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.. _concepts:priority-weight:

Priority Weights
================

``priority_weight`` defines priorities in the executor queue. The default ``priority_weight`` is ``1``, and can be
bumped to any integer. Moreover, each task has a true ``priority_weight`` that is calculated based on its
``weight_rule`` which defines weighting method used for the effective total priority weight of the task.

By default, Airflow's weighting method is ``downstream``. You can find other weighting methods in
:class:`airflow.utils.WeightRule`.

There are three weighting methods.

- downstream

  The effective weight of the task is the aggregate sum of all
  downstream descendants. 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.

- upstream

  The effective weight is the aggregate sum of all upstream ancestors.
  This is the opposite where downstream 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 DAG runs.

- absolute

  The effective weight is the exact ``priority_weight`` specified
  without additional weighting. You may want to do this when you
  know exactly what priority weight each task should have.
  Additionally, when set to ``absolute``, there is bonus effect of
  significantly speeding up the task creation process as for very
  large DAGs


The ``priority_weight`` parameter can be used in conjunction with :ref:`concepts:pool`.