http load balancing. How to keep the data up to date?

Question

Scalability

I can not understand how to build a distributed architecture with constantly up-to-date data for an individual user? Those. I changed something in the profile - it has changed, for other users some desynchronization is acceptable. In such cases, implementation is offered through sticky sessions. But what if the server crashes?
How is it implemented in VK, FB, Twitter? How is changing something in the profile relevant and synchronized instantly (!) For all users?

Answer 1

cluster database