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Run a pipeline

Runners

Runners are the execution mechanisms used to run pipelines. They all inherit from AbstractRunner.

SequentialRunner

Use SequentialRunner to execute pipeline nodes one-by-one based on their dependencies.

We recommend using SequentialRunner in cases where:

  • the pipeline has limited branching
  • the pipeline is fast
  • the resource-consuming steps require most of a scarce resource (e.g., significant RAM, disk memory or CPU)

Kedro uses SequentialRunner by default, so to execute the pipeline sequentially:

kedro run

You can also explicitly use SequentialRunner as follows:

kedro run --runner=SequentialRunner

ParallelRunner

Multiprocessing

You can alternatively run the nodes within the pipeline concurrently, using a ParallelRunner as follows:

kedro run --runner=ParallelRunner

Multithreading

While ParallelRunner uses multiprocessing, you can also run the pipeline with multithreading for concurrent execution by specifying ThreadRunner as follows:

kedro run --runner=ThreadRunner
`SparkDataset` doesn't work correctly with `ParallelRunner`. To add concurrency to the pipeline with `SparkDataset`, you must use `ThreadRunner`.

For more information on how to maximise concurrency when using Kedro with PySpark, read our guide on how to build a Kedro pipeline with PySpark.

Custom runners

If the built-in Kedro runners do not meet your requirements, you can also define your own runner within your project. For example, you may want to add a dry runner, which lists which nodes would be run without executing them:

Click to expand 
# in src/<package_name>/runner.py
from kedro.io import AbstractDataset, DataCatalog, MemoryDataset
from kedro.pipeline import Pipeline
from kedro.runner.runner import AbstractRunner
from pluggy import PluginManager


class DryRunner(AbstractRunner):
    """``DryRunner`` is an ``AbstractRunner`` implementation. It can be used to list which
    nodes would be run without actually executing anything. It also checks if all the
    neccessary data exists.
    """

    def create_default_dataset(self, ds_name: str) -> AbstractDataset:
        """Factory method for creating the default data set for the runner.

        Args:
            ds_name: Name of the missing data set
        Returns:
            An instance of an implementation of AbstractDataset to be used
            for all unregistered data sets.

        """
        return MemoryDataset()

    def _run(
        self,
        pipeline: Pipeline,
        catalog: DataCatalog,
        hook_manager: PluginManager = None,
        session_id: str = None,
    ) -> None:
        """The method implementing dry pipeline running.
        Example logs output using this implementation:

            kedro.runner.dry_runner - INFO - Actual run would execute 3 nodes:
            node3: identity([A]) -> [B]
            node2: identity([C]) -> [D]
            node1: identity([D]) -> [E]

        Args:
            pipeline: The ``Pipeline`` to run.
            catalog: The ``DataCatalog`` from which to fetch data.
            session_id: The id of the session.

        """
        nodes = pipeline.nodes
        self._logger.info(
            "Actual run would execute %d nodes:\n%s",
            len(nodes),
            pipeline.describe(),
        )
        self._logger.info("Checking inputs...")
        input_names = pipeline.inputs()
        missing_inputs = [
            input_name
            for input_name in input_names
            if not catalog._get_dataset(input_name).exists()
        ]
        if missing_inputs:
            raise KeyError(f"Datasets {missing_inputs} not found.")

And use it with kedro run through the --runner flag:

$ kedro run --runner=<package_name>.runner.DryRunner

Load and save asynchronously

`ThreadRunner` doesn't support asynchronous load-input or save-output operations.

When processing a node, both SequentialRunner and ParallelRunner perform the following steps in order:

  1. Load data based on node input(s)
  2. Execute node function with the input(s)
  3. Save the output(s)

If a node has multiple inputs or outputs (e.g., node(func, ["a", "b", "c"], ["d", "e", "f"])), you can reduce load and save time by using asynchronous mode. You can enable it by passing an --async flag to the run command as follows:

$ kedro run --async
...
2020-03-24 09:20:01,482 - kedro.runner.sequential_runner - INFO - Asynchronous mode is enabled for loading and saving data
...
All the datasets used in the run have to be [thread-safe](https://www.quora.com/What-is-thread-safety-in-Python) in order for asynchronous loading/saving to work properly.

