/*
* Copyright (c) 2024 Vaughn Nugent
* 
* Library: VNLib
* Package: ObjectCacheServer
* File: CacheNodeReplicationMaanger.cs 
*
* CacheNodeReplicationMaanger.cs is part of ObjectCacheServer which is part 
* of the larger VNLib collection of libraries and utilities.
*
* ObjectCacheServer is free software: you can redistribute it and/or modify 
* it under the terms of the GNU Affero General Public License as 
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* ObjectCacheServer is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program.  If not, see https://www.gnu.org/licenses/.
*/

using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

using VNLib.Plugins;
using VNLib.Utils.Logging;
using VNLib.Data.Caching.Extensions;
using static VNLib.Data.Caching.Constants;
using VNLib.Net.Messaging.FBM;
using VNLib.Net.Messaging.FBM.Client;
using VNLib.Plugins.Extensions.Loading;
using VNLib.Data.Caching.Extensions.Clustering;

namespace VNLib.Data.Caching.ObjectCache.Server.Clustering
{

    /*
     * This class is responsible for replicating the cache with other nodes.
     * 
     * It does this by connecting to other nodes and listening for change events. 
     * When a change event occurs, it takes action against the local cache store, 
     * to keep it consistent with the other nodes.
     * 
     * Change events are only handled first-hand, meaning that events do not 
     * propagate to other nodes, they must be connected individually to each node
     * and listen for changes.
     */

    internal sealed class CacheNodeReplicationMaanger : IAsyncBackgroundWork
    {
        private readonly PluginBase _plugin;
        private readonly ILogProvider _log;
        private readonly FBMClientFactory _clientFactory;
        private readonly ObjectCacheSystemState _sysState;

        private readonly bool _isDebug;

        private int _openConnections;

        public CacheNodeReplicationMaanger(PluginBase plugin)
        {
            _sysState = plugin.GetOrCreateSingleton<ObjectCacheSystemState>();

            //Init fbm config with fixed message size
            FBMClientConfig clientConfig = FBMDataCacheExtensions.GetDefaultConfig(
                _sysState.SharedCacheHeap,
                CacheConstants.MaxSyncMessageSize,
                debugLog: plugin.IsDebug() ? plugin.Log.CreateScope(CacheConstants.LogScopes.ReplicationFbmDebug) : null
            );

            //Init ws fallback factory and client factory
            _clientFactory = new(
                ref clientConfig, 
                new FBMFallbackClientWsFactory(), 
                (int)_sysState.ClusterConfig.MaxPeerConnections
            );

            _plugin = plugin;
            _isDebug = plugin.IsDebug();
            _log = plugin.Log.CreateScope(CacheConstants.LogScopes.RepliactionManager);
        }

        public async Task DoWorkAsync(ILogProvider pluginLog, CancellationToken exitToken)
        {
            _log.Information("Initializing node replication worker");

            try
            {
                while (true)
                {
                    //Get all new peers
                    CacheNodeAdvertisment[] peers = _sysState.PeerDiscovery.GetNewPeers();

                    if (peers.Length == 0 && _isDebug)
                    {
                        _log.Verbose("No new peers to connect to");
                    }

                    //Make sure we don't exceed the max connections
                    if(_openConnections >= _sysState.ClusterConfig.MaxPeerConnections)
                    {
                        if (_isDebug)
                        {
                            _log.Verbose("Max peer connections reached, waiting for a connection to close");
                        }
                    }
                    else
                    {
                        //Connect to each peer as a background task
                        foreach (CacheNodeAdvertisment peer in peers)
                        {
                            _ = _plugin.ObserveWork(() => OnNewPeerDoWorkAsync(peer, _log, exitToken));
                        }
                    }

                    //Wait for a new peers
                    await Task.Delay(10000, exitToken);
                }
            }
            catch (OperationCanceledException)
            {
                //Normal exit
            }
            catch
            {
                _log.Error("Node replication worker exited with an error");
                throw;
            }
            finally
            {

            }

            _log.Information("Node replication worker exited");
        }

        /*
         * This method is called when a new peer has connected (or discovered) to establish a 
         * replication connection.
         */
        private async Task OnNewPeerDoWorkAsync(CacheNodeAdvertisment newPeer, ILogProvider log, CancellationToken exitToken)
        {
            ArgumentNullException.ThrowIfNull(newPeer);
         
            FBMClient client = _clientFactory.CreateClient();

            /*
             * Notify discovery that we will be listening to this peer
             * 
             * This exists so when a new discovery happens, the work loop will produce 
             * the difference of new peers to existing peers, and we can connect to them.
             * Avoiding infinite connections to the same peer.
             */
            _sysState.PeerDiscovery.OnPeerListenerAttached(newPeer);

            Interlocked.Increment(ref _openConnections);

            try
            {
                log.Information("Establishing replication connection to peer {server}...", newPeer.NodeId);

