The production environments of machine learning pipe-lines often have more moving parts than those of traditional software, and the MLOps software to manage them is a broad and fast-moving field with many platforms, projects and tools. The underlying infrastructure may be more complex (Section 11.1), and the combination of data, code and models that makes up the pipeline is certainly more heterogeneous (Section 11.2). In addition to the tools and technologies we need to manage them, we also discuss those that we may use to complement pipelines with the dashboards and reporting capabilities that are common in data science (Section 11.3).
Successfully running a machine learning application in production goes beyond just implementing a pipeline: it involves managing different local and remote compute systems and integrating different pieces of software that communicate with each other through various APIs. Confusingly enough, the literature often refers to both as “systems”, meaning anything that requires configuration, takes some inputs and produces some outputs in response. With such an abstract definition, compute systems, the GitHub organisation that hosts our code, the Amazon AWS EC2 instances that run part of it in the cloud and the Kubernetes cluster than manages the resources of our local systems are all systems. Considering the prominent role hardware plays in a machine learning application (Chapter 2), we find this definition unhelpful because it is too abstract to reason about the architecture and the performance of the application itself (Chapter 5). The same goes for the even-more-abstracted view that “everything is just an API”.
Managing the compute systems and the software in a real-world pipeline either manually or with a few simple scripts (which would qualify as glue code, Section 5.2.3) is often too burdensome: there are too many of them, they follow different conventions (because they are produced by different vendors), they are not backward compatible and their configuration files use different languages and formats. Configuration management is the only possible approach to keep this complexity under control and to ensure that the pipeline is reproducible and auditable.
One of the most widely-used tools for this task is Terraform (HashiCorp 2022b), which defines itself as a tool to achieve “infrastructure as code”. Terraform is essentially an abstraction layer for a wide range of services (HashiCorp 2022c) and platforms including Amazon AWS, Microsoft Azure, GitHub, GitLab and Airflow. Each platform is exposed as a service “provider” that communicates through APIs that we control, effectively decoupling our infrastructure from the APIs of the original service. Terraform takes care of initialising resources in the original service and of configuring them. For instance, we can use it to create remote resources such as an EC2 instance on Amazon AWS, an object storage on Azure or a VM on a local vSphere (VmWare 2022). However, it does not handle the installation or the configuration of operating systems and software packages.
Cloud instances, VMs and development machines based on Vagrant (HashiCorp 2022d) and Packer (HashiCorp 2022a) can be installed and configured using specialised tools such as Ansible (Ansible Project 2022), Puppet (Puppet 2022) and Chef (Progress Software 2022). All three tools provide a complete solution to configuration management: we can define all resources and their configurations as code and store that code in a version control system. They also have modules for testing the configuration management code, for validating changes before applying them to a specific target environment, and for identifying manual modifications or tampering of the configuration files. As a result, they are convenient to integrate and automate in a CI/CD pipeline. Furthermore, Ansible, Puppet and Chef can all be invoked on instances and VMs created by Terraform on their first boot by software like cloud-init (Canonical 2022a). However, they have different learning curves and they require different technical skills to operate. Ansible is written in Python, uses YAML declarative configuration files and has an agentless architecture (that is, it can be run without installing anything on the instances we want to configure). Puppet and Chef use Ruby-based domain-specific languages and have a master-slave architecture (that is, we install “agents” on the instances to configure them).
As for containers, the de facto standard management tool is Kubernetes (The Kubernetes Authors 2022a), an open-source orchestration system originally developed by Google and now maintained by the Cloud Native Computing Foundation (CNCF). Kubeflow (The Kubeflow Authors 2022) extends Kubernetes by integrating it with popular machine learning frameworks like Tensorflow, notebooks like Jupyter and data pipelines like Pachyderm: the result is an integrated platform specifically geared towards managing, developing, deploying and scaling machine learning pipelines. Kubeflow can be deployed on managed Kubernetes services like Amazon EKS (Amazon Web Services 2022a), Azure AKS (Microsoft 2022b) or Google Kubernetes Engine (Google 2022c) as well as on local Kubernetes clusters. The latter, which are admittedly more complex to run, can be set up with CNCF-certified open-source solutions like the Kubernetes Fury Distribution (SIGHUP 2022) and Typhoon (Poseidon Laboratories 2022). Both are based on Terraform and Ansible and integrate with other CNCF components like software-defined networking, monitoring and logging (Section 5.3.6) to facilitate the interoperability between cloud and local deployments.
