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.

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
println!("Local schema version is: {}", session.fetch_schema_version().await?);
Ok(())
}
}

Awaiting schema agreement

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

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
session.await_schema_agreement().await?;
Ok(())
}
}

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.

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
use std::time::Duration;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
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");
}
Ok(())
}
}

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.

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::SessionBuilder;
use std::error::Error;
use std::time::Duration;
async fn check_only_compiles() -> Result<(), Box<dyn Error>> {
SessionBuilder::new()
    .known_node("127.0.0.1:9042")
    .schema_agreement_interval(Duration::from_secs(1))
    .build()
    .await?;
Ok(())
}
}

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.

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
if session.check_schema_agreement().await? { 
    println!("SCHEMA AGREED");
} else {
    println!("SCHEMA IS NOT IN AGREEMENT");
}
Ok(())
}
}