                //Connect to the server
                await client.ConnectToCacheAsync(newPeer, _sysState.NodeConfig, exitToken);

                log.Information("Connected to {server}, starting queue listeners", newPeer.NodeId);

                //Start worker tasks
                List<Task> workerTasks = [];

                for (int i = 0; i < Environment.ProcessorCount; i++)
                {
                    Task workerTask = Task.Run(() => ReplicationWorkerDoWorkAsync(client, log, exitToken), exitToken);

                    workerTasks.Add(workerTask);
                }

                //Wait for sync workers to exit
                await Task.WhenAll(workerTasks);

                log.Debug("All cache worker tasks exited successfully, disconnecting from {server}", newPeer.NodeId);

                //Disconnect client gracefully
                await client.DisconnectAsync(CancellationToken.None);
            }
            catch(FBMServerNegiationException fbm)
            {
                log.Error("Failed to negotiate buffer configuration, check your cache memory configuration. Error:{err}", fbm.Message);
            }
            catch (InvalidResponseException ie)
            {
                //See if the plugin is unloading
                if (!exitToken.IsCancellationRequested)
                {
                    log.Debug("Peer {p} responded with invalid response packet, disconnected. reason\n {reason}", newPeer.NodeId, ie);
                }
                //Disconnect client gracefully
                try
                {
                    await client.DisconnectAsync(CancellationToken.None);
                }
                catch (Exception ex)
                {
                    log.Error(ex);
                }
            }
            catch (OperationCanceledException)
            {
                //Plugin unloading, Try to disconnect 
                try
                {
                    await client.DisconnectAsync(CancellationToken.None);
                }
                catch (Exception ex)
                {
                    log.Error(ex);
                }
            }
            catch (Exception ex)
            {
                //Avoid call stacks unless debug or higher logging levels
                if (log.IsEnabled(LogLevel.Debug))
                {
                    log.Warn("Lost connection to peer {h}\n {m}", newPeer.NodeId, ex);
                }
                else
                {
                    log.Warn("Lost connection to peer {h}\n {m}", newPeer.NodeId, ex.Message);
                }
            }
            finally
            {
                Interlocked.Decrement(ref _openConnections);

                client.Dispose();

                //Notify monitor of disconnect to make it available again later
                _sysState.PeerDiscovery.OnPeerListenerDetatched(newPeer);
            }
        }

        //Wroker task callback method
        private async Task ReplicationWorkerDoWorkAsync(FBMClient client, ILogProvider log, CancellationToken exitToken)
        {
            //Reusable request message
            using FBMRequest request = new(in client.Config);

            WaitForChangeResult changedObject = new();

            //Listen for changes
            while (true)
            {
                //Wait for changes
                await client.WaitForChangeAsync(changedObject, exitToken);

                log.Debug("Object changed {typ} {obj}", changedObject.Status, changedObject.CurrentId);

                switch (changedObject.Status)
                {
                    /*
                     * During a WFC operation, if a NotFound response is received, it 
                     * means a wait queue was not found for the connection, usually meaning
                     * the server does not support replication.
                     */
                    case ResponseCodes.NotFound:
                        log.Error("Server cache not properly configured, worker exiting");
                        return;
                    case "deleted":
                        //Delete the object from the store
                        await _sysState.InternalStore.DeleteItemAsync(changedObject.CurrentId, CancellationToken.None);
                        break;
                    case "modified":
                        //Reload the record from the store
                        await UpdateRecordAsync(client, request, log, changedObject.CurrentId, changedObject.NewId, exitToken);
                        break;
                    default:
                        log.Error("Unknown status {status} received from server", changedObject.Status);
                        break;
                }

                changedObject.Status = null;
                changedObject.CurrentId = null;
                changedObject.NewId = null;

                //Reset request message
                request.Reset();
            }
        }

        private async Task UpdateRecordAsync(FBMClient client, FBMRequest modRequest, ILogProvider log, string objectId, string newId, CancellationToken cancellation)
        {
            //Set action as get/create
            modRequest.WriteHeader(HeaderCommand.Action, Actions.Get);
            //Set session-id header
            modRequest.WriteHeader(ObjectId, string.IsNullOrWhiteSpace(newId) ? objectId : newId);

            //Make request
            using FBMResponse response = await client.SendAsync(modRequest, CacheConstants.Delays.CacheSyncGetItemTimeout, cancellation);

            response.ThrowIfNotSet();

            //Check response code
            string status = response.Headers.First(static s => s.Header == HeaderCommand.Status).Value.ToString();

            if (ResponseCodes.Okay.Equals(status, StringComparison.Ordinal))
            {
                //Update the record
                await _sysState.InternalStore.AddOrUpdateBlobAsync(objectId, newId, static (t) => t.ResponseBody, response, cancellation);
                log.Debug("Updated object {id}", objectId);
            }
            else
            {
                log.Warn("Object {id} was missing on the remote server", objectId);
            }
        }
    }
}