/*
 * Copyright (c) 2015 Brocade Communications Systems, Inc. and others.  All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
 * and is available at http://www.eclipse.org/legal/epl-v10.html
 */
package org.opendaylight.controller.cluster.raft;

import akka.actor.ActorRef;
import akka.actor.ActorSelection;
import akka.actor.Cancellable;
import com.google.common.base.Preconditions;
import java.util.LinkedList;
import java.util.Queue;
import java.util.UUID;
import javax.annotation.Nullable;
import org.opendaylight.controller.cluster.raft.base.messages.ApplyState;
import org.opendaylight.controller.cluster.raft.base.messages.SnapshotComplete;
import org.opendaylight.controller.cluster.raft.behaviors.AbstractLeader;
import org.opendaylight.controller.cluster.raft.messages.AddServer;
import org.opendaylight.controller.cluster.raft.messages.AddServerReply;
import org.opendaylight.controller.cluster.raft.messages.RemoveServer;
import org.opendaylight.controller.cluster.raft.messages.RemoveServerReply;
import org.opendaylight.controller.cluster.raft.messages.ServerChangeStatus;
import org.opendaylight.controller.cluster.raft.messages.ServerRemoved;
import org.opendaylight.controller.cluster.raft.messages.UnInitializedFollowerSnapshotReply;
import org.opendaylight.controller.cluster.raft.protobuff.client.messages.Payload;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Handles server configuration related messages for a RaftActor.
 *
 * @author Thomas Pantelis
 */
class RaftActorServerConfigurationSupport {
    private static final Logger LOG = LoggerFactory.getLogger(RaftActorServerConfigurationSupport.class);

    private final OperationState IDLE = new Idle();

    private final RaftActorContext raftContext;

    private final Queue<ServerOperationContext<?>> pendingOperationsQueue = new LinkedList<>();

    private OperationState currentOperationState = IDLE;

    RaftActorServerConfigurationSupport(RaftActorContext context) {
        this.raftContext = context;
    }

    boolean handleMessage(Object message, RaftActor raftActor, ActorRef sender) {
        if(message instanceof AddServer) {
            onAddServer((AddServer) message, raftActor, sender);
            return true;
        } else if(message instanceof RemoveServer) {
            onRemoveServer((RemoveServer) message, raftActor, sender);
            return true;
        } else if (message instanceof ServerOperationTimeout) {
            currentOperationState.onServerOperationTimeout(raftActor, (ServerOperationTimeout) message);
            return true;
        } else if (message instanceof UnInitializedFollowerSnapshotReply) {
            currentOperationState.onUnInitializedFollowerSnapshotReply(raftActor,
                    (UnInitializedFollowerSnapshotReply) message);
            return true;
        } else if(message instanceof ApplyState) {
            return onApplyState((ApplyState) message, raftActor);
        } else if(message instanceof SnapshotComplete) {
            currentOperationState.onSnapshotComplete(raftActor);
            return false;
        } else {
            return false;
        }
    }

    private void onRemoveServer(RemoveServer removeServer, RaftActor raftActor, ActorRef sender) {
        LOG.debug("{}: onRemoveServer: {}, state: {}", raftContext.getId(), removeServer, currentOperationState);
        if(removeServer.getServerId().equals(raftActor.getLeaderId())){
            // Removing current leader is not supported yet
            // TODO: To properly support current leader removal we need to first implement transfer of leadership
            LOG.debug("Cannot remove {} replica because it is the Leader", removeServer.getServerId());
            sender.tell(new RemoveServerReply(ServerChangeStatus.NOT_SUPPORTED, raftActor.getLeaderId()), raftActor.getSelf());
        } else if(!raftContext.getPeerIds().contains(removeServer.getServerId())) {
            sender.tell(new RemoveServerReply(ServerChangeStatus.DOES_NOT_EXIST, raftActor.getLeaderId()), raftActor.getSelf());
        } else {
            onNewOperation(raftActor, new RemoveServerContext(removeServer, raftContext.getPeerAddress(removeServer.getServerId()), sender));
        }
    }

    private boolean onApplyState(ApplyState applyState, RaftActor raftActor) {
        Payload data = applyState.getReplicatedLogEntry().getData();
        if(data instanceof ServerConfigurationPayload) {
            currentOperationState.onApplyState(raftActor, applyState);
            return true;
        }

