In s-Server nomenclature, a source refers to any external location from which data is read. Examples of sources include the file system, Apache Kafka topics, a database (RDBMS), an AMQP message, Amazon Kinesis streams, the Apache Hadoop Distributed File System (HDFS), Apache Hive tables, network sockets, web sockets, and MQTT (Message Queuing Telemetry Transport).

Sources are an essential component of s-Server pipelines. A typical pipeline begins with a source, proceeds through a number of s-Server internal streams and views, and ends in a sink.