Just getting started with Airflow and wondering what best practices are for structuring large DAGs. For our ETL, we have a lots of tasks that fall into logical groupings, yet the groups are dependent on each other. Which of the following would be considered best practice?
One large DAG file with all tasks in that file
Splitting the DAG definition across multiple files (How to do this?)
Define multiple DAGs, one for each group of tasks, and set dependencies between them using ExternalTaskSensor
Also open to other suggestions.
DAGs are just python files. So you could split a single dag definition into multiple files. The different files should just have methods that take in a dag object and create tasks using that dag object.
Note though, you should just a single dag object in the global scope. Airflow picks up all dag objects in the global scope as separate dags.
It is often considered good practice to keep each dag as concise as possible. However if you need to set up such dependencies you could either consider using subdags. More about this here: https://airflow.incubator.apache.org/concepts.html?highlight=subdag#scope
You could also use ExternalTaskSensor but beware that as the number of dags grow, it might get harder to handle external dependencies between tasks. I think subdags might be the way to go for your use case.
With the advent of TaskGroups in Airflow 2.x, it's worth expanding on a previous answer. TaskGroups are just UI groupings for tasks, but they also serve as handy logical groupings for a bunch of related tasks. The tasks in a TaskGroup can be bundled and abstracted away to make it easier to build a DAG out of larger pieces. That being said, it may still be useful to have a file full of related tasks without bundling them into a TaskGroup.
The trick to breaking up DAGs is to have the DAG in one file, for example my_dag.py, and the logical chunks of tasks or TaskGroups in separate files, with one logical task chunk or TaskGroup per file. Each file contains functions (or methods if you want to take an OO approach) each of which returns an operator instance or a TaskGroup instance.
To illustrate, my_dag.py (below) imports operator-returning functions from foo_bar_tasks.py, and it imports a TaskGroup-returning function from xyzzy_taskgroup.py. Within the DAG context, those functions are called and their return values are assigned to task or TaskGroup variables, which can be assigned up-/downstream dependencies.
dags/my_dag.py:
# std lib imports
from airflow import DAG
# other airflow imports
from includes.foo_bar_tasks import build_foo_task, build_bar_task
from includes.xyzzy_taskgroup import build_xyzzy_taskgroup
with DAG(dag_id="my_dag", ...) as dag:
# logical chunk of tasks
foo_task = build_foo_task(dag=dag, ...)
bar_task = build_bar_task(dag=dag, ...)
# taskgroup
xyzzy_taskgroup = build_xyzzy_taskgroup(dag=dag, ...)
foo_task >> bar_task >> xyzzy_taskgroup
plugins/includes/foo_bar_tasks.py:
# std lib imports
from airflow import DAG
from airflow.operators.foo import FooOperator
from airflow.operators.bar import BarOperator
# other airflow imports
def build_foo_task(dag: DAG, ...) -> FooOperator:
# ... logic here ...
foo_task = FooOperator(..., dag=dag)
return foo_task
def build_bar_task(dag: DAG, ...) -> BarOperator:
# ... logic here ...
bar_task = BarOperator(..., dag=dag)
return bar_task
plugins/includes/xyzzy_taskgroup.py:
# std lib imports
from airflow import DAG
from airflow.operators.baz import BazOperator
from airflow.operators.qux import QuxOperator
from airflow.utils import TaskGroup
# other airflow imports
def build_xyzzy_taskgroup(dag: DAG, ...) -> TaskGroup:
xyzzy_taskgroup = TaskGroup(group_id="xyzzy_taskgroup")
# ... logic here ...
baz_task = BazOperator(task_id="baz_task", task_group=xyzzy_taskgroup, ...)
# ... logic here ...
qux_task = QuxOperator(task_id="qux_task", task_group=xyzzy_taskgroup, ...)
baz_task >> qux_task
return xyzzy_taskgroup
It seems that it is possible to place your Python modules into the plugins/ subfolder and import them from the DAG file:
https://airflow.apache.org/docs/apache-airflow/stable/plugins.html
Related
Coming from airflow, I used jinja templates such as {{ds_nodash}} to translate the date of execution of a dag within my scripts.
For example, I am able to detect and ingest a file at the first of August 2022 if it is in the format : FILE_20220801.csv. I would have a dag with a sensor and an operator that uses FILE_{{ds_nodash}}.csv within its code. In other terms I was sure my dag was idempotent in regards to its execution date.
I am now looking into dagster because of the assets abstraction that is quite attractive. Also, dagster is easy to set-up and test locally. But I cannot find similar jinja templates that can ensure the idempotency of my executions.
In other words, how do I make sure data that was sent to me during a specific date is going to be processed the same way even if I run it 1, 2 or N days later?
