private void setLeader(Source leader) {
        assert (validMember(leader));

        if (this.leader != null)
            throw new IllegalArgumentException("There can not be two default providers for the source '" + id + "'");

        this.leader = leader;
    }
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