@Override
public String saveUnacknowledged(final String channelId, final long version) throws ChannelNotFoundException {
checkNotNull(channelId, "channelId");
checkNotNull(version, "version");
final Channel channel = channels.get(channelId);
if (channel == null) {
throw new ChannelNotFoundException("Could not find channel", channelId);
}
final String uaid = channel.getUAID();
final Set<Ack> newAcks = Collections.newSetFromMap(new ConcurrentHashMap<Ack, Boolean>());
newAcks.add(new AckImpl(channelId, version));
while (true) {
final Set<Ack> currentAcks = unacked.get(uaid);
if (currentAcks == null) {