DynamoDB provisioned capacity mode

When you create a new provisioned table in DynamoDB, you must specify its provisioned throughput capacity. This is the amount of read and write throughput that the table can support. You'll be charged based on the hourly read and write capacity you have provisioned, not how much of that provisioned capacity you actually consumed.

As your application's data and access requirements change, you might need to adjust your table's throughput settings. You can use auto scaling to adjust your table’s provisioned capacity automatically in response to traffic changes. DynamoDB auto scaling uses a scaling policy in Application Auto Scaling. To configure auto scaling in DynamoDB, you set the minimum and maximum levels of read and write capacity in addition to the target utilization percentage. Application Auto Scaling creates and manages the CloudWatch alarms that trigger scaling events when the metric deviates from the target. Auto Scaling monitors your table’s activity and adjusts its capacity settings up or down based on preconfigured thresholds. Auto scaling triggers when your consumed capacity breaches the configured target utilization for two consecutive minutes. CloudWatch alarms might have a short delay of up to a few minutes before triggering auto scaling. For more information, see Managing throughput capacity automatically with DynamoDB auto scaling.

If you're using DynamoDB auto scaling, the throughput settings are automatically adjusted in response to actual workloads. You can also use the UpdateTable operation to manually adjust your table's throughput capacity. For example, you might decide to do this if you need to bulk-load data from an existing data store into your new DynamoDB table. You could create the table with a large write throughput setting and then reduce this setting after the bulk data load is complete.

You can switch tables from on-demand mode to provisioned capacity mode at any time. When you do multiple switches between capacity modes, the following conditions apply:

For more information about switching between read and write capacity modes, see Considerations when switching capacity modes in DynamoDB.

Read capacity units and write capacity units

For provisioned mode tables, you specify throughput requirements in terms of capacity units. These units represent the amount of data your application needs to read or write per second. You can modify these settings later, if needed, or enable DynamoDB auto scaling to modify them automatically.

For an item up to 4 KB, one read capacity unit (RCU) represents one strongly consistent read operation per second, or two eventually consistent read operations per second. For more information about DynamoDB read consistency models, see DynamoDB read consistency.

A write capacity unit (WCU) represents one write per second for an item up to 1 KB. For more information about the different read and write operations, see DynamoDB read and write operations.

Choosing initial throughput settings

Every application has different requirements for reading from and writing to a database. When you're determining the initial throughput settings for a DynamoDB table, consider the following:

For example, say that you want to read 80 items per second from a table. The size of these items is 3 KB, and you want strongly consistent reads. In this case, each read requires one provisioned read capacity unit. To determine this number, divide the item size of the operation by 4 KB. Then, round up to the nearest whole number, as shown in the following example:

Therefore, to read 80 items per second from a table, set the table's provisioned read throughput to 80 read capacity units as shown in the following example:

Now suppose that you want to write 100 items per second to your table and that the size of each item is 512 bytes. In this case, each write requires one provisioned write capacity unit. To determine this number, divide the item size of the operation by 1 KB. Then, round up to the nearest whole number, as shown in the following example:

To write 100 items per second to your table, set the table's provisioned write throughput to 100 write capacity units:

DynamoDB auto scaling

DynamoDB auto scaling actively manages provisioned throughput capacity for tables and global secondary indexes. With auto scaling, you define a range (upper and lower limits) for read and write capacity units. You also define a target utilization percentage within that range. DynamoDB auto scaling seeks to maintain your target utilization, even as your application workload increases or decreases.

With DynamoDB auto scaling, a table or a global secondary index can increase its provisioned read and write capacity to handle sudden increases in traffic, without request throttling. When the workload decreases, DynamoDB auto scaling can decrease the throughput so that you don't pay for unused provisioned capacity.

Utilization rate

Utilization rate can help you determine if you’re over provisioning capacity, in which case should reduce your table capacity to save costs. Conversely, it can also help you determine if you’re under provisioning capacity. In this case, you should increase table capacity to prevent potential throttling of requests during unexpected high traffic instances. For more information, see Amazon DynamoDB auto scaling: Performance and cost optimization at any scale.

If you’re using DynamoDB auto scaling, you’ll also need to set a target utilization percentage. Auto scaling will use this percentage as a target to adjust capacity upward or downward. We recommend setting target utilization to 70%. For more information, see Managing throughput capacity automatically with DynamoDB auto scaling.