code: public static <K, N> Tuple2<K, N> deserializeKeyAndNamespace(
  			byte[] serializedKeyAndNamespace,
  			TypeSerializer<K> keySerializer,
  			TypeSerializer<N> namespaceSerializer) throws IOException {
  
  		DataInputDeserializer dis = new DataInputDeserializer(
  				serializedKeyAndNamespace,
  				0,
  				serializedKeyAndNamespace.length);
  
  		try {
  			K key = keySerializer.deserialize(dis);
  			byte magicNumber = dis.readByte();
  			if (magicNumber != MAGIC_NUMBER) {
  				throw new IOException("Unexpected magic number " + magicNumber + ".");
  			}
  			N namespace = namespaceSerializer.deserialize(dis);
  
  			if (dis.available() > 0) {
  				throw new IOException("Unconsumed bytes in the serialized key and namespace.");
  			}
  
  			return new Tuple2<>(key, namespace);
  		} catch (IOException e) {
  			throw new IOException("Unable to deserialize key " +
  				"and namespace. This indicates a mismatch in the key/namespace " +
  				"serializers used by the KvState instance and this access.", e);
  		}
  	}
docs: Deserializes the key and namespace into a {@link Tuple2}.
  
  @param serializedKeyAndNamespace Serialized key and namespace
  @param keySerializer             Serializer for the key
  @param namespaceSerializer       Serializer for the namespace
  @param <K>                       Key type
  @param <N>                       Namespace
  @return Tuple2 holding deserialized key and namespace
  @throws IOException              if the deserialization fails for any reason