The most „application transparent" would be the use of a DRDA connection. This is the same protocol and method that DDM-files use, for database connections between two IBM i systems/partitions.
AFAIK there are products for Microsoft's SQL Server and Oracle available. And of course for DB2. Maybe there are other, that I don't know.
Just search for "DRDA" (Distributed Relational Database Architecture).
HTH
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Daniel Gross
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