idempotence

Query idempotence

Quick overview

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).
• can be overridden per statement Statement.setIdempotent or StatementBuilder.setIdempotence.
• retries and speculative executions only happen for idempotent statements.

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For example:

• update my_table set list_col = [1] where pk = 1 is idempotent: no matter how many times it gets executed, list_col will always end up with the value [1];
• update my_table set list_col = [1] + list_col where pk = 1 is not idempotent: if list_col was initially empty, it will contain [1] 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 false.

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.