FileSensor for file arrival detection in Apache Airflow - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the FileSensor class from Airflow sensors.

Apache Airflow

from airflow.sensors.filesystem import [1]

Drag options to blanks, or click blank then click option'

AFileSensor

BHttpSensor

CTimeSensor

DS3KeySensor

Attempts:

3 left

💡 Hint

Common Mistakes

Importing the wrong sensor class like HttpSensor or TimeSensor.

The FileSensor class is imported from airflow.sensors.filesystem to detect file arrival.

2fill in blank

medium

Complete the code to create a FileSensor that checks for a file named 'data.csv' in '/tmp'.

Apache Airflow

file_sensor = FileSensor(task_id='check_file', filepath='/tmp/[1]')

Drag options to blanks, or click blank then click option'

Ainput.txt

Breport.pdf

Cdata.csv

Dconfig.yaml

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong file name that does not exist or is not intended to be detected.

The filepath parameter should point to the file to detect, here 'data.csv'.

3fill in blank

hard

Fix the error in the FileSensor initialization by completing the missing parameter for poke interval in seconds.

Apache Airflow

file_sensor = FileSensor(task_id='check_file', filepath='/tmp/data.csv', poke_interval=[1])

Drag options to blanks, or click blank then click option'

A0.5

B5

C'five'

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of an integer.

Using a negative or zero value.

The poke_interval must be an integer number of seconds, like 5.

4fill in blank

hard

Fill both blanks to create a FileSensor that waits for 'report.txt' in '/data' and times out after 60 seconds.

Apache Airflow

file_sensor = FileSensor(task_id='wait_report', filepath='/data/[1]', timeout=[2])

Drag options to blanks, or click blank then click option'

Areport.txt

Bdata.csv

C60

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong file name or timeout value.

The filepath should be 'report.txt' and timeout set to 60 seconds.

5fill in blank

hard

Fill all three blanks to create a FileSensor that waits for 'config.json' in '/configs', checks every 10 seconds, and times out after 120 seconds.

Apache Airflow

file_sensor = FileSensor(task_id='wait_config', filepath='/configs/[1]', poke_interval=[2], timeout=[3])

Drag options to blanks, or click blank then click option'

Aconfig.json

B10

C120

D60

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up poke_interval and timeout values.

Using wrong file name.

The filepath is 'config.json', poke_interval is 10 seconds, and timeout is 120 seconds.

Practice

1. What is the main purpose of using a FileSensor in Airflow?

easy

A. To upload files to a remote server

B. To delete a file after processing

C. To wait until a specific file appears before running the next task

D. To monitor CPU usage during task execution

Solution

1. Step 1: Understand FileSensor's role

   A FileSensor is designed to pause a task until a file is detected at a given path.

2. Step 2: Compare options with FileSensor behavior

   Only To wait until a specific file appears before running the next task describes waiting for a file's arrival, which matches FileSensor's purpose.

3. Final Answer:

   To wait until a specific file appears before running the next task -> Option C

4. Quick Check:

   FileSensor waits for file = A [OK]

Hint: FileSensor pauses until file exists, then continues [OK]

Common Mistakes:

• Confusing FileSensor with file deletion or upload
• Thinking it monitors system resources
• Assuming it triggers immediately without waiting

2. Which of the following is the correct way to create a FileSensor in Airflow to check for a file named data.csv in /tmp directory with poke_interval=30 and timeout=300?

easy

A. FileSensor(task_id='check_file', filepath='/tmp/data.csv', poke_interval=30, timeout=300)

B. FileSensor(task_id='check_file', path='/tmp/data.csv', poke=30, timeout=300)

C. FileSensor(task_id='check_file', filepath='/tmp/data.csv', interval=30, timeout=300)

D. FileSensor(task_id='check_file', filepath='/tmp/data.csv', poke_interval=30)

Solution

1. Step 1: Identify correct parameter names

   The FileSensor uses 'filepath' for the file path, 'poke_interval' for checking frequency, and 'timeout' for max wait time.

