An IBM WebSphere eXtremeScale developer is responsible for creating, implementing, and managing applications that require high-speed processing and data access. They work with IBM's eXtremeScale software, a distributed, in-memory data grid that provides transactional data processing and caching functions. The developer designs systems to handle large volumes of transactions, ensuring data consistency and reliability. They optimize system performance, troubleshoot issues, and may also integrate eXtremeScale with other IBM or third-party systems. Their work helps businesses manage and process large amounts of data quickly and efficiently, supporting high-performance, scalable applications.