After a query fails the driver might decide to retry it based on its Retry Policy and the query itself.
Retry policy can be configured for Session or just for a single query.
By default there are three retry policies:
Fallthrough Retry Policy - never retries, returns all errors straight to the user
Default Retry Policy - used by default, might retry if there is a high chance of success
Downgrading Consistency Retry Policy - behaves as Default Retry Policy, but also,
in some more cases, it retries with lower Consistency.
It’s possible to implement a custom Retry Policy by implementing the traits RetryPolicy and RetrySession.
A query is idempotent if it can be applied multiple times without changing the result of the initial application
Specifying that a query is idempotent increases the chances that it will be retried in case of failure. Idempotent queries can be retried in situations where retrying non idempotent queries would be dangerous.
Idempotence has to be specified manually, the driver is not able to figure it out by itself.
use scylla::query::Query;
use scylla::prepared_statement::PreparedStatement;
// Specify that a Query is idempotent
let mut my_query: Query = Query::new("SELECT a FROM ks.tab");
my_query.set_is_idempotent(true);
// Specify that a PreparedStatement is idempotent
let mut prepared: PreparedStatement = session
.prepare("SELECT a FROM ks.tab")
.await?;
prepared.set_is_idempotent(true);