private void checkConfigSources() {
        if (configSourceSet == null || configSourceSet.getSources() == null || configSourceSet.getSources().size() == 0)
            throw new IllegalArgumentException("No config sources defined, could not check connection");

        Request req = new Request("ping");
        for (String configSource : configSourceSet.getSources()) {
            Spec spec = new Spec(configSource);
            Target target = supervisor.connect(spec);
            target.invokeSync(req, Duration.ofSeconds(60));
            if (target.isValid())
                return;

            log.log(Level.INFO, "Could not connect to config source at " + spec);
            target.close();
        }
        log.log(Level.INFO, "Could not connect to any config source in set " + configSourceSet +
                ", please make sure config server(s) are running.");
    }
Checks if config sources are available
config-proxy/src/main/java/com/yahoo/vespa/config/proxy/RpcConfigSourceClient.java