Log inspection and troubleshooting in Apache Airflow - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the primary purpose of log inspection in Airflow?

Log inspection helps you understand what happened during a task run, identify errors, and find clues to fix issues.

beginner

Where can you find task logs in the Airflow web UI?

In the Airflow web UI, click on a DAG run, then select a task instance, and click the 'Log' tab to see the logs for that task.

intermediate

What does a 'Task instance failed' message in logs usually indicate?

It means the task did not complete successfully, often due to an error in the code, missing resources, or external system failures.

intermediate

How can you access Airflow logs from the command line?

You can access logs by navigating to the logs folder on the Airflow server, usually under AIRFLOW_HOME/logs, or use the CLI command: airflow tasks logs <dag_id> <task_id> <execution_date>.

advanced

Why is it important to check both scheduler and worker logs when troubleshooting Airflow?

Scheduler logs show DAG scheduling and triggering info, while worker logs show task execution details. Checking both helps find where the problem lies.

Where do you find task logs in the Airflow web UI?

A Under the task instance's 'Log' tab

B In the DAG details page only

C In the Airflow configuration file

D In the database directly

Task logs are accessible by clicking on a task instance and then the 'Log' tab in the Airflow web UI.

What does a 'Task instance failed' log message usually mean?

A The task completed successfully

B The task encountered an error during execution

C The task did not run at all

D The task was skipped intentionally

A 'Task instance failed' message indicates the task ran but encountered an error.

Which Airflow component's logs help you understand DAG scheduling issues?

A Worker logs

B Database logs

C Webserver logs

D Scheduler logs

Scheduler logs show how DAGs are scheduled and triggered.

How can you view Airflow task logs from the command line?

A Using 'airflow tasks logs <dag_id> <task_id> <execution_date>'

B Using 'airflow webserver start'

C Using 'airflow scheduler status'

D Using 'airflow db check'

The CLI command 'airflow tasks logs' shows task logs for a specific DAG, task, and execution date.

Why check both scheduler and worker logs when troubleshooting?

A Scheduler logs show task execution details

B Worker logs show DAG scheduling info

C Both logs together help locate the problem source

D Only worker logs are useful

Scheduler logs show scheduling info; worker logs show task execution. Both are needed to find issues.

Explain how to find and interpret task logs in Airflow for troubleshooting.

Think about both UI and command line ways to see logs.

Describe why checking both scheduler and worker logs is important when troubleshooting Airflow issues.

Consider the roles of scheduler and worker in Airflow.

Practice

1. What is the main purpose of inspecting logs in Airflow tasks?

easy

A. To schedule new DAG runs

B. To change the task's code automatically

C. To understand what happened during task execution step-by-step

D. To delete old DAGs from Airflow

Solution

1. Step 1: Understand what logs represent in Airflow

   Logs record the detailed events and outputs of each task run, showing what happened inside the task.

2. Step 2: Identify the purpose of inspecting logs

   Inspecting logs helps to see the step-by-step execution and find errors or confirm success.

3. Final Answer:

   To understand what happened during task execution step-by-step -> Option C

4. Quick Check:

   Logs show task execution details = A [OK]

Hint: Logs tell the story of task steps and errors [OK]

Common Mistakes:

• Thinking logs change task code
• Confusing logs with scheduling
• Believing logs delete DAGs

2. Which command correctly shows logs for a task named process_data in DAG daily_report for date 2024-06-01?

easy

A. airflow tasks logs daily_report process_data 2024-06-01

B. airflow logs show daily_report process_data 2024-06-01

C. airflow show logs daily_report process_data 2024-06-01

D. airflow task logs daily_report process_data 2024-06-01

Solution

1. Step 1: Recall the correct airflow CLI syntax for logs

   The correct command is airflow tasks logs <dag_id> <task_id> <execution_date>.

2. Step 2: Match the command with given task and date

   airflow tasks logs daily_report process_data 2024-06-01 matches the syntax exactly with correct order and spelling.

3. Final Answer:

   airflow tasks logs daily_report process_data 2024-06-01 -> Option A

4. Quick Check:

   Correct airflow logs command = A [OK]

Hint: Use 'airflow tasks logs DAG task date' exactly [OK]

Common Mistakes:

• Swapping 'tasks' and 'logs' words
• Using 'show' or 'task' incorrectly
• Wrong command order

3. You run airflow tasks logs sales_report generate_summary 2024-06-02 and see an error about missing file. What does this log output indicate?

medium

A. The task succeeded without issues

B. The DAG did not run on that date

C. The Airflow scheduler is down

D. The task failed because it could not find a required file

Solution

1. Step 1: Analyze the log error message about missing file

   An error about a missing file means the task tried to access a file that does not exist or is not reachable.

2. Step 2: Understand the implication for task status

   This error causes the task to fail because it cannot complete its work without the file.

3. Final Answer:

   The task failed because it could not find a required file -> Option D

4. Quick Check:

   Missing file error means task failure = C [OK]

Hint: Missing file error means task failure [OK]

Common Mistakes:

• Assuming task succeeded despite error
• Confusing DAG run status with task logs
• Thinking scheduler status is shown in task logs

4. You try to view logs with airflow tasks logs my_dag my_task 2024-06-05 but get a 'No logs found' message. What is the most likely cause?

medium

A. The DAG is paused

B. The task has not run yet for that date

C. Airflow is not installed

D. The command syntax is incorrect

Solution

1. Step 1: Understand what 'No logs found' means in Airflow

   This message usually means no logs exist because the task has not executed for the given date.

2. Step 2: Check other options for plausibility

   The command syntax is correct, Airflow installed is unrelated, and paused DAGs still keep logs from past runs.

3. Final Answer:

   The task has not run yet for that date -> Option B

4. Quick Check:

   No logs means no task run yet = D [OK]

Hint: No logs means task not run yet for that date [OK]

Common Mistakes:

• Assuming command syntax error
• Thinking Airflow is missing
• Confusing paused DAG with no logs

5. You want to troubleshoot why your Airflow task data_cleanup in DAG weekly_maintenance fails intermittently. Which approach best helps you find the root cause using logs?

hard

A. Check logs for multiple recent runs to find patterns of failure

B. Delete all logs to force fresh logging

C. Restart Airflow scheduler without checking logs

D. Change the DAG schedule to run more often

Solution

1. Step 1: Understand the goal of troubleshooting intermittent failures

   Intermittent failures require comparing multiple runs to see when and why failures happen.

2. Step 2: Identify the best log inspection method

   Checking logs from several recent runs helps spot patterns or recurring errors causing failure.

3. Final Answer:

   Check logs for multiple recent runs to find patterns of failure -> Option A

4. Quick Check:

   Compare multiple logs to find failure patterns = B [OK]

Hint: Compare logs from several runs to spot failure patterns [OK]

Common Mistakes:

• Deleting logs loses important info
• Restarting scheduler ignores root cause
• Changing schedule doesn't fix errors