Machine learning applications can be designed, tested, maintained and delivered in production using integrated MLOps platforms that blend tooling and practices from DevOps (Section 5.3) with data processing (Section 5.3.3), model training and serving (Sections 5.3.4, 5.3.5 and 7.2). This is a very recent trend at the time of this writing, so the label “MLOps platform” (or “Machine Learning Platform”) has been attached to quite a variety of tools. At one end of the spectrum, we have online platforms like AWS Sagemaker (Amazon 2022d), Vertex AI (Google 2022f), Tensorflow Extended (TensorFlow 2022d), Databricks (Databricks 2022) and Neptune (Neptune Labs 2022). At the other, we have more lightweight solutions like Airflow, MLflow and DVC that are built on top of a collection of smaller open-source tools that are not specific to machine learning applications. On top of that, we have established CI/CD platforms such as GitLab that are working on MLOps features (GitLab 2022c) which overlap with those of the platforms above. We expect it will take a few years before MLOps platforms consolidate into a small number of clear categories. In the meantime, we are choosing between tools that are not mature and have different, unclear trade-offs: there certainly is no one-size-fits-all solution at the moment! However, we can safely mention one trade-off: integrated platforms are limiting because they are often opinionated (they make it difficult to support configurations and workflows other than those envisaged by the authors) and because they are opaque (their components are not visible from the outside). Adopting them early in the life of the pipeline may limit our ability to change its architecture at a later time, may prevent us from exploring different configurations to explore their trade-offs, and may limit our ability to develop software engineering skills. In contrast, manually integrating smaller open-source tools gives us more freedom but requires more work and some level of software engineering skills up front.
Solutions based on Kubernetes such as Kubeflow and Polyaxon (Polyaxon 2022) integrate and compose different tools, including Jupyter notebooks; model training (on both CPUs or GPUs) and experiment tracking for TensorFlow and other frameworks; and model serving with different solutions such as TensorFlow Serving (TensorFlow 2022b), SeldonCore (Seldon Technologies 2022) and Kserve (The KServe Authors 2022). Kubeflow focuses on managing machine learning workflows end-to-end, while Polyaxon complements it by providing distributed training, hyperparameter tuning and parallel task execution. Polyaxon can also schedule and manage Kubeflow operators and track metrics, outputs, models and resource usage to compare experiments. If a solution like Kubeflow is over-complicated for managing our pipeline, we can also consider replacing it with Argo Workflow (Argo Project 2022), a simpler orchestrator that can run parallel jobs on a Kubernetes cluster.
The architecture of Kubeflow builds on the same key ideas as Kubernetes, in particular operators and namespaces. In fact, each machine learning library that is supported by Kubeflow (TensorFlow (TensorFlow 2021a), PyTorch (Paszke et al. 2019), etc.) is encapsulated in a Kubernetes operator that can run local and distributed jobs. Pipelines are executed inside separate namespaces: each user can leverage the Kubernetes namespace isolation to prevent others from accessing notebooks, models or inference endpoints without proper authorisation (Section 5.2.2).
Seldon core (Seldon Technologies 2022) and KServe (The KServe Authors 2022) are specialised MLOps frameworks to package, deploy, monitor and manage machine learning models as custom resources on Kubernetes (The Kubernetes Authors 2022a). Both encapsulate models stored in binary artefacts or code wrappers into containers that expose the models’ capabilities via REST/gRPC APIs with auto-generated OpenAPI specification files. Furthermore, both integrate with Prometheus (Prometheus Authors and The Linux Foundation 2022) and Grafana (GrafanaLabs 2022) (for monitoring metrics), with Elasticsearch (Elasticsearch 2022) or Grafana Loki (Grafana Labs 2022) (for logging), and with other tools (for features like detecting data drift and performing progressive deployments, which we discussed in Section 5.2.1 and 7.2). Two other options with a similar architecture are BentoML (BentoML 2022) and MLEM (Iterative 2022d). The former is a Python framework with a simple object-oriented interface for packaging models into containers and creating HTTP(S) services. The latter, which is from the same authors as DVC, stores model metadata as plain text files versioned in a Git repo, which becomes the single source of truth.
