In Airflow, a DAG — or a Directed Acyclic Graph — is a collection of all the tasks you want to run, organized in a way that reflects their relationships and dependencies. It could say that A has to run successfully before B can run, but C can run anytime. It could say that task A times out after 5 minutes, and B can be restarted up to 5 times in case it fails. A, B, and C could be anything.
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Maybe A prepares data for B to analyze while C sends an email. Or perhaps A monitors your location so B can open your garage door while C turns on your house lights. Airflow will execute the code in each file to dynamically build the DAG objects. You can have as many DAGs as you want, each describing an arbitrary number of tasks. In general, each one should correspond to a single logical workflow.
Critically, that means the DAG must appear in globals. Consider the following two DAGs.
Sometimes this can be put to good use. This makes it easy to apply a common parameter to many operators without having to type it many times. A DAG run is usually created by the Airflow scheduler, but can also be created by an external trigger. For example, we might currently have two DAG runs that are in progress for and respectively.
While DAGs describe how to run a workflow, Operators determine what actually gets done by a task. An operator describes a single task in a workflow. The DAG will make sure that operators run in the correct order; other than those dependencies, operators generally run independently. In fact, they may run on two completely different machines. This is a subtle but very important point: in general, if two operators need to share information, like a filename or small amount of data, you should consider combining them into a single operator.
BashOperator - executes a bash command. PythonOperator - calls an arbitrary Python function. EmailOperator - sends an email. Sensor - an Operator that waits polls for a certain time, file, database row, S3 key, etc…. List Airflow operators. How-to guides for some Airflow operators. Operators do not have to be assigned to DAGs immediately previously dag was a required argument.
However, once an operator is assigned to a DAG, it can not be transferred or unassigned. DAG assignment can be done explicitly when the operator is created, through deferred assignment, or even inferred from other operators.
In Airflow 1.There seems to be a bit of mysticism around the scheduler in Airflow, that makes people reluctant to review PRs in this area or contribute. That should not be the case. The scheduler is the core of Airflow it needs to be the most understood and readable bit of code. So here is a small write up to get you started. A DAG consists of Tasks and obviously you need those tasks to run. We simply loop over the available tasks and see if they are runnable if ti.
If a task is runnable it will get sent to the executor. Now it is a bit more messy at the moment. That is due to the fact that DagRuns are not yet first class citizens.
This creates architectural issues but also real life ones. It is also the reason why scheduler loops start to increase over time if DAGs get more complex many tasks. The good news is work is underway to improve this see the roadmap. States are used in Airflow to understand what the different tasks and dagruns are doing. We currently know the following states. However, was it not for that fact that we have backfills In order to remove race conditions and to be able to kill any of airflows components and still be able to continue where we left off, better state handling needs to be done.
This means that at handover to a different process only certain states can get set by each process. In previous versions of the scheduler, user-supplied DAG definitions were parsed and loaded in the same process as the scheduler.
Unfortunately, this made it possible for bad user code to adversely affect the scheduler process. To help mitigate such cases, the scheduler processes DAGs in a child processes. This gives it better isolation and faster performance. The logic for scheduling is as follows:. It keeps track of which files need to be processed and ensures that once a DagFileProcessor is done, the next file in the series is processed accordingly.
It also controls the number of simultaneous DagFileProcessors and ensures that the number if simultaneous instances do not exceed the configured limit. All tasks start with State. When the executor launches a task in a separate process a TI, it eventually calls TI. That second run is the one that actually moves pooled tasks to State. Backfills are a bit of an awkward duck in the pond. They execute outside the scheduler and can therefore oversubscribe workers using more resources than assigned.
Backfills just create TaskInstances and start running them. In order to fix the scheduler and the race condition, first the scheduler and the backfills need to become aware of each other. Avoiding oversubscribing the backfills should be managed by the scheduler. Evaluate Confluence today. Page tree.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together. If nothing happens, download GitHub Desktop and try again.
If nothing happens, download Xcode and try again. If nothing happens, download the GitHub extension for Visual Studio and try again. This tutorial is loosely based on the Airflow tutorial in the official documentation.
It will walk you through the basics of setting up Airflow and creating an Airflow workflow. This tutorial was published on the blog of GoDataDriven.
You can skip this section if Airflow is already set up. Make sure that you can run airflow commands, know where to put your DAGs and have access to the web UI.
Airflow is installable with pip via a simple pip install apache-airflow. Either use a separate python virtual environment or install it in your default python environment. To use the conda virtual environment as defined in environment. Airflow used to be packaged as airflow but is packaged as apache-airflow since version 1. Make sure that you install any extra packages with the right Python package: e. Leaving out the prefix apache- will install an old version of Airflow next to your current version, leading to a world of hurt.
