tab, review your table's auto scaling settings.

    For indexes, expand the Index Capacity tab to review the index's auto scaling settings.

If your target utilization values are less than or equal to 50%, you should explore your table utilization metrics to see if they are under-provisioned or over-provisioned.

How to address workloads with seasonal variance

Consider the following scenario: your application is operating under a minimum average value most of the time, but the utilization target is low so your application can react quickly to events that happen at certain hours in the day and you have enough capacity and avoid getting throttled. This scenario is common when you have an application that is very busy during normal office hours (9 AM to 5 PM) but then it works at a base level during after hours. Since some users will start to connect before 9 am, the application uses this low threshold to ramp up quickly to get to the required capacity during peak hours.

This scenario could look like this:

  • Between 5 PM and 9 AM the ConsumedWriteCapacity units stay between 90 and 100

  • Users start to connect to the application before 9 AM and the capacity units increases considerably (the maximum value you’ve seen is 1500 WCU)

  • On average, your application usage varies between 800 to 1200 during working hours

If the previous scenario applies to you, consider using scheduled auto scaling, where your table could still have an application auto scaling rule configured, but with a less aggressive target utilization that only provisions the extra capacity at the specific intervals you require.

You can use Amazon CLI to execute the following steps to create a scheduled auto scaling rule that will execute based on the time of day and the day of the week.

  1. Register your DynamoDB table or GSI as scalable target with Application Auto Scaling. A scalable target is a resource that Application Auto Scaling can scale out or in.

    aws application-autoscaling register-scalable-target \ --service-namespace dynamodb \ --scalable-dimension dynamodb:table:WriteCapacityUnits \ --resource-id table/<table-name> \ --min-capacity 90 \ --max-capacity 1500
  2. Set up scheduled actions according to your requirements.

    We'll need two rules to cover the scenario: one to scale up and another to scale down. The first rule to scale up the scheduled action:

    aws application-autoscaling put-scheduled-action \ --service-namespace dynamodb \ --scalable-dimension dynamodb:table:WriteCapacityUnits \ --resource-id table/<table-name> \ --scheduled-action-name my-8-5-scheduled-action \ --scalable-target-action MinCapacity=800,MaxCapacity=1500 \ --schedule "cron(45 8 ? * MON-FRI *)" \ --timezone "Australia/Brisbane"

    The second rule to scale down the scheduled action:

    aws application-autoscaling put-scheduled-action \ --service-namespace dynamodb \ --scalable-dimension dynamodb:table:WriteCapacityUnits \ --resource-id table/<table-name> \ --scheduled-action-name my-5-8-scheduled-down-action \ --scalable-target-action MinCapacity=90,MaxCapacity=1500 \ --schedule "cron(15 17 ? * MON-FRI *)" \ --timezone "Australia/Brisbane"
  3. Run the following command to validate both rules have been activated:

    aws application-autoscaling describe-scheduled-actions --service-namespace dynamodb

    You should get a result like this:

    { "ScheduledActions": [ { "ScheduledActionName": "my-5-8-scheduled-down-action", "ScheduledActionARN": "arn:aws:autoscaling:<region>:<account>:scheduledAction:<uuid>:resource/dynamodb/table/<table-name>:scheduledActionName/my-5-8-scheduled-down-action", "ServiceNamespace": "dynamodb", "Schedule": "cron(15 17 ? * MON-FRI *)", "Timezone": "Australia/Brisbane", "ResourceId": "table/<table-name>", "ScalableDimension": "dynamodb:table:WriteCapacityUnits", "ScalableTargetAction": { "MinCapacity": 90, "MaxCapacity": 1500 }, "CreationTime": "2022-03-15T17:30:25.100000+10:00" }, { "ScheduledActionName": "my-8-5-scheduled-action", "ScheduledActionARN": "arn:aws:autoscaling:<region>:<account>:scheduledAction:<uuid>:resource/dynamodb/table/<table-name>:scheduledActionName/my-8-5-scheduled-action", "ServiceNamespace": "dynamodb", "Schedule": "cron(45 8 ? * MON-FRI *)", "Timezone": "Australia/Brisbane", "ResourceId": "table/<table-name>", "ScalableDimension": "dynamodb:table:WriteCapacityUnits", "ScalableTargetAction": { "MinCapacity": 800, "MaxCapacity": 1500 }, "CreationTime": "2022-03-15T17:28:57.816000+10:00" } ] }

The following picture shows a sample workload that always keeps the 70% target utilization. Notice how the auto scaling rules are still applying and the throughput will not be reduced.

Zooming in, we can see there was a spike in the application that triggered the 70% auto scaling threshold, forcing the autoscaling to kick in and provide the extra capacity required for the table. The scheduled auto scaling action will affect maximum and minimum values, and it is your responsibility to set them up.

How to address spiky workloads with unknown patterns

In this scenario, the application uses a very low utilization target because you don’t know the application patterns yet, and you want to ensure your workload is not throttled.

Consider using on-demand capacity mode instead. On-demand tables are perfect for spiky workloads where you don’t know the traffic patterns. With on-demand capacity mode, you pay per request for the data reads and writes your application performs on your tables. You do not need to specify how much read and write throughput you expect your application to perform, as DynamoDB instantly accommodates your workloads as they ramp up or down.

How to address workloads with linked applications

In this scenario, the application depends on other systems, like batch processing scenarios where you can have big spikes in traffic according to events in the application logic.

Consider developing custom auto scaling logic that reacts to those events where you can increase table capacity and TargetValues depending on your specific needs. You could benefit from Amazon EventBridge and use a combination of Amazon services like Lambda and Step Functions to react to your specific application needs.