In Akka Streams, a Source is a set of stream processing steps that has one open output. In Scala, the Source type is defined as follows:
final class Source[Out]
The Out type is the type of the elements the source produces.
It either can be an atomic source or it can comprise any number of internal sources and transformation that are wired together. Some examples of the former case is given from the following code, that shows some of the utility constructor for the Source type.
// Create a source from an Iterable
Source(List(1, 2, 3))
// Create a source from a Future
Source(Future.successful("Hello Streams!"))
// Create a source from a single element
Source.single("only one element")
// an empty source
Source.empty
An example of the latter case is when a Flow is attached to a Source, resulting in a composite source, as in the following example.
val tweets: Source[Tweet] = Source(...)
val filter: Flow[Tweet, Tweet] = Flow[Tweet].filter(t => t.hashtags.contains(hashtag))
val compositeSource: Source[Tweet] = tweets.via(filter)
The via( ) method transforms a source by appending the given processing stages, and it's the glue that enables to build composite sources.