Tensorflow Extended (TensorFlow 2022d, also known as TFX) is a platform to host end-to-end machine learning pipelines based on Tensorflow. TFX is designed to run on top of different platforms (Google Cloud via Vertex AI, Amazon AWS) and orchestration frameworks (Apache Airflow, Kubeflow and Apache Beam (The Apache Software Foundation 2022b)), supports distributed processing (with frameworks like Apache Spark), and allows for local model and data exploration using TensorBoard (TensorFlow 2022c) and Jupyter notebooks. The TFX pipeline is highly modular and is structured in different components along the lines of those we discussed in Chapter 5, all tied together by dependencies represented as a DAG. The metadata required for experiment tracking are saved using the ML metadata library (TensorFlow 2022a, also known as MLMD), along with monitoring information and the pipeline’s logs, in a data store that supports relational databases. All this functionality comes at the cost of complexity and lack of flexibility in certain areas: choosing whether to use TFX requires a careful evaluation of our use case before deciding whether to adopt it or not.
Unlike Kubeflow (built around Kubernetes) or TFX (built around Tensorflow), MLflow (Zaharia and The Linux Foundation 2022) is a library-agnostic platform written in Python that can be integrated with any machine learning library through lightweight APIs. The goal of MLflow is to support MLOps by providing four key features:
- a project packaging format built on Conda (Anaconda 2022b) and Docker (Docker 2022a) which guarantees reproducibility and which makes projects easy to share;
- an experiment tracking API to log parameters, code and results together with an interactive user interface to compare models and data across experiments;
- a model packaging format and a set of APIs for deploying models to target platforms such as Docker, Apache Spark and AWS Sagemaker; and
- a model registry with a graphical interface and a set of APIs to work collaboratively on models.
As we mentioned earlier, we can implement machine learning pipelines using general-purpose open-source orchestrators like Airflow and Luigi (Spotify 2022a) or using more integrated tools such as Dagster (Elementl 2022) and Prefect 2.0 (Prefect 2022). Both Dagster and Prefect 2.0 implement pipelines in Python as modules linked in a DAG, and they provide a web interface that makes it easy to visualise pipelines running in production, to monitor their progress and to troubleshoot them. Monitoring is outsourced to Prometheus in both Airflow and Luigi. Pachyderm, unlike Airflow and Luigi, supports unstructured data like videos and images as well as tabular data from data warehouses. Furthermore, it can trigger pipelines automatically based on data changes, version data of any type and scale resources automatically (since it is built on containers and runs on Kubernetes).
We can implement experiment tracking using more lightweight tools than Kubeflow: two examples are MLflow Tracking and DVC (integrated with a CI/CD pipeline such as Gitlab’s or Jenkins), which we discussed in Section 10.1. A related tool is CML (Iterative 2022a), which is developed by the same authors as DVC: an open-source command-line tool that can be easily integrated into any CI/CD pipeline to add auto-generated reports with plots of model metrics in each pull request/merge request. In order to do that, CML monitors changes in the data and automates model training and evaluation as well as the comparison of ML experiments across project iterations. Neptune (Neptune Labs 2022) is also designed specifically for storing and tracking metadata across multiple experiments. It implements the practices we presented in Section 5.3.4: in particular, saving model artefacts in a model registry along with references to the associated data, code, metrics and environment configurations.
The other option we have is using managed cloud platforms such as Sagemaker and Vertex AI. Their strength is the deep integration with Amazon AWS and Google Cloud, respectively, which makes it straightforward to implement progressive delivery techniques, to centralise logging and monitoring, and to train and serve models using GPUs. AWS also offers integrations with Redshift (Amazon 2022a) and with Databricks to access data; Vertex AI does the same with BigQuery (Google 2022a), and supports working with feature stores as well. Both platforms support Jupyter notebooks for interactive exploration, and both support pipelines: Sagemaker via a custom Python library, Vertex AI via Kubeflow and TFX. In addition, Vertex AI allows us to develop machine learning models in Jupyter notebooks, to deploy models saved in object storage buckets, and to upload them to a dedicated model registry. In conclusion, both platforms are chasing each other’s features, and they are very comprehensive: but they can be confusing because of that.
