# Introduction to CQRS

Learn what CQRS is, when to use it, and how Svrnty.CQRS implements the pattern.

## What is CQRS?

**CQRS** stands for **Command Query Responsibility Segregation**. It's an architectural pattern that separates read operations (queries) from write operations (commands).

### Traditional Approach

In traditional architectures, the same model handles both reads and writes:

```csharp
// Traditional approach - same service for everything
public class UserService
{
    public void CreateUser(CreateUserDto dto) { /* write */ }
    public void UpdateUser(UpdateUserDto dto) { /* write */ }
    public UserDto GetUser(int id) { /* read */ }
    public List<UserDto> SearchUsers(string criteria) { /* read */ }
}
```

### CQRS Approach

CQRS separates these responsibilities:

```csharp
// Commands (write operations)
public class CreateUserCommandHandler : ICommandHandler<CreateUserCommand, int>
{
    public Task<int> HandleAsync(CreateUserCommand command, CancellationToken cancellationToken)
    {
        // Write logic only
    }
}

// Queries (read operations)
public class GetUserQueryHandler : IQueryHandler<GetUserQuery, UserDto>
{
    public Task<UserDto> HandleAsync(GetUserQuery query, CancellationToken cancellationToken)
    {
        // Read logic only
    }
}
```

## Core Concepts

### Commands

Commands represent **write operations** that change system state.

**Characteristics:**
- ✅ Imperative names (CreateUser, UpdateOrder, DeleteProduct)
- ✅ Contain all data needed for the operation
- ✅ May or may not return a result
- ✅ Can be validated before execution
- ✅ Typically have side effects

**Example:**
```csharp
public record PlaceOrderCommand
{
    public int CustomerId { get; init; }
    public List<OrderItem> Items { get; init; } = new();
    public decimal TotalAmount { get; init; }
}
```

### Queries

Queries represent **read operations** that return data without changing state.

**Characteristics:**
- ✅ Question-based names (GetUser, SearchOrders, FetchProducts)
- ✅ Never modify state
- ✅ Always return data
- ✅ Can be cached
- ✅ Should be idempotent

**Example:**
```csharp
public record GetOrderQuery
{
    public int OrderId { get; init; }
}
```

### Handlers

Handlers contain the actual business logic for commands and queries.

**Command Handler:**
```csharp
public class PlaceOrderCommandHandler : ICommandHandler<PlaceOrderCommand, int>
{
    public async Task<int> HandleAsync(PlaceOrderCommand command, CancellationToken cancellationToken)
    {
        // Validate business rules
        // Save to database
        // Emit events
        // Return order ID
        return orderId;
    }
}
```

**Query Handler:**
```csharp
public class GetOrderQueryHandler : IQueryHandler<GetOrderQuery, OrderDto>
{
    public async Task<OrderDto> HandleAsync(GetOrderQuery query, CancellationToken cancellationToken)
    {
        // Fetch from database
        // Map to DTO
        // Return data
        return orderDto;
    }
}
```

## Why Use CQRS?

### Benefits

1. **Separation of Concerns**
   - Commands focus on business logic and validation
   - Queries focus on data retrieval and formatting
   - Easier to understand and maintain

2. **Scalability**
   - Scale reads and writes independently
   - Optimize databases differently (write DB vs read DB)
   - Use read replicas for queries

3. **Flexibility**
   - Different models for reading and writing
   - Optimize queries without affecting commands
   - Easy to add new queries without changing commands

4. **Security**
   - Fine-grained authorization (per command/query)
   - Easier to audit write operations
   - Clear boundaries for access control

5. **Testing**
   - Handlers are easy to unit test
   - Clear inputs and outputs
   - Mock dependencies easily

6. **Maintainability**
   - Small, focused handlers
   - Single Responsibility Principle
   - Easy to add new features

### Trade-offs

1. **Increased Complexity**
   - More files and classes
   - Learning curve for team
   - Might be overkill for simple CRUD

2. **Consistency Challenges**
   - With separate read/write models, eventual consistency may be required
   - Requires careful design