If a file comes in every day (or hour, or week, etc.), and some of the assets that depend on the file have a partition for each file, then the recommended way to do this is with partitions. E.g.:
from dagster import DailyPartitionsDefinition, asset, sensor, repository, define_asset_job
daily_partitions_def = DailyPartitionsDefinition(start_date="2020-01-01", fmt=%Y%m%d)
#asset(partitions_def=daily_partitions_def)
def asset1(context):
path = f"FILE_{context.partition_key}.csv"
...
#asset(partitions_def=daily_partitions_def)
def asset2(context):
...
def detect_file() -> Optional[str]:
"""Returns a value like '20220801', or None if no file is detected """
all_assets_job = define_asset_job("all_assets", partitions_def=daily_partitions_def)
#sensor(job=all_assets_job)
def my_sensor():
date_str = detect_file()
if date_str:
return all_assets_job.run_request_for_partition(run_key=None, partition_key=date_str)
#repository
def repo():
return [my_sensor, asset1, asset2]
I recently started using Apache Airflow and one of its new concept Taskflow API. I have a DAG with multiple decorated tasks where each task has 50+ lines of code. So I decided to move each task into a separate file.
After referring stackoverflow I could somehow move the tasks in the DAG into separate file per task. Now, my question is:
Does both the code samples shown below work same? (I am worried about the scope of the tasks).
How will they share data b/w them?
Is there any difference in performance? (I read Subdags are discouraged due to performance issues, though this is not Subdags just concerned).
All the code samples I see in the web (and in official documentation) put all the tasks in a single file.
Sample 1
import logging
from airflow.decorators import dag, task
from datetime import datetime
default_args = {"owner": "airflow", "start_date": datetime(2021, 1, 1)}
#dag(default_args=default_args, schedule_interval=None)
def No_Import_Tasks():
# Task 1
#task()
def Task_A():
logging.info(f"Task A: Received param None")
# Some 100 lines of code
return "A"
# Task 2
#task()
def Task_B(a):
logging.info(f"Task B: Received param {a}")
# Some 100 lines of code
return str(a + "B")
a = Task_A()
ab = Task_B(a)
No_Import_Tasks = No_Import_Tasks()
Sample 2 Folder structure:
- dags
- tasks
- Task_A.py
- Task_B.py
- Main_DAG.py
File Task_A.py
import logging
from airflow.decorators import task
#task()
def Task_A():
logging.info(f"Task A: Received param None")
# Some 100 lines of code
return "A"
File Task_B.py
import logging
from airflow.decorators import task
#task()
def Task_B(a):
logging.info(f"Task B: Received param {a}")
# Some 100 lines of code
return str(a + "B")
File Main_Dag.py
from airflow.decorators import dag
from datetime import datetime
from tasks.Task_A import Task_A
from tasks.Task_B import Task_B
default_args = {"owner": "airflow", "start_date": datetime(2021, 1, 1)}
#dag(default_args=default_args, schedule_interval=None)
def Import_Tasks():
a = Task_A()
ab = Task_B(a)
Import_Tasks_dag = Import_Tasks()
Thanks in advance!
There is virtually no difference between the two approaches - neither from logic nor performance point of view.
The tasks in Airflow share the data between them using XCom (https://airflow.apache.org/docs/apache-airflow/stable/concepts/xcoms.html) effectively exchanging data via database (or other external storage). The two tasks in Airflow - does not matter if they are defined in one or many files - can be executed anyway on completely different machines (there is no task affinity in airflow - each task execution is totally separated from other tasks. So it does not matter - again - if they are in one or many Python files.
Performance should be similar. Maybe splitting into several files is very, very little slower but it should totally negligible and possibly even not there at all - depends on the deployment you have the way you distribute files etc. etc., but I cannot imagine this can have any observable impact.
I am reading list of elements from an external file and looping over elements to create a series of tasks.
For example, if there are 2 elements in the file - [A, B]. There will be 2 series of tasks:
A1 -> A2 ..
B1 -> B2 ...
This reading elements logic is not part of any task but in the DAG itself. Thus Scheduler is calling it many times a day while reading the DAG file. I want to call it only during DAG runtime.
Wondering if there is already a pattern for such kind of use cases?
Depending on your requirements, if what you are looking for is to avoid reading a file many times, but you don't mind reading from the metadata database as many times instead, then you could change your approach to use Variables as the source of iteration to dynamically create tasks.
