An Apache Phoenix developer creates, manages, and optimizes SQL-based interfaces for large-scale data applications on top of HBase, a database in the Hadoop ecosystem. They leverage Phoenix's capabilities to provide OLTP and operational analytics for big data. Their tasks include writing Java MapReduce jobs, creating views and indexes, managing transactions, and ensuring data integrity. They handle large datasets, improve query performance, and support real-time data access. Additionally, they troubleshoot and debug issues, ensure system scalability, and maintain data security. They also collaborate with data engineers and analysts to meet data requirements.