IW4M-Admin/Data/Helpers/DataValueCache.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Data.Abstractions;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
using Timer = System.Timers.Timer;

namespace Data.Helpers
{
    public class DataValueCache<TEntityType, TReturnType> : IDataValueCache<TEntityType, TReturnType>
        where TEntityType : class
    {
        private readonly ILogger _logger;
        private readonly IDatabaseContextFactory _contextFactory;

        private readonly ConcurrentDictionary<string, Dictionary<object, CacheState<TReturnType>>> _cacheStates = new();
        private readonly object _defaultKey = new();

        private bool _autoRefresh;
        private const int DefaultExpireMinutes = 15;
        private Timer _timer;

        private class CacheState<TCacheType>
        {
            public string Key { get; set; }
            public DateTime LastRetrieval { get; set; }
            public TimeSpan ExpirationTime { get; set; }
            public Func<DbSet<TEntityType>, CancellationToken, Task<TCacheType>> Getter { get; set; }
            public TCacheType Value { get; set; }
            public bool IsSet { get; set; }

            public bool IsExpired => ExpirationTime != TimeSpan.MaxValue &&
                                     (DateTime.Now - LastRetrieval.Add(ExpirationTime)).TotalSeconds > 0;
        }

        public DataValueCache(ILogger<DataValueCache<TEntityType, TReturnType>> logger,
            IDatabaseContextFactory contextFactory)
        {
            _logger = logger;
            _contextFactory = contextFactory;
        }

        ~DataValueCache()
        {
            _timer?.Stop();
            _timer?.Dispose();
        }

        public void SetCacheItem(Func<DbSet<TEntityType>, CancellationToken, Task<TReturnType>> getter, string key,
            TimeSpan? expirationTime = null, bool autoRefresh = false)
        {
            SetCacheItem(getter, key, null, expirationTime, autoRefresh);
        }

        public void SetCacheItem(Func<DbSet<TEntityType>, CancellationToken, Task<TReturnType>> getter, string key,
            IEnumerable<object> ids = null, TimeSpan? expirationTime = null, bool autoRefresh = false)
        {
            ids ??= new[] { _defaultKey };
            
            if (!_cacheStates.ContainsKey(key))
            {
                _cacheStates.TryAdd(key, new Dictionary<object, CacheState<TReturnType>>());
            }

            foreach (var id in ids)
            {
                var cacheInstance = _cacheStates[key];

                lock (_cacheStates)
                {
                    if (cacheInstance.ContainsKey(id))
                    {
                        continue;
                    }
                }

                var state = new CacheState<TReturnType>
                {
                    Key = key,
                    Getter = getter,
                    ExpirationTime = expirationTime ?? TimeSpan.FromMinutes(DefaultExpireMinutes)
                };

                lock (_cacheStates)
                {
                    cacheInstance.Add(id, state);
                }

                _autoRefresh = autoRefresh;

                if (!_autoRefresh || expirationTime == TimeSpan.MaxValue)
                {
                    return;
                }
                
                _timer = new Timer(state.ExpirationTime.TotalMilliseconds);
                _timer.Elapsed += async (sender, args) => await RunCacheUpdate(state, CancellationToken.None);
                _timer.Start();
            }
        }

        public Task<TReturnType> GetCacheItem(string keyName, CancellationToken cancellationToken = default) =>
            GetCacheItem(keyName, null, cancellationToken);

        public async Task<TReturnType> GetCacheItem(string keyName, object id = null,
            CancellationToken cancellationToken = default)
        {
            if (!_cacheStates.ContainsKey(keyName))
            {
                throw new ArgumentException("No cache found for key {key}", keyName);
            }

            var cacheInstance = _cacheStates[keyName];

            CacheState<TReturnType> state;
            
            lock (_cacheStates)
            {
                state = id is null ? cacheInstance.Values.First() : _cacheStates[keyName][id];
            }

            // when auto refresh is off we want to check the expiration and value
            // when auto refresh is on, we want to only check the value, because it'll be refreshed automatically
            if ((state.IsExpired || !state.IsSet) && !_autoRefresh || _autoRefresh && !state.IsSet)
            {
                await RunCacheUpdate(state, cancellationToken);
            }

            return state.Value;
        }

        private async Task RunCacheUpdate(CacheState<TReturnType> state, CancellationToken token)
        {
            try
            {
                _logger.LogDebug("Running update for {ClassName} {@State}", GetType().Name, state);
                await using var context = _contextFactory.CreateContext(false);
                var set = context.Set<TEntityType>();
                var value = await state.Getter(set, token);
                state.Value = value;
                state.IsSet = true;
                state.LastRetrieval = DateTime.Now;
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Could not get cached value for {Key}", state.Key);
            }
        }
    }
}