Initial KV API concept

Core/Option/CoreContextStorageService.cs

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+#nullable enable
+using System.Linq;
+using System.Threading.Tasks;
+using Kruzya.TelegramBot.Core.Data;
+using Kruzya.TelegramBot.Core.Extensions;
+using Microsoft.EntityFrameworkCore;
+
+namespace Kruzya.TelegramBot.Core.Option;
+
+public class CoreContextStorageService : IOptionStorageAdapter
+{
+    private CoreContext _db;
+    public CoreContextStorageService(CoreContext db)
+    {
+        _db = db;
+    }
+
+    public async Task<byte[]?> LoadValueAsync(IOptionKey key)
+    {
+        var entity = await BuildQueryForKey(key).FirstOrDefaultAsync();
+
+        return entity?.Value;
+    }
+
+    public async Task SaveValueAsync(IOptionKey key, byte[] value)
+    {
+         var entity = await BuildQueryForKey(key).FirstOrDefaultAsync();
+         if (entity == null)
+         {
+             entity = await CreateEntity(key);
+         }
+
+         entity.Value = value;
+         _db.AddOrUpdate(entity);
+    }
+
+    private IQueryable<BotUserValue> BuildQueryForKey(IOptionKey key)
+    {
+        var legacyUserId = key.UserId ?? 0;
+
+        return _db.UserValues.Where(e => e.BotUser.ChatId == key.ChatId && e.BotUser.UserId == legacyUserId
+                                                                        && e.Name == key.Identifier);
+    }
+
+    private async Task<BotUserValue> CreateEntity(IOptionKey key)
+    {
+        var entity = _db.UserValues.Create()!;
+        entity.BotUser = await _db.Users.FindOrCreate(key.ChatId, key.UserId ?? 0);
+        return entity;
+    }
+}

Core/Option/IOptionItem.cs

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+using System.Threading.Tasks;
+
+namespace Kruzya.TelegramBot.Core.Option;
+
+/// <summary>
+/// Allows interact with option related with some chat.
+/// </summary>
+public interface IOptionItem<T>
+{
+    public IOptionKey Key { get; }
+    public T Value { get; set; }
+    public bool IsChanged { get; }
+
+    public Task LoadAsync(IOptionStorageAdapter adapter);
+    public Task SaveAsync(IOptionStorageAdapter adapter);
+}

Core/Option/IOptionKey.cs

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+namespace Kruzya.TelegramBot.Core.Option;
+
+/// <summary>
+/// Explains what 
+/// </summary>
+public interface IOptionKey
+{
+    /// <summary>
+    /// The chat identifier where this key is related.
+    /// </summary>
+    public long ChatId { get; }
+
+    /// <summary>
+    /// The Telegram user identifier. May be NULL if this is a simple option for all chat members.
+    /// </summary>
+    public long? UserId { get; }
+
+    /// <summary>
+    /// The unique option identifier.
+    /// </summary>
+    public string Identifier { get; }
+}

Core/Option/IOptionManagerService.cs

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+namespace Kruzya.TelegramBot.Core.Option;
+
+/// <summary>
+/// Describes what API should be implemented in Option Manager Service.
+/// </summary>
+public interface IOptionManagerService
+{
+    
+}

Core/Option/IOptionStorageAdapter.cs

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+#nullable enable
+
+using System.Threading.Tasks;
+
+namespace Kruzya.TelegramBot.Core.Option;
+
+/// <summary>
+/// Implements an interaction with storage for options.
+/// </summary>
+public interface IOptionStorageAdapter
+{
+    /// <summary>
+    /// Loads a saved value (if exists) from storage.
+    /// </summary>
+    /// <param name="key"></param>
+    /// <returns></returns>
+    public Task<byte[]?> LoadValueAsync(IOptionKey key);
+
+    /// <summary>
+    /// Saves a passed value to storage.
+    /// </summary>
+    /// <param name="key">The unique chat key</param>
+    /// <param name="value">The serialized value for saving</param>
+    /// <returns></returns>
+    public Task SaveValueAsync(IOptionKey key, byte[] value);
+}

Core/Option/OptionItem.cs

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+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Threading.Tasks;
+
+namespace Kruzya.TelegramBot.Core.Option;
+
+public class OptionItem<T> : IOptionItem<T>
+{
+    public IOptionKey Key { get; }
+
+    public T Value
+    {
+        get => _value;
+
+        set
+        {
+            _value = value;
+            _isChanged = true;
+        }
+    }
+
+    public bool IsChanged => _isChanged;
+
+    private T _value;
+    private bool _isChanged;
+
+    public OptionItem(IOptionKey key, T value)
+    {
+        Key = key;
+        Value = value;
+    }
+
+    public async Task LoadAsync(IOptionStorageAdapter adapter)
+    {
+        var savedValue = await adapter.LoadValueAsync(Key);
+        if (savedValue == null)
+        {
+            return;
+        }
+
+        // This property changed when we're loaded default value. Reset here.
+        _isChanged = false;
+        _value = JsonSerializer.Deserialize<T>(new ReadOnlySpan<byte>(savedValue));
+    }
+
+    public async Task SaveAsync(IOptionStorageAdapter adapter)
+    {
+        var data = JsonSerializer.SerializeToUtf8Bytes(Value, new JsonSerializerOptions { WriteIndented = false, 
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull });
+        await adapter.SaveValueAsync(Key, data);
+    }
+}

Core/Option/OptionKey.cs

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+using Telegram.Bot.Types;
+
+namespace Kruzya.TelegramBot.Core.Option;
+
+public class OptionKey : IOptionKey
+{
+    public long ChatId { get; }
+    public long? UserId { get; } = null;
+    public string Identifier { get; }
+
+    public OptionKey(Chat chat, User user, string identifier) : this(chat.Id, user.Id, identifier)
+    {}
+
+    public OptionKey(Chat chat, string identifier) : this(chat.Id, null, identifier)
+    {}
+
+    public OptionKey(long chat, string identifier) : this(chat, null, identifier)
+    {}
+
+    public OptionKey(long chat, long? user, string identifier)
+    {
+        ChatId = chat;
+        UserId = user;
+        Identifier = identifier;
+    }
+}