State within a durable object

[eminence]

Hi all,

I have a question about how to manage state within durable object (which is also a more general question about how these objects work).

Using the example from the README:

#[durable_object]
pub struct Chatroom {
    users: Vec<User>,
    messages: Vec<Message>,
    state: State,
    env: Env, // access `Env` across requests, use inside `fetch`
}

#[durable_object]
impl DurableObject for Chatroom {
    fn new(state: State, env: Env) -> Self {
        Self {
            users: vec![],
            messages: vec![],
            state: state,
            env,
        }
    }

    async fn fetch(&mut self, _req: Request) -> Result<Response> {
        // do some work when a worker makes a request to this DO
        Response::ok(&format!("{} active users.", self.users.len()))
    }
}

Imagine that I want to have fetch update the state of the object (for example by storing an incoming message). I see that I have mutable access to things like self.messages, and also self.state. Is it as simple as appending something to self.messages ? Or do I need to use the transactional storage API via self.state.storage() ?

If it is indeed as simple as updating self.messages, then how does that state actually get persisted?

Thanks!