Creating the Application State

The state of a Linera application consists of onchain data that are persisted between transactions.

The struct which defines your application's state can be found in src/state.rs. To represent our counter, we're going to use a u64 integer.

While we could use a plain data-structure for the entire application state:

struct Counter {
  value: u64
}

in general, we prefer to manage persistent data using the concept of "views":

Views allow an application to load persistent data in memory and stage modifications in a flexible way.

Views resemble the persistent objects of an ORM framework, except that they are stored as a set of key-value pairs (instead of a SQL row).

In this case, the struct in src/state.rs should be replaced by

extern crate linera_sdk;
extern crate async_graphql;
use linera_sdk::linera_base_types::*;
use linera_sdk::*;
use std::collections::HashSet;
use linera_sdk::views::{linera_views, RegisterView, RootView, ViewStorageContext};
use crate::linera_sdk::views::View as _;
/// The application state.
#[derive(RootView, async_graphql::SimpleObject)]
#[view(context = ViewStorageContext)]
pub struct Counter {
    pub value: RegisterView<u64>,
    // Additional fields here will get their own key in storage.
}

and the occurrences of Application in the rest of the project should be replaced by Counter.

The derive macro async_graphql::SimpleObject is related to GraphQL queries discussed in the next section.

A RegisterView<T> supports modifying a single value of type T. Other data structures available in the library linera_views include:

• LogView for a growing vector of values;
• QueueView for queues;
• MapView and CollectionView for associative maps; specifically, MapView in the case of static values, and CollectionView when values are other views.

For an exhaustive list of the different constructions, refer to the crate documentation.