3. **Code Duplication**
   - Some logic might be repeated
   - More boilerplate code

## When to Use CQRS

### ✅ Good Fit

- **Complex business logic** - Commands with validation, rules, and workflows
- **Different read/write patterns** - Complex queries vs simple writes
- **High scalability needs** - Read-heavy or write-heavy systems
- **Audit requirements** - Need to track all changes
- **Event sourcing** - Natural fit with event-driven architectures
- **Microservices** - Clear boundaries between services

### ❌ Not Recommended

- **Simple CRUD** - Basic create/read/update/delete operations
- **Small applications** - Overhead not justified
- **Tight deadlines** - Team not familiar with pattern
- **Consistent data models** - Same model for reads and writes

## How Svrnty.CQRS Works

Svrnty.CQRS provides a lightweight, production-ready implementation:

### 1. Define Commands and Queries

```csharp
// Just POCOs (Plain Old CLR Objects)
public record CreateProductCommand
{
    public string Name { get; init; } = string.Empty;
    public decimal Price { get; init; }
}
```

### 2. Implement Handlers

```csharp
public class CreateProductCommandHandler : ICommandHandler<CreateProductCommand, int>
{
    private readonly IProductRepository _repository;

    public CreateProductCommandHandler(IProductRepository repository)
    {
        _repository = repository;
    }

    public async Task<int> HandleAsync(CreateProductCommand command, CancellationToken cancellationToken)
    {
        var product = new Product { Name = command.Name, Price = command.Price };
        await _repository.AddAsync(product, cancellationToken);
        return product.Id;
    }
}
```

### 3. Register in DI

```csharp
builder.Services.AddCommand<CreateProductCommand, int, CreateProductCommandHandler>();
```

### 4. Automatic Endpoint Generation

Svrnty.CQRS automatically creates HTTP or gRPC endpoints:

**HTTP:**
```
POST /api/command/createProduct
```

**gRPC:**
```protobuf
rpc CreateProduct (CreateProductRequest) returns (CreateProductResponse);
```

### 5. Built-in Features

- ✅ **Validation** - FluentValidation integration
- ✅ **Discovery** - Metadata-driven endpoint generation
- ✅ **Authorization** - Custom authorization services
- ✅ **Protocols** - HTTP (Minimal API) and gRPC support
- ✅ **Dynamic Queries** - OData-like filtering
- ✅ **Event Streaming** - Event sourcing and projections

## Architecture Overview

```
┌─────────────────┐
│   HTTP/gRPC     │  ← Automatic endpoint generation
│   Endpoints     │
└────────┬────────┘
         │
┌────────▼────────┐
│   Validation    │  ← FluentValidation
│   (Optional)    │
└────────┬────────┘
         │
┌────────▼────────┐
│    Handler      │  ← Your business logic
│  (Command/Query)│
└────────┬────────┘
         │
┌────────▼────────┐
│  Data Layer     │  ← Database, external APIs, etc.
│  (Your choice)  │
└─────────────────┘
```

## Key Principles in Svrnty.CQRS

1. **Convention over Configuration**
   - Minimal setup required
   - Automatic endpoint naming
   - Sensible defaults

2. **Metadata-Driven Discovery**
   - Handlers registered as metadata
   - Runtime enumeration for endpoint generation
   - Type-safe at compile time

3. **Framework Agnostic**
   - Works with any data access layer (EF Core, Dapper, etc.)
   - No prescribed database or ORM
   - Integration points are interfaces

4. **Production Ready**
   - Validation, authorization, observability
   - Health checks, metrics, structured logging
   - Event sourcing and consumer groups

## What's Next?

Now that you understand CQRS, let's get your development environment set up!

**Continue to [Installation](02-installation.md) →**

## See Also

- [Architecture: CQRS Pattern](../architecture/cqrs-pattern.md) - Deeper dive into the pattern
- [Architecture: Metadata Discovery](../architecture/metadata-discovery.md) - How discovery works
- [Best Practices: Command Design](../best-practices/command-design.md) - Designing effective commands
- [Best Practices: Query Design](../best-practices/query-design.md) - Query optimization patterns