A basic example could be performing the file reading inside a PythonOperator and set the Variables you will use to iterate later on (same callable):
sample_file.json:
{
"cities": [ "London", "Paris", "BA", "NY" ]
}
Task definition:
from airflow.utils.dates import days_ago
from airflow.models import Variable
from airflow.utils.task_group import TaskGroup
import json
def _read_file():
with open('dags/sample_file.json') as f:
data = json.load(f)
Variable.set(key='list_of_cities',
value=data['cities'], serialize_json=True)
print('Loading Variable from file...')
def _say_hello(city_name):
print('hello from ' + city_name)
with DAG('dynamic_tasks_from_var', schedule_interval='#once',
start_date=days_ago(2),
catchup=False) as dag:
read_file = PythonOperator(
task_id='read_file',
python_callable=_read_file
)
Then you could read from that variable and create the dynamic tasks. (It's important to set a default_var). The TaskGroup is optional.
# Top-level code
updated_list = Variable.get('list_of_cities',
default_var=['default_city'],
deserialize_json=True)
print(f'Updated LIST: {updated_list}')
with TaskGroup('dynamic_tasks_group',
prefix_group_id=False,
) as dynamic_tasks_group:
for index, city in enumerate(updated_list):
say_hello = PythonOperator(
task_id=f'say_hello_from_{city}',
python_callable=_say_hello,
op_kwargs={'city_name': city}
)
# DAG level dependencies
read_file >> dynamic_tasks_group
In the Scheduler logs, you will only find:
INFO - Updated LIST: ['London', 'Paris', 'BA', 'NY']
Dag Graph View:
With this approach, the top-level code, hence read by the Scheduler continuously, is the call to Variable.get() method. If you need to read from many variables, it's important to remember that it's recommended to store them in one single JSON value to avoid constantly create connections to the metadata database (example in this article).
Update:
As for 11-2021 this approach is considered a "quick and dirty" kind of solution.
Does it work? Yes, totally. Is it production quality code? No.
What's wrong with it? The DB is accessed every time the Scheduler parses the file, by default every 30 seconds, and has nothing to do with your DAG execution. Full details on Airflow Best practices, top-level code.
How can this be improved? Consider if any of the recommended ways about dynamic DAG generation applies to your needs.
I'm trying to render the DAG as a tree for documentation, is there a direct way to get this ?
Right now I'm manually generating DOT files with (partial code):
for task in dag.tasks:
print("\t%s;" % task.task_id)
relatives = [r.task_id for r in task.get_direct_relatives()]
for r in relatives:
print("\t%s -- %s;" % (task.task_id, r))
Which works, but I need to dynamically import all DAGs externally..
You can use the airflow.models.DagBag object to enumerate the DAG objects.
from airflow.models import DagBag
for dag in DagBag().dags.values():
for task in dag.tasks:
[...]
I have a workflow that involves many instances of the SubDagOperator, with the tasks generated in a loop. The pattern is illustrated by the following toy dag file:
from datetime import datetime, timedelta
from airflow.models import DAG
from airflow.operators.dummy_operator import DummyOperator
from airflow.operators.subdag_operator import SubDagOperator
dag = DAG(
'subdaggy-2',
schedule_interval=None,
start_date=datetime(2017,1,1)
)
def make_sub_dag(parent_dag, N):
dag = DAG(
'%s.task_%d' % (parent_dag.dag_id, N),
schedule_interval=parent_dag.schedule_interval,
start_date=parent_dag.start_date
)
DummyOperator(task_id='task1', dag=dag) >> DummyOperator(task_id='task2', dag=dag)
return dag
downstream_task = DummyOperator(task_id='downstream', dag=dag)
for N in range(20):
SubDagOperator(
dag=dag,
task_id='task_%d' % N,
subdag=make_sub_dag(dag, N)
) >> downstream_task
I find this to be a convenient way to organize tasks, particularly since it helps keep the top-level DAG uncluttered, especially if the subdag itself contains more tasks (i.e. tens, not just a couple.)
The problem is, this approach doesn't scale very well as the number of subdags (20 in the example) increases. I find that when the total number of DAG objects created in an overall workflow surpasses about 200 (which can easily happen with a production workflow, especially if that pattern occurs several times) things grind to a halt.
So the question: is there a way to organize tasks this way (many similar subdags) that scales to hundreds or thousands of subdags? Some profiling suggests that the process spends a lot of time in the DAG object constructor. Perhaps there is a way to avoid instantiating a new DAG object for each of the SubDagOperators?
Well, it seems that at least a significant part of this problem is indeed due to the cost of the DAG constructor, which in turn is in large part due to the cost of inspect.stack(). I put up a simple patch to replace that with a cheaper method and there does indeed seem to be improvement - a flow with several thousand subdags which previously failed to load for me now loads. We'll see if this goes anywhere.