Steps

Steps are a relatively simple feature to run through a series of executable steps on Argo. They can be more flexible and powerful with parallel steps and when clauses.

Basic Steps usage involves creating a Steps object as a context manager, and referencing templates or initializing multiple Step objects within the context, which are automatically added to the Steps context manager. In Hera, when referencing a single function decorated with @script multiple times under a single Steps context, you must pass in a unique name (the step’s name) for each call.

from hera.workflows import Steps, Workflow, WorkflowsService, script


@script()
def echo(message: str):
    print(message)


with Workflow(
    generate_name="hello-world-",
    entrypoint="steps",
) as w:
    with Steps(name="steps"):
        echo(name="hello", arguments={"message": "Hello world!"})
        echo(name="goodbye", arguments={"message": "Goodbye world!"})

w.create()

Parallel Steps

As the name suggests, Parallel Steps are a way to run multiple steps concurrently. In Hera, this is done by creating a sub-context under a Steps context manager by using the context manager object’s parallel() function. We can still use sequential steps before and after the parallel context!

from hera.workflows import Steps, Workflow, script


@script()
def echo(message: str):
    print(message)


with Workflow(
    generate_name="hello-world-",
    entrypoint="steps",
) as w:
    with Steps(name="steps") as s:
        echo(name="pre-parallel", arguments={"message": "Hello world!"})

        with s.parallel():
            echo(name="parallel-1", arguments={"message": "I'm parallel-1!"})
            echo(name="parallel-2", arguments={"message": "I'm parallel-2!"})
            echo(name="parallel-3", arguments={"message": "I'm parallel-3!"})

        echo(name="post-parallel", arguments={"message": "Goodbye world!"})

w.create()

Remember any parallel steps will run indeterminately within the context, so parallel-1, parallel-2 and parallel-3 could run in any order, but pre-parallel will always run before the parallel steps and post-parallel will run after all the parallel steps have completed.

when Clauses

A when clause specifies the conditions under which the step or task will run. Examples of when clauses can be found throughout the examples, such as the Argo coinflip example.

If we consider features offered by Hera along with what we’ve learned about parameters and parallel steps, we can form a Workflow with identical behaviour to the upstream coinflip, but using only Python scripts and syntactic sugar functions, which makes for more readable and maintainable code!

from hera.workflows import Steps, Workflow, script


@script()
def flip():
    import random

    result = "heads" if random.randint(0, 1) == 0 else "tails"
    print(result)


@script()
def it_was(coin_result):
    print(f"it was {coin_result}")


with Workflow(generate_name="coinflip-", entrypoint="steps") as w:
    with Steps(name="steps") as s:
        f = flip()
        with s.parallel():
            it_was(name="heads", arguments={"coin_result": "heads"}, when=f'{f.result} == "heads"')
            it_was(name="tails", arguments={"coin_result": "tails"}, when=f'{f.result} == "tails"')

Click to see an example Workflow log

coinflip-gfrws-flip-1899249874: heads
coinflip-gfrws-it-was-2809981541: it was heads

For more about when clauses, see the Conditionals page!

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