Retry policy configuration¶

Retry policies¶

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.

Idempotence and retry policies¶

Retry policies and speculative execution treat idempotence differently:

• Speculative execution strictly requires is_idempotent = true. Without it, speculative fibers do not start at all, even if a SpeculativeExecutionPolicy is configured on the Session.

• Retry policies receive is_idempotent as one input on RequestInfo and decide for themselves. They can - and the bundled policies do - retry some non-idempotent statements when the failure mode makes re-execution safe (for example, the Default Retry Policy retries on IsBootstrapping and UnableToAllocStreamId regardless of idempotence, because the request is known not to have reached the server). Conversely, on errors where the request may have already partially executed (broken connection, Overloaded, ServerError, TruncateError), retries only happen when the statement is marked idempotent.

Marking a statement idempotent therefore unlocks speculative execution entirely and widens the set of error classes the retry policies will act on.

Query idempotence¶

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::statement::unprepared::Statement;
use scylla::statement::prepared::PreparedStatement;

// Specify that a Statement is idempotent
let mut my_query: Statement = Statement::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);