A datastream developer designs and implements data streaming systems, which process and analyze large volumes of real-time data. They use tools like Apache Kafka, Flink, or AWS Kinesis to handle real-time data feeds. Their responsibilities involve creating data processing functions, ensuring data reliability, and maintaining data storage systems. They also handle data ingestion, transformation, and delivery, optimize systems for performance and scale, and troubleshoot issues. Additionally, they may work with data scientists and analysts to understand their data needs and create solutions accordingly, thus playing a crucial role in data-driven decision making.