Yahoo! JAPAN is one of the largest e-commerce platforms in Japan, and we are constantly working on improving users' shopping experience from attempts to understand their needs and behaviors and applying that knowledge to provide better shopping experience. We have a large number of services and APIs for that, and as the number of requests to them is growing (due to the release of new services, or an increase of calls from numerous microservices) we have to be able to scale accordingly and guarantee sub-second data access.
One of the services with a high demand for being able to process tens of thousands of requests per second and grow in near future is 'Recent purchases.' Querying the service for such purchases provides you the latest n items by particular categories in a specified depth. Originally, the data was stored in RDBMS and reached 500+ million records, and the old DB infrastructure couldn't cope with increasing number of requests and prevented the service adoption by other users. That now was the time to renovate the DB to meet higher demand from users.