Abstract: A heterogeneous database system allows users to interact with multiple databases. The heterogeneous database system uses a middleware that receives client request and processes them using the databases supported in the backend. The heterogeneous database system presents a uniform interface, for example, APIs to access multiple database that may have different architectures or may be provided by different vendors. The databases connected to the middleware in the backend may include relational databases, document databases, key-value stores, and so on. The middleware stores transaction logs that represent transactions across the multiple databases supported. The heterogeneous database system provides scalability as well as backward compatibility by supporting legacy database systems as backend databases while allowing newer database architectures to be added.