        return false;
    }

    /**
     * The algorithm for AddServer is as follows:
     * <ul>
     * <li>Add the new server as a peer.</li>
     * <li>Add the new follower to the leader.</li>
     * <li>If new server should be voting member</li>
     * <ul>
     *     <li>Initialize FollowerState to VOTING_NOT_INITIALIZED.</li>
     *     <li>Initiate install snapshot to the new follower.</li>
     *     <li>When install snapshot complete, mark the follower as VOTING and re-calculate majority vote count.</li>
     * </ul>
     * <li>Persist and replicate ServerConfigurationPayload with the new server list.</li>
     * <li>On replication consensus, respond to caller with OK.</li>
     * </ul>
     * If the install snapshot times out after a period of 2 * election time out
     * <ul>
     *     <li>Remove the new server as a peer.</li>
     *     <li>Remove the new follower from the leader.</li>
     *     <li>Respond to caller with TIMEOUT.</li>
     * </ul>
     */
    private void onAddServer(AddServer addServer, RaftActor raftActor, ActorRef sender) {
        LOG.debug("{}: onAddServer: {}, state: {}", raftContext.getId(), addServer, currentOperationState);

        onNewOperation(raftActor, new AddServerContext(addServer, sender));
    }

    private void onNewOperation(RaftActor raftActor, ServerOperationContext<?> operationContext) {
        if (raftActor.isLeader()) {
            currentOperationState.onNewOperation(raftActor, operationContext);
        } else {
            ActorSelection leader = raftActor.getLeader();
            if (leader != null) {
                LOG.debug("{}: Not leader - forwarding to leader {}", raftContext.getId(), leader);
                leader.forward(operationContext.getOperation(), raftActor.getContext());
            } else {
                LOG.debug("{}: No leader - returning NO_LEADER reply", raftContext.getId());
                operationContext.getClientRequestor().tell(operationContext.newReply(
                        ServerChangeStatus.NO_LEADER, null), raftActor.self());
            }
        }
    }

    /**
     * Interface for a server operation FSM state.
     */
    private interface OperationState {
        void onNewOperation(RaftActor raftActor, ServerOperationContext<?> operationContext);

        void onServerOperationTimeout(RaftActor raftActor, ServerOperationTimeout timeout);

        void onUnInitializedFollowerSnapshotReply(RaftActor raftActor, UnInitializedFollowerSnapshotReply reply);

        void onApplyState(RaftActor raftActor, ApplyState applyState);

        void onSnapshotComplete(RaftActor raftActor);
    }

    /**
     * Interface for the initial state for a server operation.
     */
    private interface InitialOperationState {
        void initiate(RaftActor raftActor);
    }

    /**
     * Abstract base class for a server operation FSM state. Handles common behavior for all states.
     */
    private abstract class AbstractOperationState implements OperationState {
        @Override
        public void onNewOperation(RaftActor raftActor, ServerOperationContext<?> operationContext) {
            // We're currently processing another operation so queue it to be processed later.

            LOG.debug("{}: Server operation already in progress - queueing {}", raftContext.getId(),
                    operationContext.getOperation());

            pendingOperationsQueue.add(operationContext);
        }

        @Override
        public void onServerOperationTimeout(RaftActor raftActor, ServerOperationTimeout timeout) {
            LOG.debug("onServerOperationTimeout should not be called in state {}", this);
        }

        @Override
        public void onUnInitializedFollowerSnapshotReply(RaftActor raftActor, UnInitializedFollowerSnapshotReply reply) {
            LOG.debug("onUnInitializedFollowerSnapshotReply was called in state {}", this);
        }

        @Override
        public void onApplyState(RaftActor raftActor, ApplyState applyState) {
            LOG.debug("onApplyState was called in state {}", this);
        }

        @Override
        public void onSnapshotComplete(RaftActor raftActor) {
        }

        protected void persistNewServerConfiguration(RaftActor raftActor, ServerOperationContext<?> operationContext){
            raftContext.setDynamicServerConfigurationInUse();
            ServerConfigurationPayload payload = raftContext.getPeerServerInfo();
            LOG.debug("{}: New server configuration : {}", raftContext.getId(), payload.getServerConfig());

            raftActor.persistData(operationContext.getClientRequestor(), operationContext.getContextId(), payload);

            currentOperationState = new Persisting(operationContext, newTimer(
                    new ServerOperationTimeout(operationContext.getServerId())));

            sendReply(raftActor, operationContext, ServerChangeStatus.OK);
        }

        protected void operationComplete(RaftActor raftActor, ServerOperationContext<?> operationContext,
                @Nullable ServerChangeStatus replyStatus) {
            if(replyStatus != null) {
                sendReply(raftActor, operationContext, replyStatus);
            }

            operationContext.operationComplete(raftActor, replyStatus);

            currentOperationState = IDLE;

