/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * Copyright 2015 ScyllaDB
 *
 * Modified by ScyllaDB
 */

package org.apache.cassandra.streaming;

/**
 * Handles the streaming a one or more section of one of more sstables to and
 * from a specific remote node.
 *
 * Both this node and the remote one will create a similar symmetrical
 * StreamSession. A streaming session has the following life-cycle:
 *
 * 1. Connections Initialization
 *
 * (a) A node (the initiator in the following) create a new StreamSession,
 * initialize it (init()) and then start it (start()). Start will create a
 * {@link ConnectionHandler} that will create two connections to the remote node
 * (the follower in the following) with whom to stream and send a StreamInit
 * message. The first connection will be the incoming connection for the
 * initiator, and the second connection will be the outgoing. (b) Upon reception
 * of that StreamInit message, the follower creates its own StreamSession,
 * initialize it if it still does not exist, and attach connecting socket to its
 * ConnectionHandler according to StreamInit message's isForOutgoing flag. (d)
 * When the both incoming and outgoing connections are established,
 * StreamSession calls StreamSession#onInitializationComplete method to start
 * the streaming prepare phase (StreamResultFuture.startStreaming()).
 *
 * 2. Streaming preparation phase
 *
 * (a) This phase is started when the initiator onInitializationComplete()
 * method is called. This method sends a PrepareMessage that includes what
 * files/sections this node will stream to the follower (stored in a
 * StreamTransferTask, each column family has it's own transfer task) and what
 * the follower needs to stream back (StreamReceiveTask, same as above). If the
 * initiator has nothing to receive from the follower, it goes directly to its
 * Streaming phase. Otherwise, it waits for the follower PrepareMessage. (b)
 * Upon reception of the PrepareMessage, the follower records which
 * files/sections it will receive and send back its own PrepareMessage with a
 * summary of the files/sections that will be sent to the initiator (prepare()).
 * After having sent that message, the follower goes to its Streamning phase.
 * (c) When the initiator receives the follower PrepareMessage, it records which
 * files/sections it will receive and then goes to his own Streaming phase.
 *
 * 3. Streaming phase
 *
 * (a) The streaming phase is started by each node (the sender in the follower,
 * but note that each side of the StreamSession may be sender for some of the
 * files) involved by calling startStreamingFiles(). This will sequentially send
 * a FileMessage for each file of each SteamTransferTask. Each FileMessage
 * consists of a FileMessageHeader that indicates which file is coming and then
 * start streaming the content for that file (StreamWriter in
 * FileMessage.serialize()). When a file is fully sent, the fileSent() method is
 * called for that file. If all the files for a StreamTransferTask are sent
 * (StreamTransferTask.complete()), the task is marked complete
 * (taskCompleted()). (b) On the receiving side, a SSTable will be written for
 * the incoming file (StreamReader in FileMessage.deserialize()) and once the
 * FileMessage is fully received, the file will be marked as complete
 * (received()). When all files for the StreamReceiveTask have been received,
 * the sstables are added to the CFS (and 2ndary index are built,
 * StreamReceiveTask.complete()) and the task is marked complete
 * (taskCompleted()) (b) If during the streaming of a particular file an I/O
 * error occurs on the receiving end of a stream (FileMessage.deserialize), the
 * node will retry the file (up to DatabaseDescriptor.getMaxStreamingRetries())
 * by sending a RetryMessage to the sender. On receiving a RetryMessage, the
 * sender simply issue a new FileMessage for that file. (c) When all transfer
 * and receive tasks for a session are complete, the move to the Completion
 * phase (maybeCompleted()).
 *
 * 4. Completion phase
 *
 * (a) When a node has finished all transfer and receive task, it enter the
 * completion phase (maybeCompleted()). If it had already received a
 * CompleteMessage from the other side (it is in the WAIT_COMPLETE state), that
 * session is done is is closed (closeSession()). Otherwise, the node switch to
 * the WAIT_COMPLETE state and send a CompleteMessage to the other side.
 */
public class StreamSession {

    public static enum State {
        INITIALIZED, PREPARING, STREAMING, WAIT_COMPLETE, COMPLETE, FAILED,
    }

}