Hooks

Hooks let you observe pipeline execution events without modifying the pipeline itself. They are used for logging, timing, visualization, and custom instrumentation.

The Hook trait

pub trait Hook: Sync {
    // Pipeline lifecycle
    fn before_pipeline_run(&self, p: &dyn PipelineInfo) {}
    fn after_pipeline_run(&self, p: &dyn PipelineInfo) {}
    fn on_pipeline_error(&self, p: &dyn PipelineInfo, error: &str) {}

    // Node lifecycle
    fn before_node_run(&self, n: &dyn PipelineInfo) {}
    fn after_node_run(&self, n: &dyn PipelineInfo) {}
    fn on_node_error(&self, n: &dyn PipelineInfo, error: &str) {}

    // Dataset lifecycle (fired per-dataset during Node::call)
    fn before_dataset_loaded(&self, n: &dyn PipelineInfo, ds: &DatasetRef) {}
    fn after_dataset_loaded(&self, n: &dyn PipelineInfo, ds: &DatasetRef) {}
    fn before_dataset_saved(&self, n: &dyn PipelineInfo, ds: &DatasetRef) {}
    fn after_dataset_saved(&self, n: &dyn PipelineInfo, ds: &DatasetRef) {}
}

All methods have default no-op implementations, so you only override the ones you care about.

The Hooks trait

Multiple hooks are composed as a tuple:

pub trait Hooks: Sync {
    fn for_each_hook(&self, f: &mut dyn FnMut(&dyn Hook));
}

Implemented for tuples of up to 10 hooks, plus () (no hooks):

// No hooks
App::new(catalog, params).execute(pipeline)

// One hook
App::new(catalog, params)
    .with_hooks((LoggingHook::new(),))
    .execute(pipeline)

// Multiple hooks
App::new(catalog, params)
    .with_hooks((LoggingHook::new(), my_custom_hook))
    .execute(pipeline)

Hook arguments

All hook methods receive &dyn PipelineInfo, which provides:

• name() — the node or pipeline name (&'static str)
• is_leaf() — true for nodes, false for pipelines
• type_string() — the Rust type name of the underlying function
• for_each_input() / for_each_output() — iterate over dataset references

Dataset hook methods additionally receive &DatasetRef, which provides:

• id — a unique identifier (pointer-based)
• name — an Option<&str> with the resolved dataset name (from the catalog indexer, available when using std)
• meta — &dyn DatasetMeta with is_param(), type_string(), and (with std) html() and yaml()

Sync requirement

Hooks must be Sync because the ParallelRunner calls hook methods from multiple threads. For hooks that need interior mutability (e.g. to track timing), use thread-safe types like Mutex or DashMap.

This chapter covers:

• Dataset Hooks — hooks for dataset load/save events
• Node Hooks — hooks for node execution events
• Pipeline Hooks — hooks for pipeline lifecycle events
• Built-in Hooks — LoggingHook and VizHook