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Paho MQTT Client
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/*******************************************************************************
 * Copyright (c) 2009, 2014 IBM Corp.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v2.0
 * and Eclipse Distribution License v1.0 which accompany this distribution. 
 *
 * The Eclipse Public License is available at 
 *    https://www.eclipse.org/legal/epl-2.0
 * and the Eclipse Distribution License is available at 
 *   https://www.eclipse.org/org/documents/edl-v10.php
 *
 * Contributors:
 *    Dave Locke - initial API and implementation and/or initial documentation
 */
package org.eclipse.paho.mqttv5.common;

/**
 * Represents an object used to pass data to be persisted across the
 * MqttClientPersistence interface.
 * 
 * When data is passed across the interface the header and payload are
 * separated, so that unnecessary message copies may be avoided. For example, if
 * a 10 MB payload was published it would be inefficient to create a byte array
 * a few bytes larger than 10 MB and copy the MQTT message header and payload
 * into a contiguous byte array.
 * 
 * 
 * When the request to persist data is made a separate byte array and offset is
 * passed for the header and payload. Only the data between offset and length
 * need be persisted. So for example, a message to be persisted consists of a
 * header byte array starting at offset 1 and length 4, plus a payload byte
 * array starting at offset 30 and length 40000. There are three ways in which
 * the persistence implementation may return data to the client on recovery:
 * 
 * 
 * It could return the data as it was passed in originally, with the same
 * byte arrays and offsets.
 * It could safely just persist and return the bytes from the offset for the
 * specified length. For example, return a header byte array with offset 0 and
 * length 4, plus a payload byte array with offset 0 and length 40000
 * It could return the header and payload as a contiguous byte array with
 * the header bytes preceeding the payload. The contiguous byte array should be
 * set as the header byte array, with the payload byte array being null. For
 * example, return a single byte array with offset 0 and length 40004. This is
 * useful when recovering from a file where the header and payload could be
 * written as a contiguous stream of bytes.
 * 
 */
public interface MqttPersistable {

	/**
	 * Returns the header bytes in an array. The bytes start at
	 * {@link #getHeaderOffset()} and continue for {@link #getHeaderLength()}.
	 * 
	 * @return the header bytes.
	 * @throws MqttPersistenceException
	 *             if an error occurs getting the Header Bytes
	 */
    byte[] getHeaderBytes() throws MqttPersistenceException;

	/**
	 * Returns the length of the header.
	 * 
	 * @return the header length
	 * @throws MqttPersistenceException
	 *             if an error occurs getting the Header length
	 */
    int getHeaderLength() throws MqttPersistenceException;

	/**
	 * Returns the offset of the header within the byte array returned by
	 * {@link #getHeaderBytes()}.
	 * 
	 * @return the header offset.
	 * @throws MqttPersistenceException
	 *             if an error occurs getting the Header offset
	 * 
	 */
    int getHeaderOffset() throws MqttPersistenceException;

	/**
	 * Returns the payload bytes in an array. The bytes start at
	 * {@link #getPayloadOffset()} and continue for {@link #getPayloadLength()}.
	 * 
	 * @return the payload bytes.
	 * @throws MqttPersistenceException
	 *             if an error occurs getting the Payload Bytes
	 */
    byte[] getPayloadBytes() throws MqttPersistenceException;

	/**
	 * Returns the length of the payload.
	 * 
	 * @return the payload length.
	 * @throws MqttPersistenceException
	 *             if an error occurs getting the Payload length
	 */
    int getPayloadLength() throws MqttPersistenceException;

	/**
	 * Returns the offset of the payload within the byte array returned by
	 * {@link #getPayloadBytes()}.
	 * 
	 * @return the payload offset.
	 * @throws MqttPersistenceException
	 *             if an error occurs getting the Payload Offset
	 * 
	 */
    int getPayloadOffset() throws MqttPersistenceException;
}