In Cassandra, each mutation has a microsecond-precision timestamp, which is used to order operations relative to each other.

There are various ways to assign it:

If the client does not specify any timestamp, the server will assign one based on the time it receives the query.

This can be a problem when the order of the writes matter: with unlucky timing (different coordinators, network latency, etc.), two successive requests from the same client might be processed in a different order server-side, and end up with out-of-order timestamps.

You can explicitly provide the timestamp in your CQL query:

session.execute("INSERT INTO my_table(c1, c2) values (1, 1) " +
    "USING TIMESTAMP 1432815430948040");

This solves the problems of server-side generation: the client has control over the ordering of the statements it generates. On the other hand, it puts the burden of generating timestamps on client code.

Starting with version 2.1.2 of the driver, and if native protocol v3 or above is in use, a default timestamp can be sent with each query.

To enable this feature, provide an instance of TimestampGenerator at initialization:

Cluster.builder().addContactPoint("127.0.0.1")
    .withTimestampGenerator(new AtomicMonotonicTimestampGenerator())
    .build();

The default is still server-side generation. So unless you explicitly provide a generator, you get the same behavior as previous driver versions.

In addition, you can also override the default timestamp on a per-statement basis:

Statement statement = new SimpleStatement(
    "UPDATE users SET email = 'x@y.com' where id = 1");
statement.setDefaultTimestamp(1234567890);
session.execute(statement);

As shown in the previous sections, there are multiple ways to provide a timestamp, some of which overlap. The order of precedence is the following:

1. if there is a USING TIMESTAMP clause in the CQL string, use that over anything else;
2. otherwise, if a default timestamp was set on the statement and is different from Long.MIN_VALUE, use it;
3. otherwise, if a generator is specified, invoke it and use its result if it is different from Long.MIN_VALUE;
4. otherwise, let the server assign the timestamp.

Steps 2 and 3 only apply if native protocol v3 or above is in use.