Run a pipeline by name

To run the pipeline by its name, you need to add your new pipeline to the register_pipelines() function in src/<package_name>/pipeline_registry.py:

Click to expand 
def register_pipelines():
    """Register the project's pipelines.

    Returns:
        A mapping from pipeline names to ``Pipeline`` objects.
    """
    pipelines = find_pipelines()
    pipelines["__default__"] = sum(pipelines.values())
    my_pipeline = pipeline(
        [
            # your definition goes here
        ]
    )
    pipelines["my_pipeline"] = my_pipeline
    return pipelines

Then, from the command line, execute the following:

kedro run --pipeline=my_pipeline
If you specify `kedro run` without the `--pipeline` option, it runs the `__default__` pipeline from the dictionary returned by `register_pipelines()`.

Further information about kedro run can be found in the Kedro CLI documentation.

Run pipelines with IO

The above definition of pipelines only applies for non-stateful or "pure" pipelines that do not interact with the outside world. In practice, we would like to interact with APIs, databases, files and other sources of data. By combining IO and pipelines, we can tackle these more complex use cases.

By using DataCatalog from the IO module we are still able to write pure functions that work with our data and outsource file saving and loading to DataCatalog.

Through DataCatalog, we can control where inputs are loaded from, where intermediate variables get persisted and ultimately the location to which output variables are written.

In a simple example, we define a MemoryDataset called xs to store our inputs, save our input list [1, 2, 3] into xs, then instantiate SequentialRunner and call its run method with the pipeline and data catalog instances:

Click to expand 
io = DataCatalog(dict(xs=MemoryDataset()))
io.list()

Output:

Out[10]: ['xs']
io.save("xs", [1, 2, 3])
SequentialRunner().run(pipeline, catalog=io)

Output:

Out[11]: {'v': 0.666666666666667}

Output to a file

We can also use IO to save outputs to a file. In this example, we define a custom LambdaDataset that would serialise the output to a file locally:

Click to expand 
def save(value):
    with open("./data/07_model_output/variance.pickle", "wb") as f:
        pickle.dump(value, f)


def load():
    with open("./data/07_model_output/variance.pickle", "rb") as f:
        return pickle.load(f)


pickler = LambdaDataset(load=load, save=save)
io.add("v", pickler)

It is important to make sure that the data catalog variable name v matches the name v in the pipeline definition.

Next we can confirm that this LambdaDataset behaves correctly:

Click to expand 
io.save("v", 5)
io.load("v")

Ouput:

Out[12]: 5

Finally, let's run the pipeline again now and serialise the output:

Click to expand 
SequentialRunner().run(pipeline, catalog=io)

Ouput:

Out[13]: {}

The output has been persisted to a local file so we don't see it directly, but it can be retrieved from the catalog:

Click to expand 
io.load("v")

Ouput:

Out[14]: 0.666666666666667
try:
    os.remove("./data/07_model_output/variance.pickle")
except FileNotFoundError:
    pass

Configure kedro run arguments

The Kedro CLI documentation lists the available CLI options for kedro run. You can alternatively supply a configuration file that contains the arguments to kedro run.

Here is an example file named config.yml, but you can choose any name for the file:

$ kedro run --config=config.yml

where config.yml is formatted as below (for example):

run:
  tags: tag1, tag2, tag3
  pipeline: pipeline1
  runner: ParallelRunner
  node_names: node1, node2
  env: env1

The syntax for the options is different when you're using the CLI compared to the configuration file. In the CLI you use dashes, for example for kedro run --from-nodes=..., but you have to use an underscore in the configuration file:

run:
  from_nodes: ...

This is because the configuration file gets parsed by Click, a Python package to handle command line interfaces. Click passes the options defined in the configuration file to a Python function. The option names need to match the argument names in that function.

Variable names and arguments in Python may only contain alpha-numeric characters and underscores, so it's not possible to have a dash in the option names when using the configuration file.

If you provide both a configuration file and a CLI option that clashes with the configuration file, the CLI option will take precedence.