Connected.Common/Common/Distributed/DistributedLockProtector.cs

using Connected.Caching;
using Connected.Data.DataProtection;
using Connected.Data.EntityProtection;
using Connected.Entities;
using Connected.Entities.Storage;
using Connected.Middleware;

namespace Common.Distributed;
internal sealed class DistributedLockProtector : MiddlewareComponent, IEntityProtector<DistributedLock>
{
	public DistributedLockProtector(IStorageProvider storage, ICacheContext cache)
	{
		Storage = storage;
		Cache = cache;
	}

	private IStorageProvider Storage { get; }
	private ICacheContext Cache { get; }

	public async Task Invoke(EntityProtectionArgs<DistributedLock> args)
	{
		/*
		 * We don't care for deleted entities.
		 */
		if (args.State == State.Deleted)
			return;
		/*
		 * The most important thing is to prevent duplicate inserts. If the lock
		 * already exists we will reject the transaction. Each entity record can have
		 * only one entry in the distributed lock table.
		 */
		if (args.State == State.New)
		{
			/*
			 * First we'll look into the memory cache.
			 */
			var existing = Cache.Get<DistributedLock>(DistributedLock.EntityKey,
				f => string.Equals(f.Entity, args.Entity.Entity, StringComparison.OrdinalIgnoreCase)
				&& string.Equals(f.EntityId, args.Entity.EntityId, StringComparison.OrdinalIgnoreCase));
			/*
			 * If the cache entry exists and holds a valid lock, we simply reject the transaction.
			 */
			if (existing is not null && existing.Expiration > DateTime.UtcNow)
				throw new InvalidOperationException($"{SR.ValLock} ({args.Entity.Entity}, {args.Entity.EntityId})");
			/*
			 * Entry doesn't exist in the cache. Let's look in the storage. Note that this scenarios is unusual because
			 * locks tend to be short and there are only two possible schenarios that lock exists in the storage but not
			 * in the cache:
			 * - the process has rebooted
			 * - the master has changed. It this case all distributed services are redirected to the new master which must
			 * load record by record from the cache. In this case it's most probably that all locks already expired but we 
			 * must check that anyway
			 */
			var entry = await (from dc in Storage.Open<DistributedLock>()
									 where string.Equals(dc.Entity, args.Entity.Entity, StringComparison.OrdinalIgnoreCase)
									 && string.Equals(dc.EntityId, args.Entity.EntityId, StringComparison.OrdinalIgnoreCase)
									 select dc).AsEntity();
			/*
			 * It exists, we must perform additional checks.
			 */
			if (entry is not null)
			{
				/*
				 * Set it in the cache so it gets deleted by the recycling service.
				 */
				Cache.Set(DistributedLock.EntityKey, entry.Id, entry, TimeSpan.Zero);

				if (entry.Expiration > DateTime.UtcNow)
					throw new InvalidOperationException($"{SR.ValLock} ({args.Entity.Entity}, {args.Entity.EntityId})");
			}
		}
	}
}