Airflow Concepts

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Revision as of 19:13, 11 July 2022 by Ovidiu (talk | contribs) (→‎Task)
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External

Internal

Workflow

DAG

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html
Graph Concepts | Directed Acyclic Graph

The edges can be labeled in the UI.

SubDAG

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html#concepts-subdags

A DAG is made of tasks among which there are relations of dependency. The DAG is not concerned about what happens inside the tasks, it is only concerned about how to run them: order, retries, timeouts. etc.

Declaring a DAG

DAG Run

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html#concepts-dag-run

A DAG instantiates in a DAG Run at runtime.

Control Flow

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html#control-flow

Dynamic DAG

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html#dynamic-dags

The DAGs can be purely declarative, or they can be declared in Python code, by adding tasks dynamically.

Task

https://airflow.apache.org/docs/apache-airflow/stable/concepts/tasks.html

A Task is the basic unit of execution in Airflow. Every task must be assigned to a DAG to run. Tasks have dependencies on each other. There could be upstream dependencies (if B depends on A, A → B, then A is an upstream dependency of B). To be scheduled, a task have all its dependencies met.

Task Dependencies

https://airflow.apache.org/docs/apache-airflow/stable/concepts/tasks.html#relationships

The task dependencies, or their relationships, are a key part in using Tasks. If a task B has a dependency on task A (A → B), it is said that A is upstream of B and B is downstream of A. The dependencies are the directed edges of the directed acyclic graph.

The upstream term has a very strict semantics: an upstream task is the task that is directly preceding the other task. This concept does not describe the tasks that are higher in the task hierarchy (they are not a direct parent of the task). Same constrains apply to a downstream task, which need to be a direct child of the other task.

Task Types

Airflow has three types of tasks: Operator, Sensor, which is a subclass of Operator, and TaskFlow-decorated Task. All these are subclasses of Airflow's BaseOperator. Operators and Sensor are templates, and when one is called in a DAG, it is made into a Task.

Operator

https://airflow.apache.org/docs/apache-airflow/stable/concepts/operators.html

An Operator is a predefined task template.

Sensor

https://airflow.apache.org/docs/apache-airflow/stable/concepts/sensors.html

A Sensor is a subclass of Operator that wait for an external event to happen.

TaskFlow-decorated Task

https://airflow.apache.org/docs/apache-airflow/stable/concepts/taskflow.html

Decorated with @task. A custom Python function packaged up as a Task.

Task Assignment to DAG

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html#dag-assignment

Task Instance

https://airflow.apache.org/docs/apache-airflow/stable/concepts/tasks.html#task-instances

The same way a DAG is instantiated at runtime into a DAG Run, the tasks under a DAG are instantiated into Task Instances.

Task States

none

scheduled

queued

running

success

shutdown

restarting

failed

skipped

upstream_failed

up_for_retry

up_for_reschedule

sensing

deferred

removed

Passing Data between Tasks

Tasks pass data among each other using:

  • XComs, when the amount of metadata to be exchanged is small.
  • Uploading and downloading large files from a storage service.

TaskGroup

https://airflow.apache.org/docs/apache-airflow/stable/concepts/dags.html#concepts-taskgroups

This is a pure UI concept.

XComs

https://airflow.apache.org/docs/apache-airflow/stable/concepts/taskflow.html

"Cross-communications".

Workload

Scheduler

https://airflow.apache.org/docs/apache-airflow/stable/concepts/scheduler.html

Executor

https://airflow.apache.org/docs/apache-airflow/stable/executor/index.html

Worker

Metadata Database

Connections & Hooks

https://airflow.apache.org/docs/apache-airflow/stable/concepts/connections.html

Pool

https://airflow.apache.org/docs/apache-airflow/stable/concepts/pools.html