Public Extension Points

This chapter lists every trait you implement to plug your application into the framework.

The framework never imports your application. It defines public traits that your integration crate implements and calls them at defined points in the run lifecycle. Each entry below links to the corresponding chapter.

Environment and Topology

Application is the root of every integration. It bundles the backend-specific types the scenario engine is generic over: a deployment descriptor, a node client, and a node config. The three methods have working defaults: override external_node_client to support external sources, build_node_client to support deployer-discovered nodes, and node_readiness_path when your health endpoint is not /.

#[async_trait]
pub trait Application: Send + Sync + 'static {
    type Deployment: DeploymentDescriptor + Clone + 'static;
    type NodeClient: Clone + Send + Sync + 'static;
    type NodeConfig: Clone + Send + Sync + 'static;

    fn external_node_client(source: &ExternalNodeSource) -> Result<Self::NodeClient, DynError>;
    fn build_node_client(access: &NodeAccess) -> Result<Self::NodeClient, DynError>;
    fn node_readiness_path() -> &'static str; // default "/"
}

Plugs in as the E type parameter of ScenarioBuilder<E>, Workload<E>, Expectation<E>, and every deployer.

DeploymentProvider<D> builds the deployment descriptor a scenario runs against, optionally driven by a DeploymentSeed for reproducible generation. ScenarioBuilder::new accepts one; ScenarioBuilder::with_deployment wraps a fixed value in the built-in FixedDeploymentProvider.

pub trait DeploymentProvider<D: DeploymentDescriptor>: Send + Sync {
    fn build(&self, seed: Option<&DeploymentSeed>) -> Result<D, DynTopologyError>;
}

Scenario Behavior

Workload<E> describes an action sequence executed during the run. start receives the RunContext<E> (node clients, extensions, run metrics) and runs concurrently with other workloads. A workload can bundle its own checks via expectations().

#[async_trait]
pub trait Workload<E: Application>: Send + Sync {
    fn name(&self) -> &str;
    fn expectations(&self) -> Vec<Box<dyn Expectation<E>>> { Vec::new() }
    fn init(&mut self, descriptors: &E::Deployment, metrics: &RunMetrics) -> Result<(), DynError> { Ok(()) }
    async fn start(&self, ctx: &RunContext<E>) -> Result<(), DynError>;
}

Registered with with_workload / with_workload_boxed on the builder.

Expectation<E> defines a check evaluated during or after the run. start_capture records a baseline, check_during_capture is the optional fail-fast hook polled during the run, and evaluate delivers the verdict at the end.

#[async_trait]
pub trait Expectation<E: Application>: Send + Sync {
    fn name(&self) -> &str;
    fn init(&mut self, descriptors: &E::Deployment, metrics: &RunMetrics) -> Result<(), DynError> { Ok(()) }
    async fn start_capture(&mut self, ctx: &RunContext<E>) -> Result<(), DynError> { Ok(()) }
    async fn check_during_capture(&mut self, ctx: &RunContext<E>) -> Result<(), DynError> { Ok(()) }
    async fn evaluate(&mut self, ctx: &RunContext<E>) -> Result<(), DynError>;
}

Registered with with_expectation / with_expectation_boxed.

RuntimeExtensionFactory<E> prepares one typed value after deployment (node clients are available) and before workloads start. The value is stored by TypeId and retrieved in workloads via ctx.extension::<T>() / ctx.require_extension::<T>(). Return PreparedRuntimeExtension::new(value), ::with_cleanup(value, guard), or ::from_task(value, join_handle) to tie a background task’s lifetime to the run.

#[async_trait]
pub trait RuntimeExtensionFactory<E: Application>: Send + Sync {
    async fn prepare(
        &self,
        deployment: &E::Deployment,
        node_clients: NodeClients<E>,
    ) -> Result<PreparedRuntimeExtension, DynError>;
}

Registered with with_runtime_extension_factory. Registering two factories that produce the same extension type fails at prepare time with duplicate runtime extension type registered.

Observation

Observer owns the app-side logic of the continuous observation runtime: init builds retained state from the source set, poll advances it each cycle and emits delta events, snapshot renders the current view. The runtime handles scheduling, history, and error tracking.

#[async_trait]
pub trait Observer: Send + Sync + 'static {
    type Source: Clone + Send + Sync + 'static;
    type State: Send + Sync + 'static;
    type Snapshot: Clone + Send + Sync + 'static;
    type Event: Clone + Send + Sync + 'static;

    async fn init(&self, sources: &[ObservedSource<Self::Source>]) -> Result<Self::State, DynError>;
    async fn poll(&self, sources: &[ObservedSource<Self::Source>], state: &mut Self::State)
        -> Result<Vec<Self::Event>, DynError>;
    fn snapshot(&self, state: &Self::State) -> Self::Snapshot;
}

SourceProvider<S> returns the current source set before each cycle, which lets the observed population change mid-run. Use StaticSourceProvider for a fixed set.

