presto-main/src/main/java/io/prestosql/execution/scheduler/FileBasedNetworkTopology.java/FileBasedNetworkTopology/getTopology
class FileBasedNetworkTopology:    private synchronized Map<String, NetworkLocation> getTopology()
    {
        if (ticker.read() - lastUpdate >= refreshPeriodNanos) {
            try {
                refreshTopology();
            }
            catch (RuntimeException e) {
                log.error(e);
            }
        }
        return topology;
    }