            ServerOperationContext<?> nextOperation = pendingOperationsQueue.poll();
            if(nextOperation != null) {
                RaftActorServerConfigurationSupport.this.onNewOperation(raftActor, nextOperation);
            }
        }

        protected void sendReply(RaftActor raftActor, ServerOperationContext<?> operationContext,
                ServerChangeStatus status) {
            LOG.debug("{}: Returning {} for operation {}", raftContext.getId(), status, operationContext.getOperation());

            operationContext.getClientRequestor().tell(operationContext.newReply(status, raftActor.getLeaderId()),
                    raftActor.self());
        }

        Cancellable newTimer(Object message) {
            return raftContext.getActorSystem().scheduler().scheduleOnce(
                    raftContext.getConfigParams().getElectionTimeOutInterval().$times(2), raftContext.getActor(),
                            message, raftContext.getActorSystem().dispatcher(), raftContext.getActor());
        }

        @Override
        public String toString() {
            return getClass().getSimpleName();
        }
    }

    /**
     * The state when no server operation is in progress. It immediately initiates new server operations.
     */
    private class Idle extends AbstractOperationState {
        @Override
        public void onNewOperation(RaftActor raftActor, ServerOperationContext<?> operationContext) {
            operationContext.newInitialOperationState(RaftActorServerConfigurationSupport.this).initiate(raftActor);
        }

        @Override
        public void onApplyState(RaftActor raftActor, ApplyState applyState) {
            // Noop - we override b/c ApplyState is called normally for followers in the idle state.
        }
    }

    /**
     * The state when a new server configuration is being persisted and replicated.
     */
    private class Persisting extends AbstractOperationState {
        private final ServerOperationContext<?> operationContext;
        private final Cancellable timer;
        private boolean timedOut = false;

        Persisting(ServerOperationContext<?> operationContext, Cancellable timer) {
            this.operationContext = operationContext;
            this.timer = timer;
        }

        @Override
        public void onApplyState(RaftActor raftActor, ApplyState applyState) {
            // Sanity check - we could get an ApplyState from a previous operation that timed out so make
            // sure it's meant for us.
            if(operationContext.getContextId().equals(applyState.getIdentifier())) {
                LOG.info("{}: {} has been successfully replicated to a majority of followers", raftActor.getId(),
                        applyState.getReplicatedLogEntry().getData());

                timer.cancel();
                operationComplete(raftActor, operationContext, null);
            }
        }

        @Override
        public void onServerOperationTimeout(RaftActor raftActor, ServerOperationTimeout timeout) {
            LOG.warn("{}: Timeout occured while replicating the new server configuration for {}", raftContext.getId(),
                    timeout.getServerId());

            timedOut = true;

            // Fail any pending operations
            ServerOperationContext<?> nextOperation = pendingOperationsQueue.poll();
            while(nextOperation != null) {
                sendReply(raftActor, nextOperation, ServerChangeStatus.PRIOR_REQUEST_CONSENSUS_TIMEOUT);
                nextOperation = pendingOperationsQueue.poll();
            }
        }

        @Override
        public void onNewOperation(RaftActor raftActor, ServerOperationContext<?> operationContext) {
            if(timedOut) {
                sendReply(raftActor, operationContext, ServerChangeStatus.PRIOR_REQUEST_CONSENSUS_TIMEOUT);
            } else {
                super.onNewOperation(raftActor, operationContext);
            }
        }
    }

    /**
     * Abstract base class for an AddServer operation state.
     */
    private abstract class AddServerState extends AbstractOperationState {
        private final AddServerContext addServerContext;

        AddServerState(AddServerContext addServerContext) {
            this.addServerContext = addServerContext;
        }

        AddServerContext getAddServerContext() {
            return addServerContext;
        }

        Cancellable newInstallSnapshotTimer(RaftActor raftActor) {
            return newTimer(new ServerOperationTimeout(addServerContext.getOperation().getNewServerId()));
        }

        void handleInstallSnapshotTimeout(RaftActor raftActor, ServerOperationTimeout timeout) {
            String serverId = timeout.getServerId();

            LOG.debug("{}: handleInstallSnapshotTimeout for new server {}", raftContext.getId(), serverId);

            // cleanup
            raftContext.removePeer(serverId);

            boolean isLeader = raftActor.isLeader();
            if(isLeader) {
                AbstractLeader leader = (AbstractLeader) raftActor.getCurrentBehavior();
                leader.removeFollower(serverId);
            }

            operationComplete(raftActor, getAddServerContext(),
                    isLeader ? ServerChangeStatus.TIMEOUT : ServerChangeStatus.NO_LEADER);
        }

