Connected.Logistics/Logistics.Stock/StockOps.cs

using Connected.Caching;
using Connected.Collections.Queues;
using Connected.Entities;
using Connected.Entities.Storage;
using Connected.Notifications.Events;
using Connected.ServiceModel;
using Connected.Services;
using Connected.Threading;
using Logistics.Stock.Services;
using Logistics.Types.Serials;
using Logistics.Types.WarehouseLocations;

namespace Logistics.Stock;
internal sealed class StockOps
{
	public const string StockQueue = "Stock";
	static StockOps()
	{
		Locker = new();
	}

	private static AsyncLockerSlim Locker { get; }
	/// <summary>
	/// This method ensures that a stock (parent) record exists.
	/// </summary>
	/// <remarks>
	/// The stock record is not created explicitly since this would introduce unnecessary complexity. It is
	/// instead created on the fly when the first request is made. The tricky part is it must be thread safe
	/// so we need an async locker since lock statement does not support async calls.
	/// </remarks>
	private static async Task<IStock> Ensure(IStorageProvider storage, IStockService stock, EntityArgs args)
	{
		/*
		 * First check for existence so we don't need to perform a lock if the record is found.
		 */
		if (await stock.Select(args) is IStock existing)
			return existing;
		/*
		 * Doesn't exist.
		 * Perform an async lock to ensure no one else is trying to insert the item.
		 */
		return await Locker.LockAsync(async () =>
		{
			/*
			 * Read again if two or more threads were competing for the insert. The thing is this
			 * is happening quite frequently even in semi loaded warehouse systems.
			 */
			if (await stock.Select(args) is IStock existing2)
				return existing2;
			/*
			 * Still nothing. We are safe to insert a new stock descriptor. Note that in scalable environments
			 * there is still a possibillity that two requests would made it here but from different processes.
			 * Thus we should have a unique constraint on the entity ensuring only one request will win, all the others
			 * lose. This also means the provider owning the entity must support unique constraints.
			 */
			var entity = await storage.Open<Stock>().Update(args.AsEntity<Stock>(State.New));
			var result = await stock.Select(entity.Id);
			/*
			 * This should not happen anyway but we'll do it for the sake of sompiler warning.
			 */
			if (result is null)
				throw new NullReferenceException(nameof(IStock));

			return result;
		});
	}

	public sealed class Update : ServiceFunction<UpdateStockArgs, long>
	{
		public Update(IStorageProvider storage, ICacheContext cache, IEventService events, IStockService stock,
			ISerialService serials, IQueueService queue, IWarehouseLocationService locations)
		{
			Storage = storage;
			Cache = cache;
			Events = events;
			Stock = stock;
			Serials = serials;
			Queue = queue;
			Locations = locations;
		}

		private IStorageProvider Storage { get; }
		private ICacheContext Cache { get; }
		private IEventService Events { get; }
		private IStockService Stock { get; }
		private ISerialService Serials { get; }
		private IQueueService Queue { get; }
		private IWarehouseLocationService Locations { get; }
		private bool IsLeaf { get; set; }

		protected override async Task<long> OnInvoke()
		{
			/*
			 * We need this info for queueing aggregations.
			 */
			IsLeaf = (await Locations.Select(Arguments.Location)).ItemCount == 0;
			/*
			 * Validators should validate the existence. Serials don't get deleted.
			 */
			if (await Serials.Select(Arguments.Serial) is not ISerial serial)
				return 0;
			/*
			 * Ensure the stock record exists.
			 */
			var stock = await Ensure(Storage, Stock, serial.AsArguments<EntityArgs, long>());
			/*
			 * Now we must check if the stock item exists for the specified serial and
			 * warehouse location. If so we'll only update the quantity.
			 */
			if (await FindExisting(stock) is not StockItem existing)
				return await InsertItem(stock);
			else
				return await UpdateItem(existing);
		}

		private async Task<long> InsertItem(IStock stock)
		{
			return await Locker.LockAsync(async () =>
			{
				/*
				 * Query again if someone overtook us. 
				 */
				if (await FindExisting(stock) is not StockItem existing)
				{
					/*
					 * Still doesn't exist, it's safe to insert it since we are in the locked area.
					 */
					return (await Storage.Open<StockItem>().Update(Arguments.AsEntity<StockItem>(State.New))).Id;
				}
				else
				{
					/*
					 * Indeed, there was a record inserted in the meantime.
					 */
					return await UpdateItem(existing);
				}
			});
		}
		/// <summary>
		/// Performs the update on the existing stock item.
		/// </summary>
		/// <param name="item">The stock item to be updated.</param>
		private async Task<long> UpdateItem(StockItem item)
		{
			await Storage.Open<StockItem>().Update(item, Arguments, async () =>
			{
				await Cache.Remove(StockItem.EntityKey, item.Id);

				return SetState((await Stock.SelectItem(item.Id)) as StockItem);
			},
			async (e) =>
			{
				var quantity = item.Quantity + Arguments.Quantity;

				await Task.CompletedTask;

				return e.Merge(Arguments, State.Default, new { Quantity = quantity });
			});

			return item.Id;
		}

		private async Task<StockItem?> FindExisting(IStock stock)
		{
			var items = await Stock.QueryItems(new QueryStockItemsArgs
			{
				Id = stock.Id,
				Location = Arguments.Location,
				Serial = Arguments.Serial
			});

			if (items.IsEmpty || items[0] is not StockItem existing)
				return null;

			return existing;
		}

		protected override async Task OnCommitted()
		{
			await Cache.Remove(StockItem.EntityKey, Result);
			await Events.Enqueue(this, Stock, nameof(IStockService.Updated), new PrimaryKeyArgs<long> { Id = Result });

			if (IsLeaf)
				await Queue.Enqueue<StockAggregator, PrimaryKeyQueueArgs<long>>(new PrimaryKeyQueueArgs<long> { Id = Result });
		}
	}
}