Creating the Application State

The struct which defines your application's state can be found in src/state.rs.

To represent our counter, we're going to need a single u64. To persist the counter we'll be using Linera's view paradigm.

Views are a little like an ORM, however instead of mapping data structures to a relational database like Postgres, they are instead mapped onto key-value stores like RocksDB.

In vanilla Rust, we might represent our Counter as so:

// do not use this
struct Counter {
  value: u64
}

However, to persist your data, you'll need to replace the existing Application state struct in src/state.rs with the following view:

/// The application state.
#[derive(RootView, async_graphql::SimpleObject)]
#[view(context = "ViewStorageContext")]
pub struct Counter {
    pub value: RegisterView<u64>,
}

and all other occurrences of Application in your app.

The RegisterView<T> supports modifying a single value of type T. There are different types of views for different use-cases, but the majority of common data structures have already been implemented:

  • A Vec or VecDeque corresponds to a LogView
  • A BTreeMap corresponds to a MapView if its values are primitive, or to CollectionView if its values are other views;
  • A Queue corresponds to a QueueView

For an exhaustive list refer to the Views documentation.

Finally, run cargo check to ensure that your changes compile.