private void setMode(Request req) {
        dispatchRpcRequest(req, () -> {
            String suppliedMode = req.parameters().get(0).asString();
            String[] s = new String[2];
            try {
                proxyServer.setMode(suppliedMode);
                s[0] = "0";
                s[1] = "success";
            } catch (Exception e) {
                s[0] = "1";
                s[1] = e.getMessage();
            }

            req.returnValues().add(new StringArray(s));
            req.returnRequest();
        });
    }
Returns 0 if server is alive.

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