You may run into problems if you don't have the right binaries or Python packages installed for certain backends or operators. When specifying support for e. PostgreSQL when installing extra Airflow packages, make sure the database is installed; do a brew install postgresql or apt-get install postgresql before the pip install apache-airflow[postgres].
Similarly, when running into HiveOperator errors, do a pip install apache-airflow[hive] and make sure you can use Hive. Before you can use Airflow you have to initialize its database.
retry_delay not honored
Once the database is set up, Airflow's UI can be accessed by running a web server and workflows can be started. The default database is a SQLite database, which is fine for this tutorial. You'll probably want to back it up as this database stores the state of everything related to Airflow. This directory will be used after your first Airflow command. Now start the web server and go to localhost to check out the UI:.
With the web server running workflows can be started from a new terminal window. The tasks of a workflow make up a Graph; the graph is Directed because the tasks are ordered; and we don't want to get stuck in an eternal loop so the graph also has to be Acyclic.
Your workflow will automatically be picked up and scheduled to run.
First we'll configure settings that are shared by all our tasks. Settings for tasks can be passed as arguments when creating them, but we can also pass a dictionary with default values to the DAG. This allows us to share default arguments for all the tasks in our DAG is the best place to set e. These settings tell Airflow that this workflow is owned by 'me'that the workflow is valid since June 1st ofit should not send emails and it is allowed to retry the workflow once if it fails with a delay of 5 minutes.
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. In my Airflow DAG I have a task that needs to know if it's the first time it's ran or if it's a retry run. I need to adjust my logic in the task if it's a retry attempt. I have a few ideas on how I could store the number of retries for the task but I'm not sure if any of them are legitimate or if there's an easier built in way to get this information within the task.
I'm wondering if I can just have an integer variable inside the dag that I append every time the task runs. Then if the task if reran I could check the value of the variable to see that it's greater than 1 and hence would be a retry run. But I'm not sure if mutable global variables work that way in Airflow since there can be multiple workers for different tasks I'm not sure on this though.
So you could do something like:. But I didn't test that to confirm. Learn more. Asked 1 year, 8 months ago. Active 1 year, 8 months ago. Viewed 3k times. Write it in an XCOM variable? Kyle Bridenstine Kyle Bridenstine 3, 2 2 gold badges 30 30 silver badges 68 68 bronze badges. Active Oldest Votes. This is a good feature thanks. I did a simple test with the BashOperator shown here and when I go to rerun the DAG by clearing all the past, future, Updated my answer.
This is important so you always know how many times the code has been run.Apache Airflow is an open-source tool for orchestrating complex computational workflows and data processing pipelines. If you find yourself running cron task which execute ever longer scripts, or keeping a calendar of big data processing batch jobs then Airflow can probably help you. This article provides an introductory tutorial for people who want to get started writing pipelines with Airflow. An Airflow workflow is designed as a directed acyclic graph DAG.
That means, that when authoring a workflow, you should think how it could be divided into tasks which can be executed independently. You can then merge these tasks into a logical whole by combining them into a graph. The shape of the graph decides the overall logic of your workflow. An Airflow DAG can include multiple branches and you can decide which of them to follow and which to skip at the time of workflow execution.
This creates a very resilient design, because each task can be retried multiple times if an error occurs. Airflow can even be stopped entirely and running workflows will resume by restarting the last unfinished task. Each task should be idempotenti. Airflow documentation provides more information about these and other concepts. Airflow is written in Python, so I will assume you have it installed on your machine.
I will also assume that you have virtualenv installed. If the airflow version command worked, then Airflow also created its default configuration file airflow. Default configuration values stored in airflow. Take a look at the docs for more information about configuring Airflow.Airflow in Practice Stop Worrying Start Loving DAGs - Sarah Schattschneider
Next step is to issue the following command, which will create and initialize the Airflow SQLite database:. Using SQLite is an adequate solution for local testing and development, but it does not support concurrent access.
In a production environment you will most certainly want to use a more robust database solution such as Postgres or MySQL. You can start it by issuing the command:. Airflow comes with a number of example DAGs. In order to run your DAG, open a second terminal and start the Airflow scheduler by issuing the following commands:. The scheduler will send tasks for execution.
The default Airflow settings rely on an executor named SequentialExecutorwhich is started automatically by the scheduler. In production you would probably want to use a more robust executor, such as the CeleryExecutor. In order to start a DAG Run, first turn the workflow on arrow 1then click the Trigger Dag button arrow 2 and finally, click on the Graph View arrow 3 to see the progress of the run.
