> ## Documentation Index > Fetch the complete documentation index at: https://docs.prefect.io/llms.txt > Use this file to discover all available pages before exploring further. # rules # `prefect.server.orchestration.rules` Prefect's flow and task-run orchestration machinery. This module contains all the core concepts necessary to implement Prefect's state orchestration engine. These states correspond to intuitive descriptions of all the points that a Prefect flow or task can observe executing user code and intervene, if necessary. A detailed description of states can be found in our concept [documentation](https://docs.prefect.io/v3/concepts/states). Prefect's orchestration engine operates under the assumption that no governed user code will execute without first requesting Prefect REST API validate a change in state and record metadata about the run. With all attempts to run user code being checked against a Prefect instance, the Prefect REST API database becomes the unambiguous source of truth for managing the execution of complex interacting workflows. Orchestration rules can be implemented as discrete units of logic that operate against each state transition and can be fully observable, extensible, and customizable -- all without needing to store or parse a single line of user code. ## Classes ### `OrchestrationContext` A container for a state transition, governed by orchestration rules. When a flow- or task- run attempts to change state, Prefect REST API has an opportunity to decide whether this transition can proceed. All the relevant information associated with the state transition is stored in an `OrchestrationContext`, which is subsequently governed by nested orchestration rules implemented using the `BaseOrchestrationRule` ABC. `OrchestrationContext` introduces the concept of a state being `None` in the context of an intended state transition. An initial state can be `None` if a run is is attempting to set a state for the first time. The proposed state might be `None` if a rule governing the transition determines that no state change should occur at all and nothing is written to the database. **Attributes:** * `session`: a SQLAlchemy database session * `initial_state`: the initial state of a run * `proposed_state`: the proposed state a run is transitioning into * `validated_state`: a proposed state that has committed to the database * `rule_signature`: a record of rules that have fired on entry into a managed context, currently only used for debugging purposes * `finalization_signature`: a record of rules that have fired on exit from a managed context, currently only used for debugging purposes * `response_status`: a SetStateStatus object used to build the API response * `response_details`: a StateResponseDetails object use to build the API response **Args:** * `session`: a SQLAlchemy database session * `initial_state`: the initial state of a run * `proposed_state`: the proposed state a run is transitioning into **Methods:** #### `entry_context` ```python theme={null} entry_context(self) -> tuple[Optional[states.State], Optional[states.State], Self] ``` A convenience method that generates input parameters for orchestration rules. An `OrchestrationContext` defines a state transition that is managed by orchestration rules which can fire hooks before a transition has been committed to the database. These hooks have a consistent interface which can be generated with this method. #### `exit_context` ```python theme={null} exit_context(self) -> tuple[Optional[states.State], Optional[states.State], Self] ``` A convenience method that generates input parameters for orchestration rules. An `OrchestrationContext` defines a state transition that is managed by orchestration rules which can fire hooks after a transition has been committed to the database. These hooks have a consistent interface which can be generated with this method. #### `flow_run` ```python theme={null} flow_run(self) -> orm_models.FlowRun | None ``` #### `initial_state_type` ```python theme={null} initial_state_type(self) -> Optional[states.StateType] ``` The state type of `self.initial_state` if it exists. #### `model_validate_list` ```python theme={null} model_validate_list(cls, obj: Any) -> list[Self] ``` #### `proposed_state_type` ```python theme={null} proposed_state_type(self) -> Optional[states.StateType] ``` The state type of `self.proposed_state` if it exists. #### `reset_fields` ```python theme={null} reset_fields(self: Self) -> Self ``` Reset the fields of the model that are in the `_reset_fields` set. **Returns:** * A new instance of the model with the reset fields. #### `run_settings` ```python theme={null} run_settings(self) -> RP ``` Run-level settings used to orchestrate the state transition. #### `safe_copy` ```python theme={null} safe_copy(self) -> Self ``` Creates a mostly-mutation-safe copy for use in orchestration