/// The purpose of this class is to hold several data sources and search
/// through them to find one containing a zone best matching a request.
///
+/// All the data source clients should be for the same class. If you need
+/// to handle multiple classes, you need to create multiple separate containers.
+///
/// This is an abstract base class. It is not expected we would use multiple
/// implementation inside the servers (but it is not forbidden either), we
/// have it to allow easy testing. It is possible to create a mock-up class