The website and online store "Eldorado" is about 40 thousand purchases every day. There is probably no need to explain what this means for the company's business.
Historically, the store runs on the Bitrix engine with a huge amount of custom code and add-ons. The storage is a MySQL cluster with four master servers.
A significant number of companies have monolithic applications, and many have to work with them. There are plenty of ways to deal with the monolith, but, unfortunately, few people write about successful ones. I hope that the story of how we prop up our monolith (until we saw it) will be of interest to you.
We are well aware that massive architecture can bring a lot of problems. But it is easy to destroy it, by a simple strong-willed decision, it is impossible: sales are going, the site should work, changes for users should not be radical. Therefore, the transition from a monolith to a set of microservices takes time, which we need to hold out: to ensure the system is operational and its resistance to stress.