Writing Tests

Linera applications can be tested using unit tests or integration tests. Both are a bit different than usual Rust tests. Unit tests are executed inside a WebAssembly virtual machine in an environment that simulates a single microchain and a single application. System APIs are only available if they are mocked using helper functions from linera_sdk::test.

Integration tests run outside a WebAssembly virtual machine, and use a simulated validator for testing. This allows testing interactions between multiple microchains and multiple applications. However, it is also more low-level because testing the application requires interacting with microchains, so execution only happens when blocks are published.

Applications should consider having both types of tests. Unit tests should be used to focus on the application's internals and core functionality. Integration tests should be used to test how the application behaves on a more complex environment that's closer to the real network.

Unit tests

Unit tests can be written beside the application's source code (i.e., inside the src/ directory of the project). There are two major differences to normal Rust unit tests:

  • linera project test is used instead of cargo test (or the environment must be configured so that Cargo uses a custom test runner, as described below);
  • the #[webassembly_test] attribute is used instead of the usual #[test] attribute.

Manually Configuring the Environment

Running linera project test is easier, but if there's a need to run cargo test explicitly to run the unit tests, Cargo must be configured to use the custom test-runner. The test-runner can be built from the repository.

cd linera-protocol
cargo build -p linera-sdk --bin test-runner --release

The steps above build the test-runner and places the resulting binary at linera-protocol/target/release/test-runner.

With the binary available, the last step is to configure Cargo. There are a few ways to do this. A quick way is to set the CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER environment variable to the path of the binary. A more persistent way is to change one of Cargo's configuration files. As an example, the following file can be placed inside the project's directory at PROJECT_DIR/.cargo/config.toml:

runner = "PATH_TO/test-runner"

After configuring the test runner, unit tests can be executed with

cargo test --target wasm32-unknown-unknown


A simple unit test is shown below, which tests if the application's do_something method changes the application state.

mod tests {
    use crate::state::ApplicationState;
    use webassembly_test::webassembly_test;

    fn test_do_something() {
        let mut application = ApplicationState {
            // Configure the application's initial state

        let result = application.do_something();

        assert_eq!(application, ApplicationState {
            // Define the application's expected final state

Mocking System APIs

Unit tests run in a constrained environment, so things like access to the key-value store, cross-chain messages and cross-application calls can't be executed. However, they can be simulated using mock APIs. The linera-sdk::test module provides some helper functions to mock the system APIs.

Here's an example mocking the key-value store.

mod tests {
    use crate::state::ApplicationState;
    use linera_sdk::test::mock_key_value_store;
    use webassembly_test::webassembly_test;

    fn test_state_is_not_persisted() {
        let mut storage = mock_key_value_store();

        // Assuming the application uses views
        let mut application = ApplicationState::load(storage.clone())
            .expect("Mock key-value store returns immediately")
            .expect("Failed to load view from mock key-value store");

        // Assuming `do_something` changes the view, but does not persist it
        let result = application.do_something();


        // Check that the state in memory is different from the state in storage
        assert_ne!(application, ApplicatonState::load(storage));

Integration Tests

Integration tests are usually written separately from the application's source code (i.e., inside a tests directory that's beside the src directory). Integration tests are normal Rust integration tests, and they are compiled to the host target instead of the wasm32-unknown-unknown target used for unit tests. This is because unit tests run inside a WebAssembly virtual machine and integration tests run outside a virtual machine, starting isolated virtual machines to run each operation of each block added to each chain.

Integration tests use the helper types from linera_sdk::test to set up a simulated Linera network, and publish blocks to microchains in order to execute the application.


A simple test that sends a message between application instances on different chains is shown below.

async fn test_cross_chain_message() {
    let (validator, application_id) = TestValidator::with_current_application(vec![], vec![]).await;

    let mut sender_chain = validator.get_chain(application_id.creation.chain_id).await;
    let mut receiver_chain = validator.new_chain().await;

        .add_block(|block| {


            .query::<ChainId>(application_id, Query::LastSender)