Configure on-demand capacity mode - Amazon Keyspaces (for Apache Cassandra)
Read request units and write request unitsPeak traffic and scaling propertiesInitial throughput for on-demand capacity mode

Configure on-demand capacity mode

Amazon Keyspaces (for Apache Cassandra) on-demand capacity mode is a flexible billing option capable of serving thousands of requests per second without capacity planning. This option offers pay-per-request pricing for read and write requests so that you pay only for what you use.

When you choose on-demand mode, Amazon Keyspaces can scale the throughput capacity for your table up to any previously reached traffic level instantly, and then back down when application traffic decreases. If a workload’s traffic level hits a new peak, the service adapts rapidly to increase throughput capacity for your table. You can enable on-demand capacity mode for both new and existing tables.

On-demand mode is a good option if any of the following is true:

  • You create new tables with unknown workloads.

  • You have unpredictable application traffic.

  • You prefer the ease of paying for only what you use.

To get started with on-demand mode, you can create a new table or update an existing table to use on-demand capacity mode using the console or with a few lines of Cassandra Query Language (CQL) code. For more information, see Tables.

Read request units and write request units

With on-demand capacity mode tables, you don't need to specify how much read and write throughput you expect your application to use in advance. Amazon Keyspaces charges you for the reads and writes that you perform on your tables in terms of read request units (RRUs) and write request units (WRUs).

  • One RRU represents one LOCAL_QUORUM read request, or two LOCAL_ONE read requests, 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 RRUs. The total number of RRUs required depends on the row size, and whether you want to use LOCAL_QUORUM or LOCAL_ONE read consistency. For example, reading an 8 KB row requires 2 RRUs using LOCAL_QUORUM read consistency, and 1 RRU if you choose LOCAL_ONE read consistency.

  • One WRU represents one write 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 WRUs. The total number of WRUs required depends on the row size. For example, if your row size is 2 KB, you require 2 WRUs to perform one write request.

For information about supported consistency levels, see Supported Apache Cassandra read and write consistency levels and associated costs.

Peak traffic and scaling properties

Amazon Keyspaces tables that use on-demand capacity mode automatically adapt to your application’s traffic volume. On-demand capacity mode instantly accommodates up to double the previous peak traffic on a table. For example, your application's traffic pattern might vary between 5,000 and 10,000 LOCAL_QUORUM reads per second, where 10,000 reads per second is the previous traffic peak.

With this pattern, on-demand capacity mode instantly accommodates sustained traffic of up to 20,000 reads per second. If your application sustains traffic of 20,000 reads per second, that peak becomes your new previous peak, enabling subsequent traffic to reach up to 40,000 reads per second.

If you need more than double your previous peak on a table, Amazon Keyspaces automatically allocates more capacity as your traffic volume increases. This helps ensure that your table has enough throughput capacity to process the additional requests. However, you might observe insufficient throughput capacity errors if you exceed double your previous peak within 30 minutes.

For example, suppose that your application's traffic pattern varies between 5,000 and 10,000 strongly consistent reads per second, where 20,000 reads per second is the previously reached traffic peak. In this case, the service recommends that you space your traffic growth over at least 30 minutes before driving up to 40,000 reads per second.

To learn how to estimate read and write capacity consumption of a table, see Estimate capacity consumption of read and write throughput in Amazon Keyspaces.

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

Initial throughput for on-demand capacity mode

If you create a new table with on-demand capacity mode enabled or switch an existing table to on-demand capacity mode for the first time, the table has the following previous peak settings, even though it hasn't served traffic previously using on-demand capacity mode:

  • Newly created table with on-demand capacity mode: The previous peak is 2,000 WRUs and 6,000 RRUs. You can drive up to double the previous peak immediately. Doing this enables newly created on-demand tables to serve up to 4,000 WRUs and 12,000 RRUs.

  • Existing table switched to on-demand capacity mode: The previous peak is half the previous WCUs and RCUs provisioned for the table or the settings for a newly created table with on-demand capacity mode, whichever is higher.