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.
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.
OrchestrationContext
¶
Bases: PrefectBaseModel
A container for a state transition, governed by orchestration rules.
Note
An OrchestrationContext
should not be instantiated directly, instead
use the flow- or task- specific subclasses, FlowOrchestrationContext
and
TaskOrchestrationContext
.
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:
Name | Type | Description |
---|---|---|
session |
Optional[Union[Session, AsyncSession]]
|
a SQLAlchemy database session |
initial_state |
Optional[State]
|
the initial state of a run |
proposed_state |
Optional[State]
|
the proposed state a run is transitioning into |
validated_state |
Optional[State]
|
a proposed state that has committed to the database |
rule_signature |
List[str]
|
a record of rules that have fired on entry into a managed context, currently only used for debugging purposes |
finalization_signature |
List[str]
|
a record of rules that have fired on exit from a managed context, currently only used for debugging purposes |
response_status |
SetStateStatus
|
a SetStateStatus object used to build the API response |
response_details |
StateResponseDetails
|
a StateResponseDetails object use to build the API response |
Parameters:
Name | Type | Description | Default |
---|---|---|---|
session |
a SQLAlchemy database session |
required | |
initial_state |
the initial state of a run |
required | |
proposed_state |
the proposed state a run is transitioning into |
required |
Source code in prefect/server/orchestration/rules.py
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|
initial_state_type: Optional[states.StateType]
property
¶
The state type of self.initial_state
if it exists.
proposed_state_type: Optional[states.StateType]
property
¶
The state type of self.proposed_state
if it exists.
validated_state_type: Optional[states.StateType]
property
¶
The state type of self.validated_state
if it exists.
safe_copy
¶
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:
Type | Description |
---|---|
A mutation-safe copy of the |
Source code in prefect/server/orchestration/rules.py
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|
entry_context
¶
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.
Source code in prefect/server/orchestration/rules.py
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|
exit_context
¶
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.
Source code in prefect/server/orchestration/rules.py
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|
FlowOrchestrationContext
¶
Bases: OrchestrationContext
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:
Name | Type | Description |
---|---|---|
session |
a SQLAlchemy database session |
|
run |
Any
|
the flow run attempting to change state |
initial_state |
Any
|
the initial state of the run |
proposed_state |
Any
|
the proposed state the run is transitioning into |
validated_state |
Any
|
a proposed state that has committed to the database |
rule_signature |
Any
|
a record of rules that have fired on entry into a managed context, currently only used for debugging purposes |
finalization_signature |
Any
|
a record of rules that have fired on exit from a managed context, currently only used for debugging purposes |
response_status |
Any
|
a SetStateStatus object used to build the API response |
response_details |
Any
|
a StateResponseDetails object use to build the API response |
Parameters:
Name | Type | Description | Default |
---|---|---|---|
session |
a SQLAlchemy database session |
required | |
run |
the flow run attempting to change state |
required | |
initial_state |
the initial state of a run |
required | |
proposed_state |
the proposed state a run is transitioning into |
required |
Source code in prefect/server/orchestration/rules.py
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|
run_settings: Dict
property
¶
Run-level settings used to orchestrate the state transition.
validate_proposed_state
async
¶
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:
Type | Description |
---|---|
None |
Source code in prefect/server/orchestration/rules.py
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safe_copy
¶
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.
Note
self.run
is an ORM model, and even when copied is unsafe to mutate
Returns:
Type | Description |
---|---|
A mutation-safe copy of |
Source code in prefect/server/orchestration/rules.py
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TaskOrchestrationContext
¶
Bases: OrchestrationContext
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:
Name | Type | Description |
---|---|---|
session |
a SQLAlchemy database session |
|
run |
Any
|
the task run attempting to change state |
initial_state |
Any
|
the initial state of the run |
proposed_state |
Any
|
the proposed state the run is transitioning into |
validated_state |
Any
|
a proposed state that has committed to the database |
rule_signature |
Any
|
a record of rules that have fired on entry into a managed context, currently only used for debugging purposes |
finalization_signature |
Any
|
a record of rules that have fired on exit from a managed context, currently only used for debugging purposes |
response_status |
Any
|
a SetStateStatus object used to build the API response |
response_details |
Any
|
a StateResponseDetails object use to build the API response |
Parameters:
Name | Type | Description | Default |
---|---|---|---|
session |
a SQLAlchemy database session |
required | |
run |
the task run attempting to change state |
required | |
initial_state |
the initial state of a run |
required | |
proposed_state |
the proposed state a run is transitioning into |
required |
Source code in prefect/server/orchestration/rules.py
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|
run_settings: Dict
property
¶
Run-level settings used to orchestrate the state transition.
validate_proposed_state
async
¶
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:
Type | Description |
---|---|
None |
Source code in prefect/server/orchestration/rules.py
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|
safe_copy
¶
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.
