Integrating blockchain into an existing IT infrastructure can be challenging, particularly when it comes to ensuring seamless interoperability between decentralized and legacy systems. In a project I worked on, we needed to integrate Ethereum-based smart contracts into an application built on Java, which used PostgreSQL as its database. The main challenges we faced included data inconsistencies, connecting on-chain and off-chain data, security concerns, and managing gas fees. To address these issues, we implemented several solutions: hashing mechanisms to maintain data integrity, off-chain indexing solutions like The Graph to bridge on-chain and off-chain data, and secure key storage solutions such as HashiCorp Vault to enhance security. Additionally, we employed proxy contract patterns and meta-transactions to optimize gas usage. These strategies enabled us to achieve a smooth integration of blockchain technology while maintaining both performance and security.