rules. Orchestration rules govern state transitions using information stored in an `OrchestrationContext`. However, mutating objects stored on the context directly can have unintended side-effects. To guard against this, `self.safe_copy` can be used to pass information to orchestration rules without risking mutation. **Returns:** * A mutation-safe copy of the `OrchestrationContext` #### `validated_state_type` ```python theme={null} validated_state_type(self) -> Optional[states.StateType] ``` The state type of `self.validated_state` if it exists. ### `FlowOrchestrationContext` A container for a flow run state transition, governed by orchestration rules. When a flow- run attempts to change state, Prefect REST API has an opportunity to decide whether this transition can proceed. All the relevant information associated with the state transition is stored in an `OrchestrationContext`, which is subsequently governed by nested orchestration rules implemented using the `BaseOrchestrationRule` ABC. `FlowOrchestrationContext` introduces the concept of a state being `None` in the context of an intended state transition. An initial state can be `None` if a run is is attempting to set a state for the first time. The proposed state might be `None` if a rule governing the transition determines that no state change should occur at all and nothing is written to the database. **Attributes:** * `session`: a SQLAlchemy database session * `run`: the flow run attempting to change state * `initial_state`: the initial state of the run * `proposed_state`: the proposed state the run is transitioning into * `validated_state`: a proposed state that has committed to the database * `rule_signature`: a record of rules that have fired on entry into a managed context, currently only used for debugging purposes * `finalization_signature`: a record of rules that have fired on exit from a managed context, currently only used for debugging purposes * `response_status`: a SetStateStatus object used to build the API response * `response_details`: a StateResponseDetails object use to build the API response **Args:** * `session`: a SQLAlchemy database session * `run`: the flow run attempting to change state * `initial_state`: the initial state of a run * `proposed_state`: the proposed state a run is transitioning into **Methods:** #### `flow_run` ```python theme={null} flow_run(self) -> orm_models.FlowRun ``` #### `run_settings` ```python theme={null} run_settings(self) -> core.FlowRunPolicy ``` Run-level settings used to orchestrate the state transition. #### `safe_copy` ```python theme={null} safe_copy(self) -> Self ``` Creates a mostly-mutation-safe copy for use in orchestration rules. Orchestration rules govern state transitions using information stored in an `OrchestrationContext`. However, mutating objects stored on the context directly can have unintended side-effects. To guard against this, `self.safe_copy` can be used to pass information to orchestration rules without risking mutation. **Returns:** * A mutation-safe copy of `FlowOrchestrationContext` #### `task_run` ```python theme={null} task_run(self) -> None ``` #### `validate_proposed_state` ```python theme={null} validate_proposed_state(self, db: PrefectDBInterface) ``` Validates a proposed state by committing it to the database. After the `FlowOrchestrationContext` is governed by orchestration rules, the proposed state can be validated: the proposed state is added to the current SQLAlchemy session and is flushed. `self.validated_state` set to the flushed state. The state on the run is set to the validated state as well. If the proposed state is `None` when this method is called, no state will be written and `self.validated_state` will be set to the run's current state. **Returns:** * None ### `TaskOrchestrationContext` A container for a task run state transition, governed by orchestration rules. When a task- run attempts to change state, Prefect REST API has an opportunity to decide whether this transition can proceed. All the relevant information associated with the state transition is stored in an `OrchestrationContext`, which is subsequently governed by nested orchestration rules implemented using the `BaseOrchestrationRule` ABC. `TaskOrchestrationContext` introduces the concept of a state being `None` in the context of an intended state transition. An initial state can be `None` if a run is is attempting to set a state for the first time. The proposed state might be `None` if a rule governing the transition determines that no state change should occur at all and nothing is written to the database. **Attributes:** * `session`: a SQLAlchemy database session * `run`: the task run attempting to change state * `initial_state`: the initial state of the run * `proposed_state`: the proposed state the run is transitioning into * `validated_state`: a proposed state that has committed to the