In the monolithic world, doordash service only has two layers: a python Django monolithic service as the backend, and the client layer, including the web and mobile clients. The simple architecture worked well when the business and the team were small, as it enabled the product team to move fast. However it doesn’t scale as it increases the overhead maintaining the backend.

The biggest issues Doordash faced in the past few years are reliability and scalability challenges. They are connected: since we can’t scale the server as the business grows, we crash very often. In the worst outage, Doordash lost millions of dollars since it had to pay for the prepared food to the merchant and send extra apology credit to the customer besides the refund.

This is the third article discussing the challenges of implementing high concurrent web applications. In the previous two articles, we mainly focused on the challenge for handling concurrent requests and the two design patterns of handling concurrent requests. In this article, we are going to discuss the challenges in designing highly concurrent business logic.