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Overview

The Domain layer represents the core business logic and entities of PriceSignal. It contains domain models, value objects, and domain events following Domain-Driven Design (DDD) principles. This layer has no dependencies on other layers and defines the business rules.

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Base Entities

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EventEntity

The foundation for domain events, allowing entities to raise and manage events. 
public abstract class EventEntity 
{
    private readonly List<BaseEvent> _events = new();
    
    [NotMapped]
    public IReadOnlyCollection<BaseEvent> Events => _events.AsReadOnly();
    
    public void AddEvent(BaseEvent @event) { /* ... */ }
    public void RemoveEvent(BaseEvent @event) { /* ... */ }
    public void ClearEvents() { /* ... */ }
}

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BaseEntity

Provides identity tracking for all domain entities. 
public abstract class BaseEntity : EventEntity
{
    public long Id { get; set; }
    public Guid EntityId { get; set; }
}
src/Domain/Models/BaseEntity.cs:5

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BaseAuditableEntity

Extends BaseEntity with audit timestamps for tracking creation, modification, and soft deletion. 
public abstract class BaseAuditableEntity : BaseEntity
{
    public DateTime CreatedAt { get; set; }
    public DateTime? ModifiedAt { get; set; }
    public DateTime? DeletedAt { get; set; }
}
src/Domain/Models/BaseAuditableEntity.cs:3

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Core Domain Models

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Exchange

Represents a cryptocurrency exchange (e.g., Binance, Coinbase). 
public class Exchange : BaseAuditableEntity
{
    public required string Name { get; init; }
    public string? Description { get; init; }
    public IList<Instrument> Instruments { get; init; } = new List<Instrument>();
}
src/Domain/Models/Exchanges/Exchange.cs:5

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Instrument

Represents a tradable cryptocurrency pair (e.g., BTC/USDT). 
public class Instrument : BaseAuditableEntity
{
    public required string Symbol { get; init; }
    public required string Name { get; init; }
    public string? Description { get; init; }
    public required string BaseAsset { get; init; } 
    public required string QuoteAsset { get; init; }
    public Exchange Exchange { get; private set; } = default!;
    
    public void SetExchange(Exchange exchange)
    {
        Exchange = exchange;
    }
}
src/Domain/Models/Instruments/Instrument.cs:5

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InstrumentPrice

Captures real-time price data for an instrument. 
public class InstrumentPrice : EventEntity
{
    public required string Symbol { get; init; }
    public required decimal Price { get; init; }
    public decimal Volume { get; init; }
    public decimal Quantity { get; init; }
    public DateTimeOffset Timestamp { get; init; }
    public Exchange Exchange { get; private set; } = default!;
    
    public void SetExchange(Exchange exchange)
    {
        Exchange = exchange;
    }
}
src/Domain/Models/Instruments/InstrumentPrice.cs:5

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User

Represents a PriceSignal user with their subscriptions and notification preferences. 
public class User
{
    public string Id { get; set; }
    public string Email { get; set; }
    public string? StripeCustomerId { get; set; }
    public ICollection<Subscription> Subscriptions { get; set; } = new List<Subscription>();
    public ICollection<PriceRule> PriceRules { get; set; } = new List<PriceRule>();
    public ICollection<UserNotificationChannel> NotificationChannels { get; set; } = new List<UserNotificationChannel>();
}
src/Domain/Models/User/User.cs:3

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Price Rules Aggregate

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PriceRule

The core aggregate root for user-defined price alerts and conditions. 
public class PriceRule : BaseAuditableEntity
{
    public required string Name { get; set; }
    public required string Description { get; set; }
    public bool IsEnabled { get; set; }
    public DateTime? LastTriggeredAt { get; set; }
    public decimal? LastTriggeredPrice { get; set; }
    public NotificationChannelType NotificationChannel { get; set; }
    public Instrument Instrument { get; set; }
    public required long InstrumentId { get; set; }
    public ICollection<PriceCondition> Conditions { get; set; } = new List<PriceCondition>();
    public User? User { get; set; }
    public string? UserId { get; init; }
    public ICollection<PriceRuleTriggerLog> ActivationLogs { get; set; } = new List<PriceRuleTriggerLog>();
    
    [NotMapped]
    public bool HasAttempted { get; set; } = false;
    
    public void Trigger(decimal price)
    {
        LastTriggeredPrice = price;
        LastTriggeredAt = DateTime.UtcNow;
        if (Events.Count != 0) return;
        AddEvent(new PriceRuleTriggeredEvent(this));
        ActivationLogs.Add(new PriceRuleTriggerLog(this));
    }
}
src/Domain/Models/PriceRule/PriceRule.cs:8 Key features:
  • Supports multiple conditions that must all be met
  • Tracks trigger history with snapshots
  • Raises domain events when triggered
  • Supports cooldown periods to prevent spam

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PriceCondition

Defines specific conditions for price rule evaluation. 
public class PriceCondition : BaseAuditableEntity
{
    public required string ConditionType { get; set; }
    public decimal Value { get; set; }
    public JsonDocument AdditionalValues { get; set; }
    [JsonIgnore]
    public PriceRule Rule { get; set; }
}

public enum ConditionType
{
    PricePercentage,
    Price,
    PriceAction,
    PriceCrossover,
    TechnicalIndicator,
}
src/Domain/Models/PriceRule/PriceCondition.cs:6 Condition types:
  • Price: Trigger when price crosses a threshold
  • PricePercentage: Trigger on percentage change
  • PriceCrossover: Trigger when price crosses above/below a level
  • TechnicalIndicator: Trigger based on RSI, SMA, EMA

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PriceRuleTriggerLog

Immutable audit log of price rule activations. 
public class PriceRuleTriggerLog : BaseAuditableEntity
{
    public decimal? Price { get; set; }
    public decimal? PriceChange { get; set; }
    public decimal? PriceChangePercentage { get; set; }
    public DateTime TriggeredAt { get; init; }
    public PriceRule PriceRule { get; init; }
    public JsonDocument PriceRuleSnapshot { get; init; }
}
src/Domain/Models/PriceRule/PriceRuleHistory.cs:7

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Domain Events

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BaseEvent

Base class for all domain events, integrated with MediatR. 
public abstract class BaseEvent : INotification
{
}
src/Domain/Models/BaseEvent.cs:5

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PriceRuleTriggeredEvent

Raised when a price rule’s conditions are met. 
public class PriceRuleTriggeredEvent(PriceRule rule) : BaseEvent
{
    public PriceRule Rule { get; set; } = rule;
}
src/Domain/Models/PriceRule/Events/PriceRuleTriggeredEvent.cs:3 This event triggers:
  • Notification delivery to users
  • Trigger log creation
  • Analytics tracking

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Design Patterns

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Aggregate Pattern

PriceRule acts as an aggregate root, ensuring consistency:
  • Controls access to PriceCondition entities
  • Manages its own state transitions
  • Raises domain events at aggregate boundaries

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Rich Domain Model

Entities contain business logic, not just data:
  • PriceRule.Trigger() encapsulates triggering logic
  • Instrument.SetExchange() maintains invariants

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Event Sourcing (Partial)

While not full event sourcing, the system:
  • Captures all state changes as events
  • Maintains immutable trigger logs
  • Uses snapshots for historical queries

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Validation Rules

  • Instruments must have valid base and quote assets
  • Price rules require at least one condition
  • Soft deletes prevent data loss (DeletedAt pattern)
  • EntityId (GUID) used for external references, Id (long) for internal performance

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Next Steps

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