States are rich objects that contain information about the status of a particular task run or flow run. While you don't need to know the details of the states to use Prefect, you can give your workflows superpowers by taking advantage of it.
At any moment, you can learn anything you need to know about a task or flow by examining its current state or the history of its states. For example, a state could tell you:
- that a task is scheduled to make a third run attempt in an hour
- that a task succeeded and what data it produced
- that a task was scheduled to run, but later cancelled
that a task used the cached result of a previous run instead of re-running
that a task failed because it timed out
By manipulating a relatively small number of task states, Prefect flows can harness the complexity that emerges in workflows.
Only runs have states
Though we often refer to the "state" of a flow or a task, what we really mean is the state of a flow run or a task run. Flows and tasks are templates that describe what a system does; only when we run the system does it also take on a state. So while we might refer to a task as "running" or being "successful", we really mean that a specific instance of the task is in that state.
States have names and types. State types are canonical, with specific orchestration rules that apply to transitions into and out of each state type. A state's name, is often, but not always, synonymous with its type. For example, a task run that is running for the first time has a state with the name Running and the type
RUNNING. However, if the task retries, that same task run will have the name Retrying and the type
RUNNING. Each time the task run transitions into the
RUNNING state, the same orchestration rules are applied.
There are three terminal state types, from which there are no orchestrated transitions to any other state type.
|Scheduled||SCHEDULED||No||The run will begin at a particular time in the future.|
|Late||SCHEDULED||No||The run's scheduled start time has passed, but it has not transitioned to PENDING (5 seconds by default).|
|AwaitingRetry||SCHEDULED||No||The run did not complete successfully because of a code issue and had remaining retry attempts.|
|Pending||PENDING||No||The run has been submitted to run, but is waiting on necessary preconditions to be satisfied.|
|Running||RUNNING||No||The run code is currently executing.|
|Retrying||RUNNING||No||The run code is currently executing after previously not complete successfully.|
|Cancelled||CANCELLED||Yes||The run did not complete because a user determined that it should not.|
|Completed||COMPLETED||Yes||The run completed successfully.|
|Retrieved Cache||COMPLETED||Yes||The run has retrieved a previously created cached state.|
|Failed||FAILED||Yes||The run did not complete because of a code issue and had no remaining retry attempts.|
|Crashed||CRASHED||No||Coming Soon - The run did not complete because of an infrastructure issue.|
More to come