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.
Session::fetch_schema_version returns an Uuid of local node’s schema version.
# 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(())
# }
Session::await_schema_agreement returns a Future that can be awaited on as long as schema is not in an agreement.
# 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(())
# }
We can also set timeout in milliseconds 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.
# 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(())
# }
If schema is not agreed driver sleeps for a duration before checking it again. Default value is 200 milliseconds but it can be changed with SessionBuilder::schema_agreement_interval.
# 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(())
# }
If you want to check if schema is in agreement now without retrying after failure you can use Session::check_schema_agreement function.
# 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(())
# }