What is good cluster software for request distribution?

At the moment I work at a web development company as a full stack developer. We have created a system that many people use. So much so that our server has a hard time with it. Especially the database processes use the most resources.

The entire system runs on a dedicated server and is programmed in the Symfony framework that uses PHP and MySQL (MariaDB). All queries to the database are already optimized, and the tables contain indexes and full-text searches.

The server resources are also expanded in such a way that we cannot get even more speed on the server. We will probably have to move to a cluster. That there is one master server (load balancer) that distributes the requests over multiple child servers.

Of course, this also means that databases are on multiple servers and must be synchronized with each other. And it would be nice if the project is updated with eg ftp, that the project on all child servers is also updated.

TL: DR – What is a suitable cluster software for a cluster server because the load for the current server is too high.

Source: Symfony Questions