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Schema agreement¶
Sometimes after executing CQL statements some nodes have not been updated, so we need a mechanism that checks if every node have agreed on schema version.
Automated awaiting schema agreement¶
The driver automatically awaits schema agreement after a schema-altering statement is executed. Waiting for schema agreement more than necessary is never a bug, but might slow down applications which do a lot of schema changes (e.g. a migration). For instance, in case where somebody wishes to create a keyspace and then a lot of tables in it, it makes sense only to wait after creating a keyspace and after creating all the tables rather than after every statement execution. Therefore, the said behaviour can be disabled:
let session = SessionBuilder::new()
.known_node("127.0.0.1:9042")
.auto_await_schema_agreement(false)
.build()
.await?;
Manually awaiting schema agreement¶
Session::await_schema_agreement returns a Future that can be awaited as long as schema is not in an agreement.
However, it won’t wait forever; SessionConfig defines a timeout that limits the time of waiting. If the timeout elapses,
the return value is Err(SchemaAgreementError::Timeout), otherwise it is Ok(schema_version).
session.await_schema_agreement().await?;
Interval of checking for schema agreement¶
If the schema is not agreed upon, the driver sleeps for a duration before checking it again. The default value is 200 milliseconds,
but it can be changed with SessionBuilder::schema_agreement_interval.
SessionBuilder::new()
.known_node("127.0.0.1:9042")
.schema_agreement_interval(Duration::from_secs(1))
.build()
.await?;
Checking if schema is in agreement now¶
If you want to check if schema is in agreement now, without retrying after failure, you can use Session::check_schema_agreement function.
if session.check_schema_agreement().await?.is_some() {
println!("SCHEMA AGREED");
} else {
println!("SCHEMA IS NOT IN AGREEMENT");
}