prefect.server.orchestration.policies

Policies are collections of orchestration rules and transforms.

Prefect implements (most) orchestration with logic that governs a Prefect flow or task changing state. Policies organize of orchestration logic both to provide an ordering mechanism as well as provide observability into the orchestration process.

While Prefect's orchestration rules can gracefully run independently of one another, ordering can still have an impact on the observed behavior of the system. For example, it makes no sense to secure a concurrency slot for a run if a cached state exists. Furthermore, policies, provide a mechanism to configure and observe exactly what logic will fire against a transition.

BaseOrchestrationPolicy

Bases: ABC

An abstract base class used to organize orchestration rules in priority order.

Different collections of orchestration rules might be used to govern various kinds of transitions. For example, flow-run states and task-run states might require different orchestration logic.

Source code in prefect/server/orchestration/policies.py

class BaseOrchestrationPolicy(ABC):
    """
    An abstract base class used to organize orchestration rules in priority order.

    Different collections of orchestration rules might be used to govern various kinds
    of transitions. For example, flow-run states and task-run states might require
    different orchestration logic.
    """

    @staticmethod
    @abstractmethod
    def priority():
        """
        A list of orchestration rules in priority order.
        """

        return []

    @classmethod
    def compile_transition_rules(cls, from_state=None, to_state=None):
        """
        Returns rules in policy that are valid for the specified state transition.
        """

        transition_rules = []
        for rule in cls.priority():
            if from_state in rule.FROM_STATES and to_state in rule.TO_STATES:
                transition_rules.append(rule)
        return transition_rules

priority abstractmethod staticmethod

A list of orchestration rules in priority order.

Source code in prefect/server/orchestration/policies.py

@staticmethod
@abstractmethod
def priority():
    """
    A list of orchestration rules in priority order.
    """

    return []

compile_transition_rules classmethod

Returns rules in policy that are valid for the specified state transition.

Source code in prefect/server/orchestration/policies.py

@classmethod
def compile_transition_rules(cls, from_state=None, to_state=None):
    """
    Returns rules in policy that are valid for the specified state transition.
    """

    transition_rules = []
    for rule in cls.priority():
        if from_state in rule.FROM_STATES and to_state in rule.TO_STATES:
            transition_rules.append(rule)
    return transition_rules