# Engine

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# Overview

Prefect's execution model is built around two classes, FlowRunner and TaskRunner, which produce and operate on State objects. The actual execution is handled by Executor classes, which can interface with external environments.

# Flow runners

The flow runner takes a flow and attempts to run all of its tasks. It collects the resulting states and, if possible, returns a final state for the flow.

Flow runners loop over all of the tasks one time. If tasks remain unfinished after that pass -- for example, if one of them needs to be retried -- then a second loop will be required to attempt to finish them. There is no limit to the number of attempts it may take to move all tasks (and therefore the flow itself) into a finished state.

# Parameters

Flows that have parameters may require parameter values (if those parameters have no defaults). Parameter values must be passed to the flow runner when it runs.

# Task runners

The task runner is responsible for executing a single task. It receives the task's initial state as well as any upstream states, and uses these to evaluate an execution pipeline. For example:

  • the task must be in a Pending state
  • the upstream tasks must be Finished
  • the task's trigger function must pass

If these conditions (and a few others) are met, the task can move into a Running state.

Then, depending on the task, it may either be run() or it may be mapped, which involves creating dynamic children task runners.

Finally, the task moves through a post-process pipeline that checks to see if it should be retried or cached.

# Executors

The executor classes are responsible for actually running tasks. For example, the flow runner will submit each task runner to its executor, and wait for the result. We recommend Dask distributed as the preferred execution engine.

Executors have a relatively simple API - users can submit functions and wait for their results.

For testing and development, the LocalExecutor is preferred. It runs every function synchronously in the local process and is the default executor for flows unless otherwise specified.

The LocalDaskExecutor is slightly more complex. It still runs functions locally, but uses Dask to parallelize across threads or processes.

The DaskExecutor is a completely asynchronous engine that can run functions in a distributed Dask cluster. This is the recommended engine for production.

# Using a Dask Executor

An executor can be provided to a flow at runtime:



from prefect import task, Flow

def say_hello():
    print("Hello, world!")

with Flow("Run Me") as flow:
    h = say_hello()

from prefect.executors import DaskExecutor

executor = DaskExecutor(address="tcp://localhost:8786")

This DaskExecutor will connect to a Dask scheduler over the address tcp://localhost:8786 and begin submitting work to be executed on Dask workers.

Dynamic Scheduler

If no scheduler address is specified for the DaskExecutor than an in-process scheduler will be created and torn down upon completion. See the DaskExecutor API Documentation for more information.

LocalDaskExecutor vs DaskExecutor

The key difference between the LocalDaskExecutor and the DaskExecutor is the choice of scheduler. The LocalDaskExecutor is configurable to use any number of schedulers while the DaskExecutor uses the distributed scheduler. This means that the LocalDaskExecutor can help achieve some multithreading / multiprocessing however it does not provide as many distributed features as the DaskExecutor.