public JRTConnectionPool updateSources(ConfigSourceSet sourceSet) {
        synchronized (connections) {
            for (JRTConnection conn : connections.values()) {
                conn.getTarget().close();
            }
            connections.clear();
            addSources(sourceSet);
        }
        return this;
    }
Preconditions:
1. the current connection is unhealthy and should not be selected when switching
2. There is more than 1 source.
config/src/main/java/com/yahoo/vespa/config/JRTConnectionPool.java