#[async_trait]
pub trait SourceProvider<S>: Send + Sync + 'static {
    async fn sources(&self) -> Result<Vec<ObservedSource<S>>, DynError>;
}

SourceProviderFactory<E, S> builds the provider once node clients exist. Any Fn(&E::Deployment, NodeClients<E>) -> Result<BoxedSourceProvider<S>, DynError> closure implements it. All three plug into a scenario through ObservationExtensionFactory<E, O>, which is itself a RuntimeExtensionFactory; see examples/openraft_kv/testing/integration/src/observation.rs for a complete implementation.

Application Composition

AppDeployment<E, P> prepares one reusable application preset, such as a process, child cluster, or composed stack, and returns a typed access or control handle. Framework adapters register managed resource lifetime separately with scenario cleanup. AppHandle is blanket-implemented for any Clone + Send + Sync + 'static type.

#[async_trait]
pub trait AppDeployment<E, P = LocalClusterProvisioner>: Send + 'static
where
    E: Application,
{
    type Handle: AppHandle;
    async fn deploy(self, ctx: &mut DeployContext<E, P>) -> Result<Self::Handle, DynError>;
}

Registered with AppScenarioBuilderExt::with_app, which wraps it in an AppDeploymentFactory (a RuntimeExtensionFactory). Compose children inside deploy via ctx.deploy(...) / ctx.deploy_and_expose(...). See Handle Ownership and Teardown for handle access and cleanup semantics.

Deployment Backends

Deployer<E, Caps> is the contract every backend implements: turn a built Scenario into a Runner<E>. ProcessDeployer (local), ComposeDeployer, and K8sDeployer are the in-repo implementations; Caps carries capability markers such as NodeControlCapability.

#[async_trait]
pub trait Deployer<E: Application, Caps = ()>: Send + Sync {
    type Error;
    async fn deploy(&self, scenario: &Scenario<E, Caps>) -> Result<Runner<E>, Self::Error>;
}

NodeControlHandle<E> is the deployer-agnostic control surface behind node-control scenarios: start_node(_with), stop_node, restart_node(_with), wait_node_ready, node_client, and node_pid. Every method has a default that returns a “not supported by this deployer” error, so backends implement only what they support. ClusterWaitHandle<E> provides the cluster-wide wait_network_ready operation. Both are combined by ManualClusterHandle<E> in core::runtime::manual, the interface behind ManualCluster.

ObservabilityCapabilityProvider lets deployers read telemetry endpoints out of whatever capability marker a scenario was built with; it is implemented for (), NodeControlCapability, and ObservabilityCapability. You only implement it when defining a new capability marker type.

Local Binary Resolution

BinaryProvider resolves the executable path for a locally spawned node process. Implementations return Ok(None) when valid but unable to resolve, which is how FallbackBinaryProvider chains providers. The default resolve caches per process by cache_key.

pub trait BinaryProvider: Send + Sync {
    fn try_resolve(&self) -> Result<Option<PathBuf>, BinaryProviderError>;
    fn display(&self) -> String;
    fn cache_key(&self) -> String;
    // provided: resolve(), resolve_uncached()
}

Built-in implementations: PathBinaryProvider, EnvBinaryProvider, BuildBinaryProvider, DownloadBinaryProvider, FallbackBinaryProvider. DownloadProcessor post-processes a checksum-verified download (for example, unpacking an archive) into the executable; DownloadProcessorFn adapts a closure with a stable cache_key so changed preparation logic invalidates the cache.

pub trait DownloadProcessor: Send + Sync {
    fn process(&self, artifact: &Path, output: &Path) -> Result<(), DownloadProcessorError>;
    fn cache_key(&self) -> &str;
}

Attaching Sources

IntoExistingCluster converts a value into the typed ExistingCluster descriptor accepted by with_existing_cluster_from. It is implemented for ExistingCluster and &ExistingCluster; implement it for your own environment-selection types to keep attach logic in one place. External endpoints use ExternalNodeSource values directly and pair with Application::external_node_client.

The required extension points depend on the entry pattern: a uniform managed cluster needs Application and the scenario traits, an AppHost stack adds AppDeployment, and attached clusters add the source traits. See Choosing an Entry Pattern. For where each implementation should live, see Framework vs Application Boundaries and the crate-level view in Crate and API Map.