Glossary of terminology used in ZenML

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Alerters are a type of stack component that allows you to send messages to chat services (like Slack, Discord, Mattermost, etc.) from within your pipelines. This is useful to immediately get notified when failures happen, for general monitoring/reporting, and also for building human-in-the-loop ML.


Annotators are a stack component that enables the use of data annotation as part of your ZenML stack and pipelines. You can use the associated CLI command to launch annotation, configure your datasets and get stats on how many labeled tasks you have ready for use.


Artifacts are the data that power your experimentation and model training. It is actually steps that produce artifacts, which are then stored in the artifact store.

Artifacts can be serialized and deserialized (i.e. written and read from the Artifact Store) in different ways like TFRecords or saved model pickles, depending on what the step produces. The serialization and deserialization logic of artifacts is defined by Materializers.

Artifact Store

An artifact store is a stack component which is responsible for the storage of artifacts. These artifacts may have been produced by the pipeline steps, or they may be the data first ingested into a pipeline via an ingestion step.


Our command-line tool is your entry point into ZenML. You install this tool and use it to set up and configure your repository to work with ZenML. A single init command serves to get you started, and then you can provision the infrastructure that you wish to work with using the stack register command with the relevant arguments passed in.

Container Registry

A container registry in ZenML is a type of stack component tasked with storing (Docker) containers. A ZenML workflow involving a container registry would see you spinning up a Kubernetes cluster and then deploying a pipeline to be run on Kubeflow Pipelines. As part of the deployment to the cluster, the ZenML base image would be downloaded (from a cloud container registry) and used as the basis for the deployed 'run'. When you are running a local Kubeflow stack, you would therefore have a local container registry which stores the container images you create that bundle up your pipeline code. These images would in turn be built on top of a base image or custom image of your choice.


Pipelines are traditionally represented as DAGs. DAG is an acronym for Directed Acyclic Graph.

  • Directed, because the nodes of the graph (i.e. the steps of a pipeline), have a sequence. Nodes do not exist as freestanding entities in this way.

  • Acyclic, because there must be one (or more) straight paths through the graph from the beginning to the end. It is acyclic because the graph doesn't loop back on itself at any point.

  • Graph, because the steps of the pipeline are represented as nodes in a graph.

ZenML follows this paradigm, and it is a useful mental model to have in your head when thinking about how the pieces of your pipeline get executed and how dependencies between the different stages are managed.

Data Validator

Data Validators are stack components which can help you to ensure and maintain data quality not only in the initial stages of model development, but throughout the entire machine learning project lifecycle.

Experiment Tracker

Experiment trackers are stack components which let you track your ML experiments by logging extended information about your models, datasets, metrics and other parameters and allowing you to browse them, visualize them and compare them between runs.

Feature Store

In ZenML, a feature store as a stack component allows data teams to serve data via an offline store and an online low-latency store where data is kept in sync between the two. It also offers a centralized registry where features (and feature schemas) are stored for use within a team or wider organization.


An integration is a third-party tool or platform that implements a ZenML abstraction. A tool can implement many abstractions and therefore an integration can have different entrypoints for the user. We have a consistently updated integrations page which shows all current integrations supported by the ZenML core team here. However, as ZenML is a framework users are encouraged to use these as a guideline and implement their own integrations by extending the various ZenML abstractions.


A materializer defines how and where Artifacts live in between steps. It is used to convert a ZenML artifact into a specific format. They are most often used to handle the input or output of ZenML steps, and can be extended by building on the BaseMaterializer class. We care about this because steps are not just isolated pieces of work; they are linked together and the outputs of one-step might well be the inputs of the next.

We have some built-in ways to serialize and deserialize the data flowing between steps. Of course, if you are using some library or tool which doesn't work with our built-in options, you can write your own custom materializer to ensure that your data can be passed from step to step in this way. We use our fileio utilities to do the disk operations without needing to be concerned with whether we're operating on a local or cloud machine.

Model Deployer

Model deployers are stack components responsible for serving models on a real-time or batch basis.


An orchestrator manages the running of each step of the pipeline, administering the actual pipeline runs. You can think of it as the 'root' of any pipeline job that you run during your experimentation.


When we think about steps as functions, we know they receive input in the form of artifacts. We also know that they produce output (also in the form of artifacts, stored in the artifact store). But steps also take parameters. The parameters that you pass into the steps are also (helpfully!) stored by ZenML. This helps freeze the iterations of your experimentation workflow in time, so you can return to them exactly as you ran them.


Pipelines are designed as basic Python functions. They are created by using decorators appropriate to the specific use case you have. The moment it is run, a pipeline is compiled and passed directly to the orchestrator, to be run in the orchestrator environment.

Within your repository, you will have one or more pipelines as part of your experimentation workflow. A ZenML pipeline is a sequence of tasks that execute in a specific order and yield artifacts. The artifacts are stored within the artifact store and indexed via the ZenML Server. Each individual task within a pipeline is known as a step.


Every ZenML project starts inside a ZenML repository and, it is at the core of all ZenML activity. Every action that can be executed within ZenML must take place within such a repository. ZenML repositories are denoted by a local .zen folder in your project root where various information about your local configuration lives, e.g., the active Stack that you are using to run pipelines, is stored.

Runner Scripts

A runner script is a Python file, usually called and located at the root of a ZenML repository, which has the code to actually create a pipeline run. The code usually looks like this:

from pipelines.my_pipeline import my_pipeline
from steps.step_1 import step_1

if __name__ == "__main__":
    p = my_pipeline(


A ZenML Secret is a grouping of key-value pairs. These are accessed and administered via the ZenML Secret Manager (a stack component).

Secrets are distinguished by having different schemas. An AWS SecretSchema, for example, has key-value pairs for AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY as well as an optional AWS_SESSION_TOKEN. If you don't specify a schema at the point of registration, ZenML will set the schema as ArbitrarySecretSchema, a kind of default schema where things that aren't attached to a grouping can be stored.

Secrets Manager

Secrets managers as stack components provide a secure way of storing confidential information that is needed to run your ML pipelines. Most production pipelines will run on cloud infrastructure and therefore need credentials to authenticate with those services. Instead of storing these credentials in code or files, ZenML secrets managers can be used to store and retrieve these values in a secure manner.


In ZenML, a Stack represents a set of configurations for your MLOps tools and infrastructure. It is made up of various stack components, two of which are required in each stack:

  • An Artifact Store

  • An Orchestrator


A step is a single piece or stage of a ZenML pipeline. Think of each step as being one of the nodes of the DAG. Steps are responsible for one aspect of processing or interacting with the data / artifacts in the pipeline.

Step Operator

The step operator as a stack component enables the execution of individual pipeline steps in specialized runtime environments that are optimized for certain workloads. These specialized environments can give your steps access to resources like GPUs or distributed processing frameworks like Spark.


A visualizer contains logic to create visualizations within the ZenML ecosystem.

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