Finally, feature stores are increasing in popularity in MLOps for storing and cataloguing frequently used features and for enabling feature reuse across models, thus reducing coupling and duplication. They are available from open-source tools such as Feast (Feast Authors 2022) and Hopsworks (Hopsworks 2022), Vertex AI and Databricks.
Data visualisation is an essential part of data science and machine learning: it helps explain complex data and makes them understandable by users and domain experts, allowing them to participate in the design and maintenance of the pipeline (Chapter 5). As in Section 11.2, we can choose to implement it with a spectrum of solutions, from low-level libraries for data exploration to more comprehensive visualisation platforms that create interactive dashboards and data reports.
The decade-old Matplotlib (Hunter 2022) library is the most widely adopted Python package for basic data visualisation, followed by its descendant Seaborn (Waskom 2022), which tries to tackle some of the complexity of Matplotlib while producing figures with a more modern look.
At a higher level, we have Plotly (Plotly 2022c), Bokeh (Bokeh 2022) and Altair (Altair 2022) for Python, and the ggplot2 package (Wickham 2022a) for R. These libraries have similar features and aesthetics, and they can create static, animated and interactive visualisation. Plotly, Bokeh and ggplot2 are programmatic; Altair uses the declarative JSON syntax of the Vega-Lite (Satyanarayan et al. 2022) language and a simple set of APIs to implement the “Grammar of Graphics” (Wilkinson 2005), which has inspired the design of ggplot2 as well. Ggplot2 has a Python port called Plotnine (Kibirige 2022) and Altair has an R wrapper (Lyttle, Jeppson, and Altair Developers 2022) based on the reticulate package (Ushey, Allaire, and Tang 2022).
These packages are the foundation upon which more advanced web dashboards like Dash (Plotly 2022b), Bokeh Server and Shiny (Chang et al. 2022) are built. Dash provides interfaces for Jupyter notebooks and for multiple languages such as Python, R and Julia, while Bokeh only supports Python. Both libraries are good starting points for creating dashboards, although Plotly has a faster learning curve. Shiny, on the other hand, is the de facto standard for creating web-based interactive visualisations in R due to its deep integration with RStudio and R Markdown. Other open-source options are Voilà (Voilà Dashboards 2022), Streamlit (Streamlit 2022), and Panel (Holoviz 2022). Voilà can turn Jupyter notebooks into standalone applications and dashboards, which is useful when generating quick data analysis reports. Streamlit and Panel build web dashboards that interact with data by composing widgets, tables and plots from Plotly, Bokeh and Altair, as well as viewable objects and controls. Panel has better support for Jupyter notebooks compared to Streamlit and Voilà.
Applications for visual analytics and business intelligence like Tableau (Tableau Software 2022) and Microsoft PowerBI (Microsoft 2022e) are also suitable for creating dashboards, and are especially useful to management or domain experts who need to create their own dashboards but who may not be as familiar with programming. Tableau can execute Python code on the fly and display its outputs within Tableau visualisations via TabPy (Tableau 2022). PowerBI, on the other hand, does not yet have a complete integration with Python: it only allows reports to be placed within Jupyter notebooks but without a direct connection between the data in the notebook and the PowerBI report.
Finally, we can leverage standard monitoring and reporting tools such as Prometheus (Prometheus Authors and The Linux Foundation 2022) and Grafana (GrafanaLabs 2022) to display metrics related to data, features and models. We discussed in Section 5.3.6 how important it is to monitor every part of the pipeline: this makes it likely that we are already using Prometheus and Grafana to monitor other things, and we may as well use them to track and compare data and models across environments in addition to other metrics. This approach is certainly robust: it uses highly-tested components. However, it requires a significant engineering effort to integrate the training and serving modules with Prometheus and to integrate the dashboards into the server-side infrastructure. We can build a similar setup with a more opinionated approach using the TFX validation module on Google Vertex AI, which implements training-serving skew detection (TensorFlow 2022d), or using Amazon Sagemaker with its Monitor (Amazon 2022b).
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