Building scalable dApps with Ethereum, Polygon and Fauna` discusses the architectural pattern for scaling decentralized applications (dApps) using Ethereum as the primary chain, Polygon as a sidechain, and Fauna as an off-chain database. This pattern aims to achieve high throughput and low expenses by minimizing reads from Ethereum and leveraging off-chain index accelerators like The Graph. The architecture involves partitioning dApp data into three components: on-chain data stored directly on Ethereum, off-chain data stored in Fauna's document-relational database, and sidechain data stored on Polygon. Fauna provides fine-grained data security, multi-variable indexing, attribute-based access control, and distributed databases that guarantee consistency, making it an ideal solution for dApps requiring secure and performant storage of sensitive user information and metadata.