database * `rule_signature`: a record of rules that have fired on entry into a managed context, currently only used for debugging purposes * `finalization_signature`: a record of rules that have fired on exit from a managed context, currently only used for debugging purposes * `response_status`: a SetStateStatus object used to build the API response * `response_details`: a StateResponseDetails object use to build the API response **Args:** * `session`: a SQLAlchemy database session * `run`: the task run attempting to change state * `initial_state`: the initial state of a run * `proposed_state`: the proposed state a run is transitioning into **Methods:** #### `flow_run` ```python theme={null} flow_run(self) -> orm_models.FlowRun | None ``` #### `run_settings` ```python theme={null} run_settings(self) -> core.TaskRunPolicy ``` Run-level settings used to orchestrate the state transition. #### `safe_copy` ```python theme={null} safe_copy(self) -> Self ``` Creates a mostly-mutation-safe copy for use in orchestration rules. Orchestration rules govern state transitions using information stored in an `OrchestrationContext`. However, mutating objects stored on the context directly can have unintended side-effects. To guard against this, `self.safe_copy` can be used to pass information to orchestration rules without risking mutation. **Returns:** * A mutation-safe copy of `TaskOrchestrationContext` #### `task_run` ```python theme={null} task_run(self) -> orm_models.TaskRun ``` #### `validate_proposed_state` ```python theme={null} validate_proposed_state(self, db: PrefectDBInterface) ``` Validates a proposed state by committing it to the database. After the `TaskOrchestrationContext` is governed by orchestration rules, the proposed state can be validated: the proposed state is added to the current SQLAlchemy session and is flushed. `self.validated_state` set to the flushed state. The state on the run is set to the validated state as well. If the proposed state is `None` when this method is called, no state will be written and `self.validated_state` will be set to the run's current state. **Returns:** * None ### `BaseOrchestrationRule` An abstract base class used to implement a discrete piece of orchestration logic. An `OrchestrationRule` is a stateful context manager that directly governs a state transition. Complex orchestration is achieved by nesting multiple rules. Each rule runs against an `OrchestrationContext` that contains the transition details; this context is then passed to subsequent rules. The context can be modified by hooks that fire before and after a new state is validated and committed to the database. These hooks will fire as long as the state transition is considered "valid" and govern a transition by either modifying the proposed state before it is validated or by producing a side-effect. A state transition occurs whenever a flow- or task- run changes state, prompting Prefect REST API to decide whether or not this transition can proceed. The current state of the run is referred to as the "initial state", and the state a run is attempting to transition into is the "proposed state". Together, the initial state transitioning into the proposed state is the intended transition that is governed by these orchestration rules. After using rules to enter a runtime context, the `OrchestrationContext` will contain a proposed state that has been governed by each rule, and at that point can validate the proposed state and commit it to the database. The validated state will be set on the context as `context.validated_state`, and rules will call the `self.after_transition` hook upon exiting the managed context. Examples: Create a rule: ```python theme={null} class BasicRule(BaseOrchestrationRule): # allowed initial state types FROM_STATES = [StateType.RUNNING] # allowed proposed state types TO_STATES = [StateType.COMPLETED, StateType.FAILED] async def before_transition(initial_state, proposed_state, ctx): # side effects and proposed state mutation can happen here ... async def after_transition(initial_state, validated_state, ctx): # operations on states that have been validated can happen here ... async def cleanup(intitial_state, validated_state, ctx): # reverts side effects generated by `before_transition` if necessary ... ``` Use a rule: ```python theme={null} intended_transition = (StateType.RUNNING, StateType.COMPLETED) async with BasicRule(context, *intended_transition): # context.proposed_state has been governed by BasicRule ... ``` Use multiple rules: ```python theme={null} rules = [BasicRule, BasicRule] intended_transition = (StateType.RUNNING, StateType.COMPLETED) async with contextlib.AsyncExitStack() as stack: for rule in rules: stack.enter_async_context(rule(context, *intended_transition)) # context.proposed_state has been governed by all rules ... ``` **Attributes:** * `FROM_STATES`: list of valid initial