public void validate() {
        if (leader == null)
            throw new IllegalArgumentException("Missing leader for the source " + getComponentId() +
                                               ". One of the sources must use the attribute id instead of idref.");
    }
A set of sources with the same name, each associated with a different provider, that fills the same role.

@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/container/search/searchchain/SourceGroup.java