Painless Model Deployment Using PFA
By Brandon Williams
Originally published on Aug 24, 2017
Tags: data science, pfa
Like most companies in predictive analytics, Arena has two groups of experts with differing desires and skill sets: our data scientists constantly design, test, and refine our algorithms that make predictions to improve health care operations our engineers build efficient, scalable infrastructure to productionalize these algorithms, allowing us to score new input data in real-time. Because of these different goals, there is often a disconnect between model training and scoring. In the past, when it came time to move a new algorithm to production, engineers had to manually reimplement it in another system. Reimplementing anything in software is painful — it comes with more bugs, more iteration, new library requirements, and (because multiple teams are involved) more communication. Any tweaks from the data science team would require additional work by engineering. This situation was becoming a hindrance to our productivity, and we knew there had to be a better way.
The solution we found was the Portable Format for Analytics (PFA). Arena has been using PFA as the interface between model training and scoring in our products. One can think of it as a language-independent data format to define modeling workflows, or, JSON for models. The technology is new enough that we couldn’t find others writing about the implementation process or their day-to-day experience working with it. As such, I thought it would be useful to share our experience adopting and working with PFA. So far, it has solved all of our use cases, and we have been happy working with it.
PFA is an easier-to-use, customizable evolution of PMML
The data scientists primarily write algorithms in Python or R. Until last year, if we wanted the resulting models to be used for scoring, they would have to be reimplemented by engineering in the language of our scoring system. Whenever we wanted to tweak the model, the code would have to be modified in two different places. This process was laborious and open to human error, subtle bugs, and implementation differences.
We made it a priority to find a better solution that would
- allow data scientists to investigate, train, test, and deploy different types of models,
- have the ability to run different models for different subsets of users,
- remove the need for engineers having to manually implement models,
- allow data scientists to view all properties of historically deployed models, and
- have easy “roll back” functionality.
Early on, we considered letting data scientists deploy arbitrary Python or R code. However, managing Python/R library dependencies in multiple environments and embedding Python/R models into production applications is painful. (In fact, there are many services that offer to do this for you.)
The Predictive Model Markup Language (PMML) seemed like a promising tool for exchanging model descriptions between different environments, but it did not meet all our needs since customizing and extending models with it is a slow, arduous process.
When we came across PFA, we were excited by the potential of the solution. In many ways, PFA is a nice evolution of PMML: it is easier to use, it supports custom modeling logic, it’s composable, and the user doesn’t have to write JSON.
The team for this project consisted of me (a data scientist) and two engineers.
First, we did a proof of concept to demonstrate that PFA would meet our requirements. We took one of our existing production models and reimplemented it in PFA, feeding the same inputs to both versions of the model. The resulting scores were identical, and the process took less than two days.
Next, we built a new scoring system — we implemented all of our models in PFA and wrote the necessary infrastructure in OCaml. Tools like PrettyPFA made writing PFA simple.
We shadow deployed this system alongside our existing scoring engine and monitored the scores returned by each. After a few months of matching scores, we flipped the switch to start using the new scoring system in production.
We knew that PFA was not going to be an off-the-shelf solution. PFA provides a standard format and a way to evaluate algorithms defined in that format, but we had to build the infrastructure for managing the deployment of models and scoring new data in production. Although the implementation was generally smooth, we ran into a few problems while building this system.
Because PFA is a relatively new technology, it still has a few bugs. One of the main benefits of PFA is that it is supposed to produce the same result from the same input and model definition, even when they are evaluated in different languages. But in one of our models, because of a slight difference between PFA’s static typing for the Python (Titus) and JVM (Hadrian) implementations, Titus was returning a float while Hadrian was returning an integer. For this bug, we submitted an issue on Github and one of the senior maintainers promptly fixed it. Since then, we have run into two additional bugs and have started maintaining our own fork of Hadrian while we wait for our upstream pull requests to be accepted. These types of bugs are expected when using any new software; overall, the experience was better than we had hoped, with quick turnarounds on diagnosing and fixing issues when they arose.
Simpler, faster, and more effective workflow from experimentation to production
Prior to implementing PFA, we struggled to keep up with the innovations of our data science team in production. In large part, this was due to the clunkiness of our old scoring system. Since implementing PFA, we have been able to design a few new algorithms and easily deploy them to production. This allows the data science team to have confidence that any experiments we run will not go to waste — if a new model shows good results, we implement it in PFA. The benefits of this new workflow cannot be overstated.
Furthermore, we are currently developing new products to predict additional client-specific outcomes. Our use of PFA has halved the time that this project has taken. This means that not only are these products easier for our technical teams to implement, but also that our clients save money.
Close cross-team collaboration and personal development
A great side effect of this project is that now both the engineering and data science teams are intimately familiar with our scoring engine. Previously, we each had limited and siloed understandings of the system; now, data scientists have the agency to tweak our production models and deploy new ones without engineering support. Additionally, this project was a good excuse for me to learn OCaml, which has made me a better engineer. Finding opportunities for data scientists and engineers to develop overlapping skills is always worthwhile and facilitates better communication and better products.