Kustomer is a SaaS-based customer relationship platform designed to integrate conversations, transactions, and proprietary data in one single system, capturing all aspects of the customer journey. Kustomer's application allows users to combine conversations, custom objects, and track events to provide a complete picture of their customer's activity from the first interaction through the entire journey. The company chose MongoDB as its primary database due to its flexible data model and ease of use with JavaScript-based technologies like Node.js and React.js. With MongoDB, Kustomer is able to store complicated data in a simple way, have independent tenants, and leverage features like WiredTiger storage engine for improved performance and concurrency. Kustomer's application stack includes microservices architecture with MongoDB as the primary database, AWS infrastructure, and services like CircleCI, Amazon ECS, Elasticsearch, and Terraform.