A Hazelcast developer primarily works with Hazelcast IMDG (In-Memory Data Grid), a software that allows data to be stored in RAM across multiple machines and accessed in real-time. They design, implement, and maintain distributed systems and applications that require high availability, scalability, and performance. They also handle data partitioning, replication, and server-side programming. Moreover, they work with Hazelcast's operational tools for monitoring, management, and data manipulation. Their responsibilities may also include troubleshooting issues, ensuring data consistency, and optimizing system performance. They may also need to collaborate with other developers, architects, and product managers to meet specific project requirements.