Building a scalable leaderboard service can be challenging, but using a serverless platform like Momento can simplify the process significantly. Momento, developed by the team behind DynamoDB, offers a serverless cache and real-time data platform that is highly scalable and efficient, charging only for what is used. This guide explains how to create a leaderboard service using Momento's Sorted Set collections, which are ideal for ranking users based on scores, similar to Redis sorted sets but without the infrastructure management overhead. The architecture involves using AWS API Gateway and Lambda functions to handle API routes for submitting scores, retrieving user standings, and listing leaderboards by rank. Key steps include securing API keys with AWS Systems Manager Parameter Store and using the AWS Cloud Development Kit (CDK) to manage infrastructure as code. The guide also highlights the benefits of ephemeral environments for development and testing, leveraging Momento's pay-per-use pricing model. The implementation details emphasize security best practices, such as avoiding storing sensitive data in environment variables and using middleware to manage API key access securely.