Defining the database struct

First, we need to create the database struct. Typically it is only used by the "driver" of your application; the one which starts up the program, supplies the inputs, and relays the outputs.

In calc, the database struct is in the db module, and it looks like this:

#![allow(unused)]
fn main() {
#[salsa::db]
#[derive(Default, Clone)]
pub struct CalcDatabaseImpl {
    storage: salsa::Storage<Self>,

    // The logs are only used for testing and demonstrating reuse:
    logs: Arc<Mutex<Option<Vec<String>>>>,
}
}

The #[salsa::db] attribute marks the struct as a database. It must have a field named storage whose type is salsa::Storage<Self>, but it can also contain whatever other fields you want.

Implementing the salsa::Database trait

In addition to the struct itself, we must add an impl of salsa::Database:

#![allow(unused)]
fn main() {
#[salsa::db]
impl salsa::Database for CalcDatabaseImpl {
    fn salsa_event(&self, event: &dyn Fn() -> salsa::Event) {
        let event = event();
        eprintln!("Event: {event:?}");
        // Log interesting events, if logging is enabled
        if let Some(logs) = &mut *self.logs.lock().unwrap() {
            // only log interesting events
            if let salsa::EventKind::WillExecute { .. } = event.kind {
                logs.push(format!("Event: {event:?}"));
            }
        }
    }
}
}

Implementing the salsa::ParallelDatabase trait

If you want to permit accessing your database from multiple threads at once, then you also need to implement the ParallelDatabase trait:

#![allow(unused)]

fn main() {
}