XComs¶
XComs (short for “cross-communications”) are a mechanism that let Tasks talk to each other, as by default Tasks are entirely isolated and may be running on entirely different machines.
An XCom is identified by a key (essentially its name), as well as the task_id and dag_id it came from. They can have any (serializable) value, but they are only designed for small amounts of data; do not use them to pass around large values, like dataframes.
XComs are explicitly “pushed” and “pulled” to/from their storage using the xcom_push and xcom_pull methods on Task Instances. Many operators will auto-push their results into an XCom key called return_value if the do_xcom_push argument is set to True (as it is by default), and @task functions do this as well.
xcom_pull defaults to using this key if no key is passed to it, meaning it’s possible to write code like this:
# Pulls the return_value XCOM from "pushing_task"
value = task_instance.xcom_pull(task_ids='pushing_task')
You can also use XComs in templates:
SELECT * FROM {{ task_instance.xcom_pull(task_ids='foo', key='table_name') }}
XComs are a relative of Variables, with the main difference being that XComs are per-task-instance and designed for communication within a DAG run, while Variables are global and designed for overall configuration and value sharing.
Custom Backends¶
The XCom system has interchangeable backends, and you can set which backend is being used via the xcom_backend configuration option.
If you want to implement your own backend, you should subclass BaseXCom, and override the serialize_value and deserialize_value methods.
There is also an orm_deserialize_value method that is called whenever the XCom objects are rendered for UI or reporting purposes; if you have large or expensive-to-retrieve values in your XComs, you should override this method to avoid calling that code (and instead return a lighter, incomplete representation) so the UI remains responsive.
You can also override the clear method and use it when clearing results for given dags and tasks. This allows the custom XCom backend process the data lifecycle easier.