You can reload the graph view until both tasks reach the status Success. If everything worked as expected, the log should show a number of lines and among them something like this:. The code you should have at this stage is available in this commit on GitHub.
An Operator is an atomic block of workflow logic, which performs a single action. The execute method may also raise the AirflowSkipException from airflow. In such a case the task instance would transition to the Skipped status.
If another exception is raised, the task will be retried until the maximum number of retries is reached. Remember that since the execute method can retry many times, it should be idempotent.
In this file we are defining a new operator named MyFirstOperator. We are also defining an Airflow plugin named MyFirstPlugin. In the docs, you can read more about Airflow plugins.
All the code you should have at this stage is available in this commit on GitHub. Debugging would quickly get tedious if you had to trigger a DAG run and wait for all upstream tasks to finish before you could retry your new operator.Bases: airflow. Abstract base class for all operators. Since operators create objects that become nodes in the dag, BaseOperator contains many recursive methods for dag crawling behavior. Operators derived from this class should perform or trigger certain tasks synchronously wait for completion.
Instances of these operators tasks target specific operations, running specific scripts, functions or data transfers. Instantiating a class derived from this one results in the creation of a task object, which ultimately becomes a node in DAG objects. This can be a single email or multiple ones. Multiple addresses can be specified as a comma or semi-colon separated string or by passing a list of strings. This is useful if the different instances of a task X alter the same asset, and this asset is used by tasks downstream of task X.
Also note that only tasks immediately downstream of the previous task instance are waited for; the statuses of any tasks further downstream are ignored. DAG — a reference to the dag the task is attached to if any. This allows the executor to trigger higher priority tasks before others when things get backed up. As a result, upstream tasks will have higher weight and will be scheduled more aggressively when using positive weight values.
This is useful when you have multiple dag run instances and desire to have all upstream tasks to complete for all runs before each dag can continue processing downstream tasks. When set to upstream the effective weight is the aggregate sum of all upstream ancestors.
This is the opposite where downtream tasks have higher weight and will be scheduled more aggressively when using positive weight values. This is useful when you have multiple dag run instances and prefer to have each dag complete before starting upstream tasks of other dags.
You may want to do this when you know exactly what priority weight each task should have. Additionally, when set to absolutethere is bonus effect of significantly speeding up the task creation process as for very large DAGS. Options can be set as string or using the constants defined in the static class airflow. Not all executors implement queue management, the CeleryExecutor does support targeting specific queues. Note that this represents the timedelta after the period is closed.
For example if you set an SLA of 1 hour, the scheduler would send an email soon after AM on the if the instance has not succeeded yet. The scheduler pays special attention for jobs with an SLA and sends alert emails for sla misses. SLA misses are also recorded in the database for future reference. All tasks that share the same SLA time get bundled in a single email, sent soon after that time. SLA notification are sent once and only once for each task instance. Context contains references to related objects to the task instance and is documented under the macros section of the API.
Parameters are namespaced by the name of executor.This tutorial walks you through some of the fundamental Airflow concepts, objects, and their usage while writing your first pipeline. Here is an example of a basic pipeline definition. Do not worry if this looks complicated, a line by line explanation follows below. The actual tasks defined here will run in a different context from the context of this script. Different tasks run on different workers at different points in time, which means that this script cannot be used to cross communicate between tasks.
Note that for this purpose we have a more advanced feature called XCom. People sometimes think of the DAG definition file as a place where they can do some actual data processing - that is not the case at all!
It needs to evaluate quickly seconds, not minutes since the scheduler will execute it periodically to reflect the changes if any. BaseOperator documentation. Also, note that you could easily define different sets of arguments that would serve different purposes. An example of that would be to have different settings between a production and development environment. Tasks are generated when instantiating operator objects. An object instantiated from an operator is called a constructor.
This is simpler than passing every argument for every constructor call. Also, notice that in the second task we override the retries parameter with 3. Airflow leverages the power of Jinja Templating and provides the pipeline author with a set of built-in parameters and macros.
Airflow also provides hooks for the pipeline author to define their own parameters, macros and templates. For more information regarding custom filters have a look at the Jinja Documentation. For more information on the variables and macros that can be referenced in templates, make sure to read through the Macros reference.
We can add documentation for DAG or each single task. DAG documentation only support markdown so far and task documentation support plain text, markdown, reStructuredText, json, yaml. We have tasks t1t2 and t3 that do not depend on each other.
Note that when executing your script, Airflow will raise exceptions when it finds cycles in your DAG or when a dependency is referenced more than once. This is the logical date, which simulates the scheduler running your task or dag at a specific date and time, even though it physically will run now or as soon as its dependencies are met.
Now remember what we did with templating earlier?