state types this rule governs * `TO_STATES`: list of valid proposed state types this rule governs * `context`: the orchestration context * `from_state_type`: the state type a run is currently in * `to_state_type`: the intended proposed state type prior to any orchestration **Args:** * `context`: A `FlowOrchestrationContext` or `TaskOrchestrationContext` that is passed between rules * `from_state_type`: The state type of the initial state of a run, if this state type is not contained in `FROM_STATES`, no hooks will fire * `to_state_type`: The state type of the proposed state before orchestration, if this state type is not contained in `TO_STATES`, no hooks will fire **Methods:** #### `abort_transition` ```python theme={null} abort_transition(self, reason: str) -> None ``` Aborts a proposed transition before the transition is validated. This method will abort a proposed transition, expecting no further action to occur for this run. The proposed state is set to `None`, signaling to the `OrchestrationContext` that no state should be written to the database. A reason for aborting the transition is also provided. Rules that abort the transition will not fizzle, despite the proposed state type changing. **Args:** * `reason`: The reason for aborting the transition #### `after_transition` ```python theme={null} after_transition(self, initial_state: Optional[states.State], validated_state: Optional[states.State], context: OrchestrationContext[T, RP]) -> None ``` Implements a hook that can fire after a state is committed to the database. **Args:** * `initial_state`: The initial state of a transition * `validated_state`: The governed state that has been committed to the database * `context`: A safe copy of the `OrchestrationContext`, with the exception of `context.run`, mutating this context will have no effect on the broader orchestration environment. **Returns:** * None #### `before_transition` ```python theme={null} before_transition(self, initial_state: Optional[states.State], proposed_state: Optional[states.State], context: OrchestrationContext[T, RP]) -> None ``` Implements a hook that can fire before a state is committed to the database. This hook may produce side-effects or mutate the proposed state of a transition using one of four methods: `self.reject_transition`, `self.delay_transition`, `self.abort_transition`, and `self.rename_state`. **Args:** * `initial_state`: The initial state of a transition * `proposed_state`: The proposed state of a transition * `context`: A safe copy of the `OrchestrationContext`, with the exception of `context.run`, mutating this context will have no effect on the broader orchestration environment. **Returns:** * None #### `cleanup` ```python theme={null} cleanup(self, initial_state: Optional[states.State], validated_state: Optional[states.State], context: OrchestrationContext[T, RP]) -> None ``` Implements a hook that can fire after a state is committed to the database. The intended use of this method is to revert side-effects produced by `self.before_transition` when the transition is found to be invalid on exit. This allows multiple rules to be gracefully run in sequence, without logic that keeps track of all other rules that might govern a transition. **Args:** * `initial_state`: The initial state of a transition * `validated_state`: The governed state that has been committed to the database * `context`: A safe copy of the `OrchestrationContext`, with the exception of `context.run`, mutating this context will have no effect on the broader orchestration environment. **Returns:** * None #### `delay_transition` ```python theme={null} delay_transition(self, delay_seconds: int, reason: str) -> None ``` Delays a proposed transition before the transition is validated. This method will delay a proposed transition, setting the proposed state to `None`, signaling to the `OrchestrationContext` that no state should be written to the database. The number of seconds a transition should be delayed is passed to the `OrchestrationContext`. A reason for delaying the transition is also provided. Rules that delay the transition will not fizzle, despite the proposed state type changing. **Args:** * `delay_seconds`: The number of seconds the transition should be delayed * `reason`: The reason for delaying the transition #### `fizzled` ```python theme={null} fizzled(self) -> bool ``` Determines if a rule is fizzled and side-effects need to be reverted. Rules are fizzled if the transitions were valid on entry (thus firing `self.before_transition`) but are invalid upon exiting the governed context, most likely caused by another rule mutating the transition. **Returns:** * True if the rule is fizzled, False otherwise. #### `invalid` ```python theme={null} invalid(self) -> bool ``` Determines if a rule is invalid. Invalid rules do nothing and no hooks fire upon entering or exiting a governed context. Rules