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/*
* Copyright (c) 2024 Vaughn Nugent
*
* Library: VNLib
* Package: VNLib.Data.Caching.Providers.VNCache
* File: ClusterNodeIndex.cs
*
* ClusterNodeIndex.cs is part of VNLib.Data.Caching.Providers.VNCache which is part of the larger
* VNLib collection of libraries and utilities.
*
* VNLib.Data.Caching.Providers.VNCache 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.
*
* VNLib.Data.Caching.Providers.VNCache 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.Threading;
using System.Text.Json;
using System.Reflection;
using System.Threading.Tasks;
using VNLib.Utils.Logging;
using VNLib.Utils.Resources;
using VNLib.Data.Caching.Extensions;
using VNLib.Data.Caching.Extensions.Clustering;
using VNLib.Plugins.Extensions.Loading.Events;
namespace VNLib.Data.Caching.Providers.VNCache.Clustering
{
internal static class ClusterNodeIndex
{
const string APP_DOMAIN_KEY = "vnlib.data.caching.providers.vncache";
/*
* Safeley determines if an instance of a node index already exists in the app domain
* if so it returns that instance, otherwise it creates a new index instance and stores
* it in the app domain.
*/
public static IClusterNodeIndex CreateIndex(VNCacheClusterManager cluster)
{
/* TEMPORARY:
* Named semaphores are only supported on Windows, which allowed synchronized communication between
* plugins, but this is not supported on Linux. This will be replaced with a more robust solution
* in the future. For now they will just need to be separate instances.
*
* Remember while plugins are in the same app-domain, they do not share an assembly
* load context which means unless the default ALC contains the desired types, types won't unify
* so we have to use "ghetto" features to avoid interprocess communication, in the same process...
*/
if (OperatingSystem.IsWindows())
{
//Create a named semaphore to ensure only one index is created per app domain
using Semaphore sm = new (1, 1, APP_DOMAIN_KEY, out _);
if (!sm.WaitOne(500))
{
throw new TimeoutException("Failed to access the Cluster index shared semaphore");
}
try
{
//Try to get an existing index from the app domain global storage pool
object? remoteIndex = AppDomain.CurrentDomain.GetData(APP_DOMAIN_KEY);
if (remoteIndex == null)
{
//Create a new index and store it in the app domain
IClusterNodeIndex index = new LocalHandler(cluster);
AppDomain.CurrentDomain.SetData(APP_DOMAIN_KEY, index);
return index;
}
else
{
//Use the existing index
return new RemoteHandler(remoteIndex);
}
}
finally
{
sm.Release();
}
}
else
{
return new LocalHandler(cluster);
}
}
/*
* So a bit of explaination.
*
* Plugins don't share types. Each plugin will load this package into its own ALC. Which will
* cause n instances of the cluster indext manager. Which can cause unecessary http traffic
* building the cluster index multiple times. In an attemt to avoid this, I try to share a single
* cluster index instance across all plugins in the same app domain.
*
* To do this a local handler instance is loaded into whichever plugin accuires the named semaphore
* first, and then the instance is stored in the app domain global storage pool. If its found,
* then other plugins will use the remote handler to access the index.
*
* The remote handler, attempts to use reflection to get function delegates and call the local
* handler functions via reflection.
*
* Unless VNLib.Core supports a new way to safley share types across ALCs, this is my solution.
*/
sealed class LocalHandler(VNCacheClusterManager cluster) : IClusterNodeIndex, IIntervalScheduleable
{
private Task _currentUpdate = Task.CompletedTask;
///<inheritdoc/>
public CacheNodeAdvertisment? GetNextNode()
{
//Get all discovered nodes
CacheNodeAdvertisment[] ads = cluster.DiscoveredNodes.GetAllNodes();
//Just get a random node from the collection for now
return ads.Length > 0 ? ads.SelectRandom() : null;
}
///<inheritdoc/>
public Task WaitForDiscoveryAsync(CancellationToken cancellationToken) => _currentUpdate;
///<inheritdoc/>
public Task OnIntervalAsync(ILogProvider log, CancellationToken cancellationToken)
{
//Run discovery operation and update the task
_currentUpdate = cluster.DiscoverNodesAsync(cancellationToken);
return Task.CompletedTask;
}
/*
* Private methods that are called via reflection
* by remote instances of the index
*/
internal string? SerializeNextNode()
{
CacheNodeAdvertisment? nextNode = GetNextNode();
return nextNode == null ? null : JsonSerializer.Serialize(nextNode);
}
}
sealed class RemoteHandler(object RemoteIndex) : IClusterNodeIndex
{
private readonly Func<string?> _remoteSerializer = ManagedLibrary.GetMethod<Func<string?>>(RemoteIndex, nameof(LocalHandler.SerializeNextNode), BindingFlags.NonPublic);
private readonly Func<CancellationToken, Task> _waitTask = ManagedLibrary.GetMethod<Func<CancellationToken, Task>>(RemoteIndex, nameof(WaitForDiscoveryAsync), BindingFlags.Public);
///<inheritdoc/>
public CacheNodeAdvertisment? GetNextNode()
{
//Deserialize the next node from the remote index
string? nexNode = _remoteSerializer();
return nexNode == null ? null : JsonSerializer.Deserialize<CacheNodeAdvertisment>(nexNode);
}
///<inheritdoc/>
public Task WaitForDiscoveryAsync(CancellationToken cancellationToken) => _waitTask(cancellationToken);
}
}
}
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