Note
self.run
is an ORM model, and even when copied is unsafe to mutate
Returns:
Type | Description |
---|---|
A mutation-safe copy of |
Source code in prefect/server/orchestration/rules.py
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BaseOrchestrationRule
¶
Bases: AbstractAsyncContextManager
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:
>>> 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:
>>> intended_transition = (StateType.RUNNING, StateType.COMPLETED)
>>> async with BasicRule(context, *intended_transition):
>>> # context.proposed_state has been governed by BasicRule
>>> ...
Use multiple rules:
>>> 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:
Name | Type | Description |
---|---|---|
FROM_STATES |
Iterable
|
list of valid initial state types this rule governs |
TO_STATES |
Iterable
|
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 |
Parameters:
Name | Type | Description | Default |
---|---|---|---|
context |
OrchestrationContext
|
A |
required |
from_state_type |
Optional[StateType]
|
The state type of the initial state of a run, if this
state type is not contained in |
required |
to_state_type |
Optional[StateType]
|
The state type of the proposed state before orchestration, if
this state type is not contained in |
required |
Source code in prefect/server/orchestration/rules.py
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before_transition
async
¶
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
.
Note
As currently implemented, the before_transition
hook is not
perfectly isolated from mutating the transition. It is a standard instance
method that has access to self
, and therefore self.context
. This should
never be modified directly. Furthermore, context.run
is an ORM model, and
mutating the run can also cause unintended writes to the database.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
initial_state |
Optional[State]
|
The initial state of a transition |
required |
proposed_state |
Optional[State]
|
The proposed state of a transition |
required |
context |
OrchestrationContext
|
A safe copy of the |
required |
Returns:
Type | Description |
---|---|
None
|
None |
Source code in prefect/server/orchestration/rules.py
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after_transition
async
¶
Implements a hook that can fire after a state is committed to the database.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
initial_state |
Optional[State]
|
The initial state of a transition |
required |
validated_state |
Optional[State]
|
The governed state that has been committed to the database |
required |
context |
OrchestrationContext
|
A safe copy of the |
required |
Returns:
Type | Description |
---|---|
None
|
None |
Source code in prefect/server/orchestration/rules.py
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cleanup
async
¶
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.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
initial_state |
Optional[State]
|
The initial state of a transition |
required |
validated_state |
Optional[State]
|
The governed state that has been committed to the database |
required |
context |
OrchestrationContext
|
A safe copy of the |
required |
Returns:
Type | Description |
---|---|
None
|
None |
Source code in prefect/server/orchestration/rules.py
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|
invalid
async
¶
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:
Type | Description |
---|---|
bool
|
True if the rules in invalid, False otherwise. |
Source code in prefect/server/orchestration/rules.py
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fizzled
async
¶
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:
Type | Description |
---|---|
bool
|
True if the rule is fizzled, False otherwise. |
Source code in prefect/server/orchestration/rules.py
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invalid_transition
async
¶
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:
Type | Description |
---|---|
bool
|
True if the transition is invalid, False otherwise. |
Source code in prefect/server/orchestration/rules.py
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reject_transition
async
¶
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.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
state |
Optional[State]
|
The new proposed state. If |
required |
reason |
str
|
The reason for rejecting the transition |
required |
Source code in prefect/server/orchestration/rules.py
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delay_transition
async
¶
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.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
delay_seconds |
int
|
The number of seconds the transition should be delayed |
required |
reason |
str
|
The reason for delaying the transition |
required |
Source code in prefect/server/orchestration/rules.py
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abort_transition
async
¶
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.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
reason |
str
|
The reason for aborting the transition |
required |
Source code in prefect/server/orchestration/rules.py
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rename_state
async
¶
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.
Source code in prefect/server/orchestration/rules.py
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|
update_context_parameters
async
¶
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.
Source code in prefect/server/orchestration/rules.py
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BaseUniversalTransform
¶
Bases: AbstractAsyncContextManager
An abstract base class used to implement privileged bookkeeping logic.
Warning
In almost all cases, use the BaseOrchestrationRule
base class instead.
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:
Name | Type | Description |
---|---|---|
FROM_STATES |
Iterable
|
for compatibility with |
TO_STATES |
Iterable
|
for compatibility with |
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 |
Parameters:
Name | Type | Description | Default |
---|---|---|---|
context |
OrchestrationContext
|
A |
required |
Source code in prefect/server/orchestration/rules.py
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|
before_transition
async
¶
Implements a hook that fires before a state is committed to the database.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
context |
the |
required |
Returns:
Type | Description |
---|---|
None
|
None |
Source code in prefect/server/orchestration/rules.py
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|
after_transition
async
¶
Implements a hook that can fire after a state is committed to the database.
Parameters:
Name | Type | Description | Default |
---|---|---|---|
context |
the |
required |
Returns:
Type | Description |
---|---|
None
|
None |
Source code in prefect/server/orchestration/rules.py
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|
nullified_transition
¶
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:
Type | Description |
---|---|
bool
|
True if the transition is nullified, False otherwise. |
Source code in prefect/server/orchestration/rules.py
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|
exception_in_transition
¶
Determines if the transition has encountered an exception.
Returns:
Type | Description |
---|---|
bool
|
True if the transition is encountered an exception, False otherwise. |
Source code in prefect/server/orchestration/rules.py
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|