    }

    /**
     * The initial state for the AddServer operation. It adds the new follower as a peer and initiates
     * snapshot capture, if necessary.
     */
    private class InitialAddServerState extends AddServerState implements InitialOperationState {
        InitialAddServerState(AddServerContext addServerContext) {
            super(addServerContext);
        }

        @Override
        public void initiate(RaftActor raftActor) {
            AbstractLeader leader = (AbstractLeader) raftActor.getCurrentBehavior();
            AddServer addServer = getAddServerContext().getOperation();

            LOG.debug("{}: Initiating {}", raftContext.getId(), addServer);

            if(raftContext.getPeerInfo(addServer.getNewServerId()) != null) {
                operationComplete(raftActor, getAddServerContext(), ServerChangeStatus.ALREADY_EXISTS);
                return;
            }

            VotingState votingState = addServer.isVotingMember() ? VotingState.VOTING_NOT_INITIALIZED :
                    VotingState.NON_VOTING;
            raftContext.addToPeers(addServer.getNewServerId(), addServer.getNewServerAddress(), votingState);

            leader.addFollower(addServer.getNewServerId());

            if(votingState == VotingState.VOTING_NOT_INITIALIZED){
                // schedule the install snapshot timeout timer
                Cancellable installSnapshotTimer = newInstallSnapshotTimer(raftActor);
                if(leader.initiateCaptureSnapshot(addServer.getNewServerId())) {
                    LOG.debug("{}: Initiating capture snapshot for new server {}", raftContext.getId(),
                            addServer.getNewServerId());

                    currentOperationState = new InstallingSnapshot(getAddServerContext(), installSnapshotTimer);
                } else {
                    LOG.debug("{}: Snapshot already in progress - waiting for completion", raftContext.getId());

                    currentOperationState = new WaitingForPriorSnapshotComplete(getAddServerContext(),
                            installSnapshotTimer);
                }
            } else {
                LOG.debug("{}: New follower is non-voting - directly persisting new server configuration",
                        raftContext.getId());

                persistNewServerConfiguration(raftActor, getAddServerContext());
            }
        }
    }

    /**
     * The AddServer operation state for when the catch-up snapshot is being installed. It handles successful
     * reply or timeout.
     */
    private class InstallingSnapshot extends AddServerState {
        private final Cancellable installSnapshotTimer;

        InstallingSnapshot(AddServerContext addServerContext, Cancellable installSnapshotTimer) {
            super(addServerContext);
            this.installSnapshotTimer = Preconditions.checkNotNull(installSnapshotTimer);
        }

        @Override
        public void onServerOperationTimeout(RaftActor raftActor, ServerOperationTimeout timeout) {
            handleInstallSnapshotTimeout(raftActor, timeout);

            LOG.warn("{}: Timeout occured for new server {} while installing snapshot", raftContext.getId(),
                    timeout.getServerId());
        }

        @Override
        public void onUnInitializedFollowerSnapshotReply(RaftActor raftActor, UnInitializedFollowerSnapshotReply reply) {
            LOG.debug("{}: onUnInitializedFollowerSnapshotReply: {}", raftContext.getId(), reply);

            String followerId = reply.getFollowerId();

            // Sanity check to guard against receiving an UnInitializedFollowerSnapshotReply from a prior
            // add server operation that timed out.
            if(getAddServerContext().getOperation().getNewServerId().equals(followerId) && raftActor.isLeader()) {
                AbstractLeader leader = (AbstractLeader) raftActor.getCurrentBehavior();
                raftContext.getPeerInfo(followerId).setVotingState(VotingState.VOTING);
                leader.updateMinReplicaCount();

                persistNewServerConfiguration(raftActor, getAddServerContext());

                installSnapshotTimer.cancel();
            } else {
                LOG.debug("{}: Dropping UnInitializedFollowerSnapshotReply for server {}: {}",
                        raftContext.getId(), followerId,
                        !raftActor.isLeader() ? "not leader" : "server Id doesn't match");
            }
        }
    }

    /**
     * The AddServer operation state for when there is a snapshot already in progress. When the current
     * snapshot completes, it initiates an install snapshot.
     */
    private class WaitingForPriorSnapshotComplete extends AddServerState {
        private final Cancellable snapshotTimer;

        WaitingForPriorSnapshotComplete(AddServerContext addServerContext, Cancellable snapshotTimer) {
            super(addServerContext);
            this.snapshotTimer = Preconditions.checkNotNull(snapshotTimer);
        }

        @Override
        public void onSnapshotComplete(RaftActor raftActor) {
            LOG.debug("{}: onSnapshotComplete", raftContext.getId());

            if(!raftActor.isLeader()) {
                LOG.debug("{}: No longer the leader", raftContext.getId());
                return;
            }

