Passing Custom Data Types through Steps (Materializers)

How to use materializers to pass custom data types through steps
A ZenML pipeline is built in a data-centric way. The outputs and inputs of steps define how steps are connected and the order in which they are executed. Each step should be considered as its very own process that reads and writes its inputs and outputs from and to the Artifact Store. This is where Materializers come into play.
A materializer dictates how a given artifact can be written to and retrieved from the artifact store and also contains all serialization and deserialization logic.
Whenever you pass artifacts as outputs from one pipeline step to other steps as inputs, the corresponding materializer for the respective data type defines how this artifact is first serialized and written to the artifact store, and then deserialized and read in the next step.
For most data types, ZenML already includes built-in materializers that automatically handle artifacts of those data types. For instance, all of the examples from the Steps and Pipelines section were using built-in materializers under the hood to store and load artifacts correctly.
However, if you want to pass custom objects between pipeline steps, such as a PyTorch model that does not inherit from torch.nn.Module, then you need to define a custom Materializer to tell ZenML how to handle this specific data type.

Building a Custom Materializer

Base Implementation

Before we dive into how custom materializers can be built, let us briefly discuss how materializers in general are implemented. In the following, you can see the implementation of the abstract base class BaseMaterializer, which defines the interface of all materializers:
from typing import Type, Any
from zenml.materializers.base_materializer import BaseMaterializerMeta
class BaseMaterializer(metaclass=BaseMaterializerMeta):
"""Base Materializer to realize artifact data."""
def __init__(self, artifact: "BaseArtifact"):
"""Initializes a materializer with the given artifact."""
self.artifact = artifact
def handle_input(self, data_type: Type[Any]) -> Any:
"""Write logic here to handle input of the step function.
data_type: What type the input should be materialized as.
Any object that is to be passed into the relevant artifact in the
# read from self.artifact.uri
def handle_return(self, data: Any) -> None:
"""Write logic here to handle return of the step function.
data: Any object that is specified as an input artifact of the step.
# write `data` to self.artifact.uri

Which Data Type to Handle?

Each materializer has an ASSOCIATED_TYPES attribute that contains a list of data types that this materializer can handle. ZenML uses this information to call the right materializer at the right time. I.e., if a ZenML step returns a pd.DataFrame, ZenML will try to find any materializer that has pd.DataFrame in its ASSOCIATED_TYPES. List the data type of your custom object here to link the materializer to that data type.

What Type of Artifact to Generate

Each materializer also has an ASSOCIATED_ARTIFACT_TYPES attribute, which defines what types of artifacts are being stored.
In most cases, you should choose either DataArtifact or ModelArtifact here. If you are unsure, just use DataArtifact. The exact choice is not too important, as the artifact type is only used as a tag in the visualization tools of some certain integrations like Facets.
You can find a full list of available artifact types in the API Docs.

Where to Store the Artifact

Each materializer has an artifact object. The most important property of an artifact object is the uri. The uri is automatically created by ZenML whenever you run a pipeline and points to the directory of a file system where the artifact is stored (location in the artifact store). This should not be modified.

How to Store and Retrieve the Artifact

The handle_input() and handle_return() methods define the serialization and deserialization of artifacts.
  • handle_input() defines how data is read from the artifact store and deserialized,
  • handle_return() defines how data is serialized and saved to the artifact store.
These methods you will need to overwrite according to how you plan to serialize your objects. E.g., if you have custom PyTorch classes as ASSOCIATED_TYPES, then you might want to use torch.save() and torch.load() here. For example, have a look at the materializer in the Neural Prophet integration.

Using a Custom Materializer

ZenML automatically scans your source code for definitions of materializers and registers them for the corresponding data type, so just having a custom materializer definition in your code is enough to enable the respective data type to be used in your pipelines.
Alternatively, you can also explicitly define which materializer to use for a specific step
def my_first_step(...) -> ...:
Or you can use the configure() method of the step. E.g.:
When there are multiple outputs, a dictionary of type {<OUTPUT_NAME>:<MATERIALIZER_CLASS>} can be supplied to the .configure(output_materializers=...).
Note that .configure(output_materializers=...) only needs to be called for the output of the first step that produced an artifact of a given data type, all downstream steps will use the same materializer by default.

Configuring Materializers at Runtime

As briefly outlined in the Runtime Configuration section, which materializer to use for the output of what step can also be configured within YAML config files.
For each output of your steps, you can define custom materializers to handle the loading and saving. You can configure them like this in the config:
materializer_source: __main__.MyMaterializer # or full source path to materializer class
The name of the output can be found in the function declaration, e.g. my_step() -> Output(a: int, b: float) has a and b as available output names.
Similar to other configuration entries, the materializer name refers to the class name of your materializer, and the file should contain a path to the module where the materializer is defined.