2. Step 2: Match options with correct parameters

   FileSensor(task_id='check_file', filepath='/tmp/data.csv', poke_interval=30, timeout=300) uses correct parameter names and values. FileSensor(task_id='check_file', filepath='/tmp/data.csv', poke_interval=30) misses 'timeout', which is optional but often needed.

3. Final Answer:

   FileSensor(task_id='check_file', filepath='/tmp/data.csv', poke_interval=30, timeout=300) -> Option A

4. Quick Check:

   Use 'filepath' and 'poke_interval' exactly [OK]

Hint: Use 'filepath' and 'poke_interval' parameters exactly [OK]

Common Mistakes:

• Using 'path' instead of 'filepath'
• Using 'poke' or 'interval' instead of 'poke_interval'
• Omitting required parameters

3. Given this Airflow task using FileSensor:

file_sensor = FileSensor(
    task_id='wait_for_file',
    filepath='/data/input.txt',
    poke_interval=10,
    timeout=60
)

What will happen if the file /data/input.txt does NOT appear within 60 seconds?

medium

A. The task will skip and continue

B. The task will fail with a timeout error

C. The task will keep waiting indefinitely

D. The task will succeed immediately

Solution

1. Step 1: Understand timeout behavior

   The 'timeout' parameter sets the max seconds the sensor waits before failing.

2. Step 2: Apply to given values

   With timeout=60, if the file doesn't appear in 60 seconds, the task fails with a timeout error.

3. Final Answer:

   The task will fail with a timeout error -> Option B

4. Quick Check:

   Timeout reached = task fails [OK]

Hint: Timeout causes failure if file missing too long [OK]

Common Mistakes:

• Assuming task succeeds without file
• Thinking sensor waits forever ignoring timeout
• Believing task skips automatically on timeout

4. You wrote this FileSensor task:

file_sensor = FileSensor(
    task_id='wait_file',
    filepath='/tmp/data.csv',
    poke_interval='30',
    timeout=300
)

But the task fails immediately with a TypeError. What is the likely cause?

medium

A. task_id cannot contain underscores

B. filepath must be a directory, not a file

C. timeout cannot be set to 300

D. poke_interval should be an integer, not a string

Solution

1. Step 1: Check parameter types

   poke_interval expects an integer number of seconds, not a string.

2. Step 2: Identify error cause

   Setting poke_interval='30' (string) causes TypeError because Airflow expects int.

3. Final Answer:

   poke_interval should be an integer, not a string -> Option D

4. Quick Check:

   poke_interval type must be int [OK]

Hint: poke_interval must be int, not quoted string [OK]

Common Mistakes:

• Passing poke_interval as string instead of int
• Misunderstanding filepath type requirements
• Thinking task_id restrictions cause error

5. You want to create a DAG that waits for multiple files file1.csv and file2.csv to appear in /data before starting processing. Which approach correctly uses FileSensor to achieve this?

hard

A. Create two FileSensor tasks for each file, then set a downstream task that depends on both sensors

B. Create one FileSensor with filepath='/data/file1.csv,/data/file2.csv'

C. Use a single FileSensor with filepath='/data' and wait for directory

D. Create one FileSensor for file1.csv and ignore file2.csv

Solution

1. Step 1: Understand FileSensor scope

   FileSensor waits for one file path; it cannot wait for multiple files at once.

2. Step 2: Design DAG dependencies

   Create two FileSensor tasks, each waiting for one file, then a downstream task that runs after both succeed.

3. Final Answer:

   Create two FileSensor tasks for each file, then set a downstream task that depends on both sensors -> Option A

4. Quick Check:

   Multiple files = multiple sensors + downstream task [OK]

Hint: Use one sensor per file, then join downstream [OK]

Common Mistakes:

• Trying to list multiple files in one filepath
• Waiting on directory instead of files
• Ignoring one of the required files