Implement a Real-Time ML Prediction Serving System

Mid November, 2026

# Problem At DoorDash we run hundreds of machine learning models at any given time to power decisions like fraud checks, ETAs, and delivery risk. Models change over time, and the data they need to score a live entity — an order, a delivery, a customer — is scattered and constantly changing. Support and ops tooling needs a way to ask "what does the model say about this entity right now?" and get back a correct, up-to-date prediction — without the caller needing to know which model version that is or where its inputs live. We would like to build a **working end-to-end real time prediction system**. Given an entity (for example, an order), it leverages a feature store to assemble whatever inputs a model needs. It then uses a model registry to select and fetch the correct model to run. Finally, the inference service uses the model and entity features to return a prediction. In practical terms, consider a classifier that predicts whether an order is going to be delivered late as an example scenario. Decide how the order is represented, where its input data comes from, how the right model version gets selected, and how a real-time prediction gets generated and served back. **The system should use real API calls between independently running components and keep running continuously — not just execute once — so we can observe it live.** You decide the component boundaries, the data model, the API shapes, how many models or entity types to support, and how to handle edge cases. **Out of scope: model architecture, training, and the data used to train it.** Use a mock model (a function or a trivial classifier is fine) and mock/fake input data — we are not evaluating model quality or ML sophistication. The interesting part of this exercise is the serving system around the model: getting the right data to the right model and getting a prediction back out. This is unreleased interview content. If you or any tool you're using — including AI assistants — has prior exposure to this exact problem, please let us know before you start. **What we care about:** - A working end-to-end predictive system - Reasonable scope decisions under time pressure - Using AI effectively to accelerate implementation - Components that interact as separate systems **Less important:** model quality, training, model architecture, perfect abstractions, elaborate class hierarchies, low-level polish. Use Python and only the Python standard library. Do not use external services or third-party packages.