A request is idempotent if executing it multiple times leaves the database in the same state as executing it only once.
basic.request.default-idempotence in the configuration (defaults to false).
retries and speculative executions only happen for idempotent statements.
update my_table set list_col =  where pk = 1 is idempotent: no matter how many times it gets
list_col will always end up with the value
update my_table set list_col =  + list_col where pk = 1 is not idempotent: if
initially empty, it will contain
 after the first execution,
[1, 1] after the second, etc.
Idempotence matters because the driver sometimes re-runs requests automatically:
retries: if we’re waiting for a response from a node and the connection gets dropped, the default retry policy automatically retries on another node. But we can’t know what went wrong with the first node: maybe it went down, or maybe it was just a network issue; in any case, it might have applied the changes already. Therefore non-idempotent requests are never retried.
speculative executions: if they are enabled and a node takes too long to respond, the driver queries another node to get the response faster. But maybe both nodes will eventually apply the changes. Therefore non-idempotent requests are never speculatively executed.
In most cases, you need to flag your statements manually:
SimpleStatement statement = SimpleStatement.newInstance("SELECT first_name FROM user WHERE id=1") .setIdempotent(true); // Or with a builder: SimpleStatement statement = SimpleStatement.builder("SELECT first_name FROM user WHERE id=1") .setIdempotence(true) .build();
If you don’t, they default to the value defined in the configuration by the
basic.request.default-idempotence option; out of the box, it is set to
When you prepare a statement, its idempotence carries over to bound statements:
PreparedStatement pst = session.prepare( SimpleStatement.newInstance("SELECT first_name FROM user WHERE id=?") .setIdempotent(true)); BoundStatement bs = pst.bind(1); assert bs.isIdempotent();
The query builder tries to infer idempotence automatically; refer to its manual for more details.