I have worked on a project that had millions of users which began to implement a microservice architechture over a monolith. It was completely unnecessary since the monolith could easily support a load of a lot more (we did test that).
I stopped working there (partly because they wouldn't hire anyone to work on the monolith) and afaik the monolith is still in production (or at least the majority of it) since the microservices took so long time to develop I guess someone pulled funding.
I have to say my experience with microservices is a bit limited, but every time I have seen someone do it I just think it is too complicated for very little gain. It takes massive resources to develop it and I am not convinced it's the design that makes it scale better.
As a rule, if you will need your transactions spread around different datacenters over the world (AKA, you are Google), you do need microservices. If you don't have those multiple active datacenters, or don't need coherence, you don't need microservices.
If your cluster of 7 database servers is still too slow when you increase it to 9, you probably need to take a look into database optimization, not microservices. If any of those numbers look laughable to you, you should laugh about microservices too.
I stopped working there (partly because they wouldn't hire anyone to work on the monolith) and afaik the monolith is still in production (or at least the majority of it) since the microservices took so long time to develop I guess someone pulled funding.
I have to say my experience with microservices is a bit limited, but every time I have seen someone do it I just think it is too complicated for very little gain. It takes massive resources to develop it and I am not convinced it's the design that makes it scale better.