How to make a distributed data switch?

Question

API

Good day.
Please advise which way to look?
The task is this. There are several remote data systems (for example, offices), but direct access from the Internet to them is not possible for certain reasons. The data is stored in different types of database. It is necessary to organize a single service accessible from the Internet that allows you to request data from remote resources.
At the same time, it is impossible to replicate data to a central resource (otherwise it would be easy).
We see such an implementation: lists of data that are stored on remote databases are published (configured) in the central service. Listener clients work in remote offices (we will develop them ourselves), connecting to the central one and able to receive requests from the central server and transfer data back. That is, all packages go through the central service.
Transmission speed and stability is not important.
Roughly speaking, this is a kind of public data node (switch).
In order not to invent a bicycle, I would like to use the existing solution. Where to dig?

Answer 1

If all databases are relational, see SymmetricDS .

Answer 2

You can use the message queue in any implementation for asynchronous work.