debugging-dags

name

debugging-dags

description

Comprehensive DAG failure diagnosis and root-cause analysis with structured investigation and prevention recommendations. Use when deep failure investigation is needed, a DAG fails to import/parse or 'airflow dags list' errors on a file; a task or run is failing and must be diagnosed and fixed; requests like 'why did X fail', 'my dag keeps failing — find and fix it', or fixing a broken DAG so it loads cleanly. For simple 'why did it fail / show logs', the airflow skill handles it directly.

DAG Diagnosis

You are a data engineer debugging a failed Airflow DAG. Follow this systematic approach to identify the root cause and provide actionable remediation.

Running the CLI

These commands assume af is on PATH. Run via astro otto to get it automatically, or install standalone with uv tool install astro-airflow-mcp.

---

Step 1: Identify the Failure

If a specific DAG was mentioned:

• Run af runs diagnose <dag_id> <dag_run_id> (if run_id is provided)
• If no run_id specified, run af dags stats to find recent failures

If no DAG was specified:

• Run af health to find recent failures across all DAGs
• Check for import errors with af dags errors
• Show DAGs with recent failures
• Ask which DAG to investigate further

Step 2: Get the Error Details

Once you have identified a failed task:

1. Get task logs using af tasks logs <dag_id> <dag_run_id> <task_id>
2. Look for the actual exception - scroll past the Airflow boilerplate to find the real error
3. Categorize the failure type:
   • Data issue: Missing data, schema change, null values, constraint violation
   • Code issue: Bug, syntax error, import failure, type error
   • Infrastructure issue: Connection timeout, resource exhaustion, permission denied
   • Dependency issue: Upstream failure, external API down, rate limiting

Step 3: Check Context

Gather additional context to understand WHY this happened:

1. Recent changes: Was there a code deploy? Check git history if available
2. Package version changes: Was a package upgraded — in the image, in a venv-style operator, or at the index? See Package version changes below.
3. Data volume: Did data volume spike? Run a quick count on source tables
4. Upstream health: Did upstream tasks succeed but produce unexpected data?
5. Historical pattern: Is this a recurring failure? Check if same task failed before
6. Timing: Did this fail at an unusual time? (resource contention, maintenance windows)

Use af runs get <dag_id> <dag_run_id> to compare the failed run against recent successful runs.

Package version changes

A common cause of failures with no git activity is dependency drift — the user's code didn't change, but a package they depend on did. Check in this order:

1. Worker image diff (preferred when available). Every Astro deploy = new image tag, so the registry has a "before" and "after". Diff pip freeze between current and previous image — that's ground truth for what changed:

   docker run --rm <current_image> pip freeze > /tmp/now.txt
   docker run --rm <previous_image> pip freeze > /tmp/prev.txt
   diff /tmp/prev.txt /tmp/now.txt
   

   Also compare docker run --rm <image> python --version between the two — a Python minor-version bump (3.11 → 3.12, or even a patch) can break wheel compatibility even when pip freeze looks identical. af config providers lists currently installed provider versions, useful for cross-checking against modules named in the traceback.

2. Venv-style operators bypass the worker image. @task.virtualenv, PythonVirtualenvOperator, ExternalPythonOperator, and KubernetesPodOperator build their environment per task run, so an image diff won't catch failures inside them. If the failed task is one of these, read its requirements / image / python_version / python args directly:

   • Unbounded specifier (e.g. pandas>=2.0.0 with no upper bound, or no specifier at all) → a new upstream release is the prime suspect.
   • image="foo:latest" or no tag → the image moved underneath you.
   • python_version="3.11" (on @task.virtualenv / PythonVirtualenvOperator) or a python path (on ExternalPythonOperator) resolving to a different interpreter than it used to — a Python minor-version change can break wheel compatibility for unchanged requirements. Same vector applies to the worker image itself if the base Python changed there.

   Fix is to pin: pandas>=2.0.0,<3.0.0, a lockfile, a specific image SHA, or a fully-qualified Python version (python_version="3.11.7" instead of "3.11").

3. Index lookup when image diff isn't conclusive (no image history, or a venv-style operator). Identify the configured index first — it may not be PyPI:

   • Env vars: UV_INDEX_URL, PIP_INDEX_URL, PIP_EXTRA_INDEX_URL

A release timestamp landing between the last green run and the first red run, for a package named in the traceback, is the answer.

On Astro

If you're running on Astro, these additional tools can help with diagnosis:

• Deployment activity log: Check the Astro UI for recent deploys — a failed deploy or recent code change is often the cause of sudden failures
• Astro alerts: Configure alerts in the Astro UI for proactive failure monitoring (DAG failure, task duration, SLA miss)
• Observability: Use the Astro observability dashboard to track DAG health trends and spot recurring issues

On OSS Airflow

• Airflow UI: Use the DAGs page, Graph view, and task logs to inspect recent runs and failures

Step 4: Provide Actionable Output

Structure your diagnosis as:

Root Cause

What actually broke? Be specific - not "the task failed" but "the task failed because column X was null in 15% of rows when the code expected 0%".

Impact Assessment

• What data is affected? Which tables didn't get updated?
• What downstream processes are blocked?
• Is this blocking production dashboards or reports?

Immediate Fix

Specific steps to resolve RIGHT NOW:

1. If it's a data issue: SQL to fix or skip bad records
2. If it's a code issue: The exact code change needed
3. If it's infra: Who to contact or what to restart

Prevention

How to prevent this from happening again:

• Add data quality checks?
• Add better error handling?
• Add alerting for edge cases?
• Update documentation?
• Pin dependencies (constraints file, lockfile, or upper-bound specifiers on venv/external/pod operators) to avoid silent upstream drift?

Quick Commands

Provide ready-to-use commands:

• To clear and rerun the entire DAG run: af runs clear <dag_id> <run_id>
• To clear and rerun specific failed tasks: af tasks clear <dag_id> <run_id> <task_ids> -D
• To delete a stuck or unwanted run: af runs delete <dag_id> <run_id>