Configure provisioned throughput capacity mode - Amazon Keyspaces (for Apache Cassandra)
Read capacity units and write capacity units

Configure provisioned throughput capacity mode

If you choose provisioned throughput capacity mode, you specify the number of reads and writes per second that are required for your application. This helps you manage your Amazon Keyspaces usage to stay at or below a defined request rate to maintain predictability. To learn more about automatic scaling for provisioned throughput see Manage throughput capacity automatically with Amazon Keyspaces auto scaling.

Provisioned throughput capacity mode is a good option if any of the following is true:

  • You have predictable application traffic.

  • You run applications whose traffic is consistent or ramps up gradually.

  • You can forecast capacity requirements.

Read capacity units and write capacity units

For provisioned throughput capacity mode tables, you specify throughput capacity in terms of read capacity units (RCUs) and write capacity units (WCUs):

  • One RCU represents one LOCAL_QUORUM read per second, or two LOCAL_ONE reads per second, for a row up to 4 KB in size. If you need to read a row that is larger than 4 KB, the read operation uses additional RCUs.

    The total number of RCUs required depends on the row size, and whether you want LOCAL_QUORUM or LOCAL_ONE reads. For example, if your row size is 8 KB, you require 2 RCUs to sustain one LOCAL_QUORUM read per second, and 1 RCU if you choose LOCAL_ONE reads.

  • One WCU represents one write per second for a row up to 1 KB in size. All writes are using LOCAL_QUORUM consistency, and there is no additional charge for using lightweight transactions (LWTs). If you need to write a row that is larger than 1 KB, the write operation uses additional WCUs.

    The total number of WCUs required depends on the row size. For example, if your row size is 2 KB, you require 2 WCUs to sustain one write request per second. For more information about how to estimate read and write capacity consumption of a table, see Estimate capacity consumption of read and write throughput in Amazon Keyspaces.

If your application reads or writes larger rows (up to the Amazon Keyspaces maximum row size of 1 MB), it consumes more capacity units. To learn more about how to estimate the row size, see Estimate row size in Amazon Keyspaces. For example, suppose that you create a provisioned table with 6 RCUs and 6 WCUs. With these settings, your application could do the following:

  • Perform LOCAL_QUORUM reads of up to 24 KB per second (4 KB × 6 RCUs).

  • Perform LOCAL_ONE reads of up to 48 KB per second (twice as much read throughput).

  • Write up to 6 KB per second (1 KB × 6 WCUs).

Provisioned throughput is the maximum amount of throughput capacity an application can consume from a table. If your application exceeds your provisioned throughput capacity, you might observe insufficient capacity errors.

For example, a read request that doesn’t have enough throughput capacity fails with a Read_Timeout exception and is posted to the ReadThrottleEvents metric. A write request that doesn’t have enough throughput capacity fails with a Write_Timeout exception and is posted to the WriteThrottleEvents metric.

You can use Amazon CloudWatch to monitor your provisioned and actual throughput metrics and insufficient capacity events. For more information about these metrics, see Amazon Keyspaces metrics and dimensions.

Note

Repeated errors due to insufficient capacity can lead to client-side driver specific exceptions, for example the DataStax Java driver fails with a NoHostAvailableException.

To change the throughput capacity settings for tables, you can use the AWS Management Console or the ALTER TABLE statement using CQL, for more information see ALTER TABLE.

To learn more about default quotas for your account and how to increase them, see Quotas for Amazon Keyspaces (for Apache Cassandra).