Basic Example

Let's see how materialization works with a basic example. Let's say you have a custom class called MyObject that flows between two steps in a pipeline:
import logging
from zenml.steps import step
from zenml.pipelines import pipeline
class MyObj:
def __init__(self, name: str):
self.name = name
def my_first_step() -> MyObj:
"""Step that returns an object of type MyObj"""
return MyObj("my_object")
def my_second_step(my_obj: MyObj) -> None:
"""Step that logs the input object and returns nothing."""
f"The following object was passed to this step: `{my_obj.name}`"
def first_pipeline(step_1, step_2):
output_1 = step_1()
Running the above without a custom materializer will result in the following error:
zenml.exceptions.StepInterfaceError: Unable to find materializer for output 'output' of type <class '__main__.MyObj'> in step 'step1'. Please make sure to either explicitly set a materializer for step outputs using step.with_return_materializers(...) or registering a default materializer for specific types by subclassing BaseMaterializer and setting its ASSOCIATED_TYPES class variable. For more information, visit https://docs.zenml.io/advanced-guide/pipelines/materializers
The error message basically says that ZenML does not know how to persist the object of type MyObj (how could it? We just created this!). Therefore, we have to create our own materializer. To do this, you can extend the BaseMaterializer by sub-classing it, listing MyObj in ASSOCIATED_TYPES, and overwriting handle_input() and handle_return():
import os
from typing import Type
from zenml.artifacts import DataArtifact
from zenml.io import fileio
from zenml.materializers.base_materializer import BaseMaterializer
class MyMaterializer(BaseMaterializer):
def handle_input(self, data_type: Type[MyObj]) -> MyObj:
"""Read from artifact store"""
with fileio.open(os.path.join(self.artifact.uri, 'data.txt'), 'r') as f:
name = f.read()
return MyObj(name=name)
def handle_return(self, my_obj: MyObj) -> None:
"""Write to artifact store"""
with fileio.open(os.path.join(self.artifact.uri, 'data.txt'), 'w') as f:
Pro-tip: Use the ZenML fileio module to ensure your materialization logic works across artifact stores (local and remote like S3 buckets).
Now ZenML can use this materializer to handle outputs and inputs of your customs object. Edit the pipeline as follows to see this in action:
Due to the typing of the inputs and outputs and the ASSOCIATED_TYPES attribute of the materializer, you won't necessarily have to add .configure(output_materializers=MyMaterializer) to the step. It should automatically be detected. It doesn't hurt to be explicit though.
This will now work as expected and yield the following output:
Creating run for pipeline: `first_pipeline`
Cache enabled for pipeline `first_pipeline`
Using stack `default` to run pipeline `first_pipeline`...
Step `my_first_step` has started.
Step `my_first_step` has finished in 0.081s.
Step `my_second_step` has started.
The following object was passed to this step: `my_object`
Step `my_second_step` has finished in 0.048s.
Pipeline run `first_pipeline-22_Apr_22-10_58_51_135729` has finished in 0.153s.

Code Summary

Code Example for Materializing Custom Objects

Skipping Materialization

Using artifacts directly might have unintended consequences for downstream tasks that rely on materialized artifacts. Only skip materialization if there is no other way to do what you want to do.
While materializers should in most cases be used to control how artifacts are returned and consumed from pipeline steps, you might sometimes need to have a completely non-materialized artifact in a step, e.g., if you need to know the exact path to where your artifact is stored.
A non-materialized artifact is a BaseArtifact (or any of its subclasses) and has a property uri that points to the unique path in the artifact store where the artifact is stored. One can use a non-materialized artifact by specifying it as the type in the step:
from zenml.artifacts import DataArtifact
from zenml.steps import step
def my_step(my_artifact: DataArtifact): # rather than pd.DataFrame
When using artifacts directly, one must be aware of which type they are by looking at the previous step's materializer: if the previous step produces a ModelArtifact then you should specify ModelArtifact in a non-materialized step.
Materializers link pythonic types to artifact types implicitly. E.g., a keras.model or torch.nn.Module are pythonic types that are both linked to ModelArtifact implicitly via their materializers.
You can find a full list of available artifact types in the API Docs.


The following shows an example how non-materialized artifacts can be used in the steps of a pipeline. The pipeline we define will look like this:
s1 -> s3
s2 -> s4
s1 and s2 produce identical artifacts, however s3 consumes materialized artifacts while s4 consumes non-materialized artifacts. s4 can now use the dict_.uri and list_.uri paths directly rather than their materialized counterparts.
from typing import Dict, List
from zenml.artifacts import DataArtifact, ModelArtifact
from zenml.pipelines import pipeline
from zenml.steps import Output, step
def step_1() -> Output(dict_=Dict, list_=List):
return {"some": "data"}, []
def step_2() -> Output(dict_=Dict, list_=List):
return {"some": "data"}, []
def step_3(dict_: Dict, list_: List) -> None:
assert isinstance(dict_, dict)
assert isinstance(list_, list)
def step_4(dict_: DataArtifact, list_: ModelArtifact) -> None:
assert hasattr(dict_, "uri")
assert hasattr(list_, "uri")
def example_pipeline(step_1, step_2, step_3, step_4):
example_pipeline(step_1(), step_2(), step_3(), step_4()).run()