public boolean isAvailableMember(Member knownMember, long now) {
        Date memberHeartbeat = knownMember.getStatus()
                .getLastHeartbeatDate();
        return memberHeartbeat != null
                && (now - memberHeartbeat.getTime()) < heartbeatTimeoutMillis;
    }
@return null if the local node isn't active
turms-server-common/src/main/java/im/turms/server/common/infra/cluster/service/discovery/DiscoveryService.java