A MADlib developer works on the MADlib open-source library, designed to provide scalable in-database algorithms for machine learning, statistics, and other data analysis tasks. Their responsibilities include developing, testing, and improving these algorithms. They might also work on creating new features or tools within the library, and ensure that it integrates seamlessly with various database systems. MADlib developers also handle debugging issues, and they collaborate with data scientists, statisticians, and other developers to enhance the library's capabilities. They also document their work, making it easier for others to understand and use MADlib. They require strong skills in SQL, Python, and data analysis.