This document describes Cortex, a multi-tenant horizontally scalable Prometheus as a service. It retrieves metrics from applications using Prometheus scrapers, distributes the metrics across ingesters using consistent hashing, stores metrics in DynamoDB with indexes and chunks in S3, and provides a Prometheus compatible query API. The goal is to build a proof of concept quickly to monitor tens of thousands of users sending tens of millions of samples per second in a cost effective and scalable way, reusing Prometheus where possible. There is still work to be done on features like recording rules, alerting, reliability, performance, and code cleanup before it is production ready.