I've a base web application developed with .net technologies (asp.net) used into our LAN by 30 users simultanousley. From this web application I've developed two verticalization used from online users. In future i expect hundreds users simultanousley.

Our company has different locations. Each site use its own database. The web application needs to retrieve information from all existing databases. Currently there are 3 database, but it's not excluded in the future expansion of new offices.

My question then is: What is the best strategy for a web application to retrieve information from different databases (which have the same schema) whereas the main objective performance data access and high fault tolerance?

There are case studies in the literature that I can take as an example? Do you know some good documents to study? Do you have any tips to implement this task so efficient?

Intuitively I would say that two possible strategy are:

1. perform queries from different sources in real time and aggregate data on the fly;
2. create a repository that contains the union of the entities of interest and perform queries directly on repository;