The diagram is a simple analogy of a real-life case where data in three databases, each sitting in its own environment and secured from all others by firewalls, need to be replicated to a central server so that outside users will be able to access these data without breaching into all other three secured environments.
All the fancy features of a modern RDBMS possesses, such as replication, log-shipping, database mirroring, are not applicable here to help achieve the goal, simply because the firewall restrictions: only a one-way access (from the three environments to the external server) is allowed (while bi-way access/communication is required by all the features)!
My proposal to this is a low-technological solution based on some old-style tools and ideas:
1) On each of the three servers behind the firewalls create a OS batch to execute a SQL script to back up the database to a local disk path (delete the old file if exists of course);
2) In the same OS batch using xcopy to move the database backup file to the external server at a central directory which contains three sub-directories, one for each secured environment.
3) On the external server using a generic OS batch, with an input parameter to identify which environment the data is from, the backup file will be restored to the database which is a replica of the corresponding database in a secured environment
And this works beautifully:
It meets the security request of one-way access from the secured environments to the external servers only (by pushing files one-way to the external server)
It is automated, too
It is simple to maintain and support
It is reliable
It is easy to re-run if a need raised from the external user.