Supported Apache Cassandra read and write consistency levels and associated costs

The topics in this section describe which Apache Cassandra consistency levels are supported for read and write operations in Amazon Keyspaces (for Apache Cassandra).

Write consistency levels

Amazon Keyspaces replicates all write operations three times across multiple Availability Zones for durability and high availability. Writes are durably stored before they are acknowledged using the LOCAL_QUORUM consistency level. For each 1 KB write, you are billed 1 write capacity unit (WCU) for tables using provisioned capacity mode or 1 write request unit (WRU) for tables using on-demand mode.

You can use cqlsh to set the consistency for all queries in the current session to LOCAL_QUORUM using the following code.

CONSISTENCY LOCAL_QUORUM;

To configure the consistency level programmatically, you can set the consistency with the appropriate Cassandra client drivers. For example, the 4.x version Java drivers allow you to set the consistency level in the app config file as shown below.

basic.request.consistency = LOCAL_QUORUM

If you're using a 3.x version Java Cassandra driver, you can specify the consistency level for the session by adding .withQueryOptions(new QueryOptions().setConsistencyLevel(ConsistencyLevel.LOCAL_QUORUM) as shown in the following code example.

Session session = Cluster.builder()  
                      .addContactPoint(endPoint)  
                      .withPort(portNumber)  
                      .withAuthProvider(new SigV4AuthProvider("us-east-2"))  
                      .withSSL()
                      .withQueryOptions(new QueryOptions().setConsistencyLevel(ConsistencyLevel.LOCAL_QUORUM)
                      .build()  
                      .connect();

To configure the consistency level for specific write operations, you can define the consistency when you call QueryBuilder.insertInto with a setConsistencyLevel argument when you're using the Java driver.

Read consistency levels

Amazon Keyspaces supports three read consistency levels: ONE, LOCAL_ONE, and LOCAL_QUORUM. During a LOCAL_QUORUM read, Amazon Keyspaces returns a response reflecting the most recent updates from all prior successful write operations. Using the consistency level ONE or LOCAL_ONE can improve the performance and availability of your read requests, but the response might not reflect the results of a recently completed write.

For each 4 KB read using ONE or LOCAL_ONE consistency, you are billed 0.5 read capacity units (RCUs) for tables using provisioned capacity mode or 0.5 read request units (RRUs) for tables using on-demand mode. For each 4 KB read using LOCAL_QUORUM consistency, you are billed 1 read capacity unit (RCU) for tables using provisioned capacity mode or 1 read request units (RRU) for tables using on-demand mode.

To specify a different consistency for read operations, call QueryBuilder.select with a setConsistencyLevel argument when you're using the Java driver.

Unsupported consistency levels

The following consistency levels are not supported by Amazon Keyspaces and will result in exceptions.