private void listSourceConnections(Request req) {
        dispatchRpcRequest(req, () -> {
            String[] ret = new String[2];
            ret[0] = "Current source: " + proxyServer.getActiveSourceConnection();
            ret[1] = "All sources:\n" + printSourceConnections();
            req.returnValues().add(new StringArray(ret));
            req.returnRequest();
        });
    }
Returns 0 if server is alive.

@param req a Request
config-proxy/src/main/java/com/yahoo/vespa/config/proxy/ConfigProxyRpcServer.java