Scylla Rust Driver Making queries Schema agreement

Schema agreement¶

Sometimes after performing queries some nodes have not been updated so we need a mechanism that checks if every node have agreed schema version. There are four methods in Session that assist us. Every method raise QueryError if something goes wrong, but they should never raise any errors, unless there is a DB or connection malfunction.

Checking schema version¶

Session::fetch_schema_version returns an Uuid of local node’s schema version.

println!("Local schema version is: {}", session.fetch_schema_version().await?);

Awaiting schema agreement¶

Session::await_schema_agreement returns a Future that can be awaited on as long as schema is not in an agreement.

session.await_schema_agreement().await?;

Awaiting with timeout¶

We can also set timeout in miliseconds with Session::await_timed_schema_agreement. It takes one argument, an std::time::Duration value that tells how long our driver should await for schema agreement. If the timeout is met the return value is false otherwise it is true.

if session.await_timed_schema_agreement(Duration::from_secs(5)).await? { // wait for 5 seconds
    println!("SCHEMA AGREED");
} else {
    println!("SCHEMA IS NOT IN AGREEMENT - TIMED OUT");
}

Checking for schema interval¶

If schema is not agreed driver sleeps for a duration before checking it again. Default value is 200 miliseconds 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? { 
    println!("SCHEMA AGREED");
} else {
    println!("SCHEMA IS NOT IN AGREEMENT");
}