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Create a pipeline with @step-decorated functions

You can create a pipeline by converting Python functions into pipeline steps using the @step decorator, creating dependencies between those functions to create a pipeline graph (or directed acyclic graph (DAG)), and passing the leaf nodes of that graph as a list of steps to the pipeline. The following sections explain this procedure in detail with examples.

Convert a function to a step

To create a step using the @step decorator, annotate the function with @step. The following example shows a @step-decorated function that preprocesses the data.

from sagemaker.workflow.function_step import step

@step
def preprocess(raw_data):
    df = pandas.read_csv(raw_data)
    ...
    return procesed_dataframe
    
step_process_result = preprocess(raw_data)

When you invoke a @step-decorated function, SageMaker returns a DelayedReturn instance instead of running the function. A DelayedReturn instance is a proxy for the actual return of that function. The DelayedReturn instance can be passed to another function as an argument or directly to a pipeline instance as a step. For information about the DelayedReturn class, see sagemaker.workflow.function_step.DelayedReturn.

Create dependencies between the steps

When you create a dependency between two steps, you create a connection between the steps in your pipeline graph. The following sections introduce multiple ways you can create a dependency between your pipeline steps.

Data dependencies through input arguments

Passing in the DelayedReturn output of one function as an input to another function automatically creates a data dependency in the pipeline DAG. In the following example, passing in the DelayedReturn output of the preprocess function to the train function creates a dependency between preprocess and train.

from sagemaker.workflow.function_step import step

@step
def preprocess(raw_data):
    df = pandas.read_csv(raw_data)
    ...
    return procesed_dataframe

@step
def train(training_data):
    ...
    return trained_model

step_process_result = preprocess(raw_data)    
step_train_result = train(step_process_result)

The previous example defines a training function which is decorated with @step. When this function is invoked, it receives the DelayedReturn output of the preprocessing pipeline step as input. Invoking the training function returns another DelayedReturn instance. This instance holds the information about all the previous steps defined in that function (i.e, the preprocess step in this example) which form the pipeline DAG.

In the previous example, the preprocess function returns a single value. For more complex return types like lists or tuples, refer to Limitations.

Define custom dependencies

In the previous example, the train function received the DelayedReturn output of preprocess and created a dependency. If you want to define the dependency explicitly without passing the previous step output, use the add_depends_on function with the step. You can use the get_step() function to retrieve the underlying step from its DelayedReturn instance, and then call add_depends_on_on with the dependency as input. To view the get_step() function definition, see sagemaker.workflow.step_outputs.get_step. The following example shows you how to create a dependency between preprocess and train using get_step() and add_depends_on().

from sagemaker.workflow.step_outputs import get_step

@step
def preprocess(raw_data):
    df = pandas.read_csv(raw_data)
    ...
    processed_data = ..
    return s3.upload(processed_data)

@step
def train():
    training_data = s3.download(....)
    ...
    return trained_model

step_process_result = preprocess(raw_data)    
step_train_result = train()

get_step(step_train_result).add_depends_on([step_process_result])

Pass data to and from a @step-decorated function to a traditional pipeline step

You can create a pipeline that includes a @step-decorated step and a traditional pipeline step and passes data between them. For example, you can use ProcessingStep to process the data and pass its result to the @step-decorated training function. In the following example, a @step-decorated training step references the output of a processing step.

# Define processing step

from sagemaker.sklearn.processing import SKLearnProcessor
from sagemaker.processing import ProcessingInput, ProcessingOutput
from sagemaker.workflow.steps import ProcessingStep

sklearn_processor = SKLearnProcessor(
    framework_version='1.2-1',
    role='arn:aws:iam::123456789012:role/SagemakerExecutionRole',
    instance_type='ml.m5.large',
    instance_count='1',
)

inputs = [
    ProcessingInput(source=input_data, destination="/opt/ml/processing/input"),
]
outputs = [
    ProcessingOutput(output_name="train", source="/opt/ml/processing/train"),
    ProcessingOutput(output_name="validation", source="/opt/ml/processing/validation"),
    ProcessingOutput(output_name="test", source="/opt/ml/processing/test")
]

process_step = ProcessingStep(
    name="MyProcessStep",
    step_args=sklearn_processor.run(inputs=inputs, outputs=outputs,code='preprocessing.py'),
)

