presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/metadata/AssignmentLimiter.java/AssignmentLimiter/start
class AssignmentLimiter:    @PostConstruct
    public void start()
    {
        if (!started.getAndSet(true)) {
            scheduler.scheduleWithFixedDelay(() -> {
                try {
                    clearOnlineNodes();
                }
                catch (Throwable t) {
                    log.error(t, "Error clearing online nodes");
                }
            }, 2, 2, SECONDS);
        }
    }