Fallbacks

Suppose a Book can be fetched either from LocalLibraryBook, RemoteLibraryBook1, or RemoteLibraryBook2 endpoints. Normally we would want to try the local endpoint first.

This is expressed like so:

def needBook(id: String): Resolvable[Book] =
  Source[Book].from(LocalLibraryBook(id)) orElse
  Source[Book].from(RemoteLibraryBook1(id)) orElse
  Source[Book].from(RemoteLibraryBook2(id))

If any of the sources fails with an exception, the next one will be tried.

Note that the endpoints do not have to be of the same data type. For example, RemoteLibraryBook1 may return JSON, while RemoteLibraryBook2 may return XML. In this case it is enough to define two deserialization Rules:

• Rule[JsValue, Resolvable[Book]]
• Rule[xml.Node, Resolvable[Book]]

It is also possible to grab different pieces of what the endpoints return. For example (assuming JSON):

def needBook(id: String): Resolvable[Book] =
  Source[Book].fromPath(RemoteLibraryBook1(id))(_ \ "theBook" \ "entry") orElse
  Source[Book].fromPath(RemoteLibraryBook2(id))(_ \ "data")