are invalid if the transition states types are not contained in `self.FROM_STATES` and `self.TO_STATES`, or if the context is proposing a transition that differs from the transition the rule was instantiated with. **Returns:** * True if the rules in invalid, False otherwise. #### `invalid_transition` ```python theme={null} invalid_transition(self) -> bool ``` Determines if the transition proposed by the `OrchestrationContext` is invalid. If the `OrchestrationContext` is attempting to manage a transition with this rule that differs from the transition the rule was instantiated with, the transition is considered to be invalid. Depending on the context, a rule with an invalid transition is either "invalid" or "fizzled". **Returns:** * True if the transition is invalid, False otherwise. #### `reject_transition` ```python theme={null} reject_transition(self, state: Optional[states.State], reason: str) -> None ``` Rejects a proposed transition before the transition is validated. This method will reject a proposed transition, mutating the proposed state to the provided `state`. A reason for rejecting the transition is also passed on to the `OrchestrationContext`. Rules that reject the transition will not fizzle, despite the proposed state type changing. **Args:** * `state`: The new proposed state. If `None`, the current run state will be returned in the result instead. * `reason`: The reason for rejecting the transition #### `rename_state` ```python theme={null} rename_state(self, state_name: str) -> None ``` Sets the "name" attribute on a proposed state. The name of a state is an annotation intended to provide rich, human-readable context for how a run is progressing. This method only updates the name and not the canonical state TYPE, and will not fizzle or invalidate any other rules that might govern this state transition. #### `update_context_parameters` ```python theme={null} update_context_parameters(self, key: str, value: Any) -> None ``` Updates the "parameters" dictionary attribute with the specified key-value pair. This mechanism streamlines the process of passing messages and information between orchestration rules if necessary and is simpler and more ephemeral than message-passing via the database or some other side-effect. This mechanism can be used to break up large rules for ease of testing or comprehension, but note that any rules coupled this way (or any other way) are no longer independent and the order in which they appear in the orchestration policy priority will matter. ### `FlowRunOrchestrationRule` ### `TaskRunOrchestrationRule` ### `GenericOrchestrationRule` ### `BaseUniversalTransform` An abstract base class used to implement privileged bookkeeping logic. Beyond the orchestration rules implemented with the `BaseOrchestrationRule` ABC, Universal transforms are not stateful, and fire their before- and after- transition hooks on every state transition unless the proposed state is `None`, indicating that no state should be written to the database. Because there are no guardrails in place to prevent directly mutating state or other parts of the orchestration context, universal transforms should only be used with care. **Attributes:** * `FROM_STATES`: for compatibility with `BaseOrchestrationPolicy` * `TO_STATES`: for compatibility with `BaseOrchestrationPolicy` * `context`: the orchestration context * `from_state_type`: the state type a run is currently in * `to_state_type`: the intended proposed state type prior to any orchestration **Args:** * `context`: A `FlowOrchestrationContext` or `TaskOrchestrationContext` that is passed between transforms **Methods:** #### `after_transition` ```python theme={null} after_transition(self, context: OrchestrationContext[T, RP]) -> None ``` Implements a hook that can fire after a state is committed to the database. **Args:** * `context`: the `OrchestrationContext` that contains transition details **Returns:** * None #### `before_transition` ```python theme={null} before_transition(self, context: OrchestrationContext[T, RP]) -> None ``` Implements a hook that fires before a state is committed to the database. **Args:** * `context`: the `OrchestrationContext` that contains transition details **Returns:** * None #### `exception_in_transition` ```python theme={null} exception_in_transition(self) -> bool ``` Determines if the transition has encountered an exception. **Returns:** * True if the transition is encountered an exception, False otherwise. #### `nullified_transition` ```python theme={null} nullified_transition(self) -> bool ``` Determines if the transition has been nullified. Transitions are nullified if the proposed state is `None`, indicating that nothing should be written to the database. **Returns:** * True if the transition is nullified, False otherwise. ### `TaskRunUniversalTransform` ### `FlowRunUniversalTransform` ### `GenericUniversalTransform`