how to set cassandra read and write consistency

Question

I can not find the documentation for this. I know there is a consistency command in cqlsh, but there is no distinction between read and write consistency. How would I set different consistency levels for read and write?

Furthermore, there is a mention of a "default" consistency level. Where is that default set? and is it for read or write?

Answer 1

How would I set different consistency levels for read and write?

If you just want to change consistency level for your current session, use CONSISTENCY.

If you want to change the consistency level programamtically, use the cassandra driver for your client language.

Since the Consistency Level can be set per-statement, you can either set it on every statement, or use PreparedStatements. If you're using the Java driver, you can configure a global Consistency level for BOTH reads and writes (but not only reads and only writes).

Where is that default [consistency level] set? and is it for read or write?

If you want to set up a different consistency level for reads than writes, you'll have to do it on a per-statement basis. Use QueryOptions().setConsistencyLevel to set a global consistency level (for the Java driver). It is for BOTH reads and writes.

To set the consistency level for your current session, use the CONSISTENCY command from the cassandra shell (CQLSH).

For example:

Set CONSISTENCY to force the majority of the nodes to respond: CONSISTENCY QUORUM

To see your current consistency level, just run CONSISTENCY; from the shell:

ty@cqlsh> consistency; Current consistency level is ONE.

For programming client applications, set the consistency level using an appropriate driver.

To set a per-insert consistency level using the Java driver, for example, call QueryBuilder.insertInto with setConsistencyLevel.

For example:

PreparedStatement pstmt = session.prepare( "INSERT INTO product (sku, description) VALUES (?, ?)"); pstmt.setConsistencyLevel(ConsistencyLevel.QUORUM);

To set a global consistency level for reads AND writes using the Java driver, do something like:

QueryOptions qo = new QueryOptions().setConsistencyLevel(ConsistencyLevel.ALL);

• Since the Java driver only executes CQL statements, which can be either reads or writes to Cassandra, it is not possible to globally configure the Consistency Level for only reads or only writes. To do so, since the Consistency Level can be set per-statement, you can either set it on every statement, or use PreparedStatements. More info in the Queries and Results document.

More resources on read/write consistency levels

• How consistency level is configured
  • The different consistency levels are also explained here ^
• Cassandra's Java Driver documentation
• See the "Consistency levels" section in configuring consistency with the Java Driver

Other resources on consistency: You may also want to look into your replica placement strategy and replication factor (which are other forms of consistency). I included links below for good measure:

• Background on data replication in Cassandra
• How to update replica strategy and replication factor using CREATE KEYSPACE
• How consistency affects performance