Pattern: Asset-to-Task

An asset produces cached data. A task consumes that data to perform a side-effect. Use barca get for the asset, barca run for the task.

The right way

from barca import asset, task

@asset
def trained_model():
    return train(load_data("training_set.csv"))

@task(inputs={"model": trained_model})
def deploy(model):
    upload_to_s3(model, bucket="models", key="latest.pkl")
    notify_slack("Model deployed")

# Materialize the asset (cached)
barca get trained_model pipeline.py

# Execute the task (always runs)
barca run deploy pipeline.py

Why this works

• Assets are cached data. trained_model only re-runs when its code or upstream inputs change. Expensive computation is done once and reused.
• Tasks are side-effects. deploy always re-runs because side-effects (uploading, notifying) are not idempotent by default. Barca treats tasks as “always stale.”
• Clean separation. The DAG edge from asset to task means the task is guaranteed to see a fully materialized, up-to-date model before it executes.

Common mistakes

Making a task an input to an asset

# Wrong -- DAG validation will reject this
@task
def migrate_db():
    run_migrations()

@asset(inputs={"_done": migrate_db})
def user_counts():
    return query("SELECT count(*) FROM users")

Tasks always re-run, so any asset downstream of a task would have its cache perpetually invalidated. Barca’s DAG validation rejects task-to-asset edges to prevent this. If you need ordering without data, see Ordering-Only Dependencies and use a task-to-task edge or an asset-to-asset edge instead.

Using barca get on a task

# Wrong -- tasks are not cacheable artifacts
barca get deploy pipeline.py
# Error: 'deploy' is a task -- use `barca run` instead

barca get targets assets; naming a task explicitly is rejected with a clear error pointing you to barca run. Note that running barca get pipeline.py on a whole file (no target) still executes every node in the file, tasks included – it does not silently skip them. Scope to barca get <asset> pipeline.py if you want to materialize just the asset without side effects firing.