Pools¶

Some systems can get overwhelmed when too many processes hit them at the same time. Airflow pools can be used to limit the execution parallelism on arbitrary sets of tasks. The list of pools is managed in the UI (Menu -> Admin -> Pools) by giving the pools a name and assigning it a number of worker slots.

Tasks can then be associated with one of the existing pools by using the pool parameter when creating tasks:

aggregate_db_message_job = BashOperator(
    task_id='aggregate_db_message_job',
    execution_timeout=timedelta(hours=3),
    pool='ep_data_pipeline_db_msg_agg',
    bash_command=aggregate_db_message_job_cmd,
    dag=dag,
)
aggregate_db_message_job.set_upstream(wait_for_empty_queue)

Tasks will be scheduled as usual while the slots fill up. The number of slots occupied by a task can be configured by pool_slots. Once capacity is reached, runnable tasks get queued and their state will show as such in the UI. As slots free up, queued tasks start running based on the Priority Weights of the task and its descendants.

Note that if tasks are not given a pool, they are assigned to a default pool default_pool. default_pool is initialized with 128 slots and can be modified through the UI or CLI (but cannot be removed).

Warning

Pools and SubDAGs do not interact as you might first expect. SubDAGs will not honor any pool you set on them at the top level; pools must be set on the tasks inside the SubDAG directly.