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Consumer (Properties) {

  // Subscribe to a list of topics, return immediately. 
  // Throws an exception if the new subscription is not valid with existing subscriptions.
  void subscribe(String...) throws SubscriptionNotValidException;

  // Subscribe to a specified topic partition, return immediately.
  // Throws an exception if the new subscription is not valid with existing subscriptions.
  void subscribe(String, Int) throws SubscriptionNotValidException;

  // Subscribe to a specified topic partition at the given offset, return immediately.
  // Throws an exception if the new subscription is not valid with existing subscriptions.
  void subscribe(String, Int, Long) throws SubscriptionNotValidException;

  // Try to fetch messages from the topic/partitions it subscribed to. 
  // Return whatever messages are available to be fetched or empty after specified timeout.
  List<Record> poll(Long) throws SubscriptionIsEmptyException, ConsumeInitOffsetNotKnownException;

  // Commit latest offsets for all partitions currently consuming from.
  // If the sync flag is set to true, commit call will be sync and blocking.
  // Otherwise the commit call will be async and best-effort.
  void commit(Boolean) throws SubscriptionIsEmptyException; 

  // Commit the specified offset for the specified topic/partition.
  // If the sync flag is set to true, commit call will be sync and blocking.
  // Otherwise the commit call will be async and best-effort.
  // Throws an exception if the partitions specified are not currently consuming from.
  void commit(List[(String, Int, Long)], Boolean) throws InvalidPartitionsToCommitOffsetException;

  // Specify the fetch starting offset for the specified topic/partition.
  void pos(List[(String, Int, Long)])

  // Get the currently consuming partitions.
  // Block wait if the current partitions are not known yet.
  List[(String, Int)] getPartitions() throws SubscriptionIsEmptyException;

  // Get the last committed offsets of the partitions currently consuming.
  // Block wait if the current partitions are not known yet.
  Map[(String, Int), Long] getOffsets() throws SubscriptionIsEmptyException;

  // Get the last committed offsets of the specified topic/partition.
  Long getOffset(String, Int) throws SubscriptionIsEmptyException, InvalidPartitionsToGetOffsetException;
  
  // --------- Call-back Below, not part of API ------------ //

  // Call-back function upon partition de-assignment.
  // Default implementation will commit offset depending on auto.commit config.
  void onPartitionDeassigned(List[(String, Int)]);

  // Call-back function upon partition re-assignment.
  // Default implementation is No-Op.
  void onPartitionAssigned(List[(String, Int)])

  // --------- Optional Call-back Below ------------ //

  // Call-back function upon partition reassignment given the group member list and the subscribed topic partition info.
  // Return the partitions that would like to consume, and need to make sure each partition is covered by just one member in the group.
  List[(String, Int)] partitionsToConsume(List[String], List[TopicPartition]);
}