# Define a @step-decorated train step which references the 
# output of a processing step

@step
def train(train_data_path, test_data_path):
    ...
    return trained_model
    
step_train_result = train(
   process_step.properties.ProcessingOutputConfig.Outputs["train"].S3Output.S3Uri,
   process_step.properties.ProcessingOutputConfig.Outputs["test"].S3Output.S3Uri,
)

Use ConditionStep with @step-decorated steps

SageMaker Pipelines supports a ConditionStep class which evaluates the results of preceding steps to decide what action to take in the pipeline. You can use ConditionStep with a @step-decorated step as well. To use the output of any @step-decorated step with ConditionStep, enter the output of that step as an argument to ConditionStep. In the following example, the condition step receives the output of the @step-decorated model evaluation step.

# Define steps

@step(name="evaluate")
def evaluate_model():
    # code to evaluate the model
    return {
        "rmse":rmse_value
    }
    
@step(name="register")
def register_model():
    # code to register the model
    ...

# Define ConditionStep

from sagemaker.workflow.condition_step import ConditionStep
from sagemaker.workflow.conditions import ConditionGreaterThanOrEqualTo
from sagemaker.workflow.fail_step import FailStep

conditionally_register = ConditionStep(
    name="conditional_register",
    conditions=[
        ConditionGreaterThanOrEqualTo(
            # Output of the evaluate step must be json serializable
            left=evaluate_model()["rmse"],  # 
            right=5,
        )
    ],
    if_steps=[FailStep(name="Fail", error_message="Model performance is not good enough")],
    else_steps=[register_model()],
)

Define a pipeline using the DelayedReturn output of steps

You define a pipeline the same way whether or not you use a @step decorator. When you pass a DelayedReturn instance to your pipeline, you don't need to pass a full list of steps to build the pipeline. The SDK automatically infers the previous steps based on the dependencies you define. All the previous steps of the Step objects you passed to the pipeline or DelayedReturn objects are included in the pipeline graph. In the following example, the pipeline receives the DelayedReturn object for the train function. SageMaker adds the preprocess step, as a previous step of train, to the pipeline graph.

from sagemaker.workflow.pipeline import Pipeline

pipeline = Pipeline(
    name="<pipeline-name>",
    steps=[step_train_result],
    sagemaker_session=<sagemaker-session>,
)

If there are no data or custom dependencies between the steps and you run multiple steps in parallel, the pipeline graph has more than one leaf node. Pass all of these leaf nodes in a list to the steps argument in your pipeline definition, as shown in the following example:

@step
def process1():
    ...
    return data
    
@step
def process2():
   ...
   return data
   
step_process1_result = process1()
step_process2_result = process2()

pipeline = Pipeline(
    name="<pipeline-name>",
    steps=[step_process1_result, step_process2_result],
    sagemaker_session=sagemaker-session,
)

When the pipeline runs, both steps run in parallel.

You only pass the leaf nodes of the graph to the pipeline because the leaf nodes contain information about all the previous steps defined through data or custom dependencies. When it compiles the pipeline, SageMaker also infers of all of the subsequent steps that form the pipeline graph and adds each of them as a separate step to the pipeline.

Create a pipeline

Create a pipeline by calling pipeline.create(), as shown in the following snippet. For details about create(), see sagemaker.workflow.pipeline.Pipeline.create.

role = "pipeline-role"
pipeline.create(role)

When you call pipeline.create(), SageMaker compiles all of the steps defined as part of the pipeline instance. SageMaker uploads the serialized function, arguments, and all the other step-related artifacts to Amazon S3.

Data resides in the S3 bucket according to the following structure:

s3_root_uri/
    pipeline_name/
        sm_rf_user_ws/
            workspace.zip  # archive of the current working directory (workdir)
        step_name/
            timestamp/
                arguments/                # serialized function arguments
                function/                 # serialized function
                pre_train_dependencies/   # any dependencies and pre_execution scripts provided for the step       
        execution_id/
            step_name/
                results     # returned output from the serialized function including the model

s3_root_uri is defined in the SageMaker config file and applies to the entire pipeline. If undefined, the default SageMaker bucket is used.