            AbstractLeader leader = (AbstractLeader) raftActor.getCurrentBehavior();
            if(leader.initiateCaptureSnapshot(getAddServerContext().getOperation().getNewServerId())) {
                LOG.debug("{}: Initiating capture snapshot for new server {}", raftContext.getId(),
                        getAddServerContext().getOperation().getNewServerId());

                currentOperationState = new InstallingSnapshot(getAddServerContext(),
                        newInstallSnapshotTimer(raftActor));

                snapshotTimer.cancel();
            }
        }

        @Override
        public void onServerOperationTimeout(RaftActor raftActor, ServerOperationTimeout timeout) {
            handleInstallSnapshotTimeout(raftActor, timeout);

            LOG.warn("{}: Timeout occured for new server {} while waiting for prior snapshot to complete",
                    raftContext.getId(), timeout.getServerId());
        }
    }

    /**
     * Stores context information for a server operation.
     *
     * @param <T> the operation type
     */
    private static abstract class ServerOperationContext<T> {
        private final T operation;
        private final ActorRef clientRequestor;
        private final String contextId;

        ServerOperationContext(T operation, ActorRef clientRequestor){
            this.operation = operation;
            this.clientRequestor = clientRequestor;
            contextId = UUID.randomUUID().toString();
        }

        String getContextId() {
            return contextId;
        }

        T getOperation() {
            return operation;
        }

        ActorRef getClientRequestor() {
            return clientRequestor;
        }

        abstract Object newReply(ServerChangeStatus status, String leaderId);

        abstract InitialOperationState newInitialOperationState(RaftActorServerConfigurationSupport support);

        abstract void operationComplete(RaftActor raftActor, ServerChangeStatus serverChangeStatus);

        abstract String getServerId();
    }

    /**
     * Stores context information for an AddServer operation.
     */
    private static class AddServerContext extends ServerOperationContext<AddServer> {
        AddServerContext(AddServer addServer, ActorRef clientRequestor) {
            super(addServer, clientRequestor);
        }

        @Override
        Object newReply(ServerChangeStatus status, String leaderId) {
            return new AddServerReply(status, leaderId);
        }

        @Override
        InitialOperationState newInitialOperationState(RaftActorServerConfigurationSupport support) {
            return support.new InitialAddServerState(this);
        }

        @Override
        void operationComplete(RaftActor raftActor, ServerChangeStatus serverChangeStatus) {

        }

        @Override
        String getServerId() {
            return getOperation().getNewServerId();
        }
    }

    private abstract class RemoveServerState extends AbstractOperationState {
        private final RemoveServerContext removeServerContext;


        protected RemoveServerState(RemoveServerContext removeServerContext) {
            this.removeServerContext = Preconditions.checkNotNull(removeServerContext);

        }

        public RemoveServerContext getRemoveServerContext() {
            return removeServerContext;
        }
    }

    private class InitialRemoveServerState extends RemoveServerState implements InitialOperationState{

        protected InitialRemoveServerState(RemoveServerContext removeServerContext) {
            super(removeServerContext);
        }

        @Override
        public void initiate(RaftActor raftActor) {
            raftContext.removePeer(getRemoveServerContext().getOperation().getServerId());
            persistNewServerConfiguration(raftActor, getRemoveServerContext());
        }
    }

    private static class RemoveServerContext extends ServerOperationContext<RemoveServer> {
        private final String peerAddress;

        RemoveServerContext(RemoveServer operation, String peerAddress, ActorRef clientRequestor) {
            super(operation, clientRequestor);
            this.peerAddress = peerAddress;
        }

        @Override
        Object newReply(ServerChangeStatus status, String leaderId) {
            return new RemoveServerReply(status, leaderId);
        }

        @Override
        InitialOperationState newInitialOperationState(RaftActorServerConfigurationSupport support) {
            return support.new InitialRemoveServerState(this);
        }

        @Override
        void operationComplete(RaftActor raftActor, ServerChangeStatus serverChangeStatus) {
            if(peerAddress != null) {
                raftActor.context().actorSelection(peerAddress).tell(new ServerRemoved(getOperation().getServerId()), raftActor.getSelf());
            }
        }

        @Override
        String getServerId() {
            return getOperation().getServerId();
        }
    }

    static class ServerOperationTimeout {
        private final String serverId;

        ServerOperationTimeout(String serverId){
           this.serverId = Preconditions.checkNotNull(serverId, "serverId should not be null");
        }

        String getServerId() {
            return serverId;
        }
    }
}