# Storage

Storage backends for event streaming.

## Overview

Svrnty.CQRS provides two storage implementations for event streams: **PostgreSQL** for production deployments and **In-Memory** for development and testing.

## Storage Backends

| Backend | Persistence | Use Case | Package |
|---------|-------------|----------|---------|
| **PostgreSQL** | Durable | Production | `Svrnty.CQRS.Events.PostgreSQL` |
| **In-Memory** | Volatile | Development/Testing | `Svrnty.CQRS.Events` |

## Quick Comparison

### PostgreSQL Storage

**Pros:**
- ✅ Durable persistence
- ✅ ACID transactions
- ✅ Concurrent access
- ✅ Consumer groups support
- ✅ Retention policies
- ✅ Event replay
- ✅ Stream configuration
- ✅ High performance (SKIP LOCKED)

**Cons:**
- ❌ Requires PostgreSQL instance
- ❌ Network latency
- ❌ More complex setup

**When to use:**
- Production deployments
- Multi-instance scenarios
- Long-term event storage
- Consumer group coordination

### In-Memory Storage

**Pros:**
- ✅ Zero setup
- ✅ Fast (no I/O)
- ✅ Simple configuration
- ✅ Great for testing

**Cons:**
- ❌ No persistence (lost on restart)
- ❌ Limited to single process
- ❌ No consumer groups
- ❌ No retention policies

**When to use:**
- Unit testing
- Local development
- Prototyping
- Learning the framework

## Installation

### PostgreSQL

```bash
dotnet add package Svrnty.CQRS.Events.PostgreSQL
dotnet add package Svrnty.CQRS.Events.ConsumerGroups
```

### In-Memory

```bash
dotnet add package Svrnty.CQRS.Events
```

## Configuration

### PostgreSQL

```csharp
using Svrnty.CQRS.Events.PostgreSQL;

var builder = WebApplication.CreateBuilder(args);

// Register PostgreSQL event streaming
builder.Services.AddPostgresEventStreaming(
    builder.Configuration.GetConnectionString("EventStore"));

// Optional: Consumer groups
builder.Services.AddPostgresConsumerGroups(
    builder.Configuration.GetSection("EventStreaming:ConsumerGroups"));

// Optional: Retention policies
builder.Services.AddPostgresRetentionPolicies(options =>
{
    options.Enabled = true;
    options.CleanupInterval = TimeSpan.FromHours(1);
});

var app = builder.Build();
app.Run();
```

**appsettings.json:**
```json
{
  "ConnectionStrings": {
    "EventStore": "Host=localhost;Database=eventstore;Username=postgres;Password=postgres"
  },
  "EventStreaming": {
    "ConsumerGroups": {
      "HeartbeatInterval": "00:00:10",
      "SessionTimeout": "00:00:30",
      "CleanupInterval": "00:01:00"
    }
  }
}
```

### In-Memory

```csharp
using Svrnty.CQRS.Events;

var builder = WebApplication.CreateBuilder(args);

// Register in-memory event streaming
builder.Services.AddInMemoryEventStreaming();

var app = builder.Build();
app.Run();
```

## Features by Backend

| Feature | PostgreSQL | In-Memory |
|---------|-----------|-----------|
| Persistent Streams | ✅ | ✅ |
| Ephemeral Streams | ✅ | ✅ |
| Consumer Groups | ✅ | ❌ |
| Retention Policies | ✅ | ❌ |
| Event Replay | ✅ | ✅ |
| Stream Configuration | ✅ | ❌ |
| gRPC Streaming | ✅ | ✅ |
| Health Checks | ✅ | ❌ |
| Metrics | ✅ | ✅ |
| Durability | ✅ | ❌ |
| Multi-Instance | ✅ | ❌ |

## Storage Operations

### Common Interface

Both backends implement the same interface:

```csharp
public interface IEventStreamStore
{
    // Persistent streams
    Task AppendAsync(string streamName, object[] events, CancellationToken ct = default);
    IAsyncEnumerable<StoredEvent> ReadStreamAsync(string streamName, long fromOffset, CancellationToken ct = default);

    // Ephemeral streams
    Task EnqueueAsync(string streamName, object message, CancellationToken ct = default);
    Task<StoredMessage?> DequeueAsync(string streamName, TimeSpan visibilityTimeout, CancellationToken ct = default);
    Task AcknowledgeAsync(string streamName, string messageId, CancellationToken ct = default);
    Task NackAsync(string streamName, string messageId, TimeSpan redeliverAfter, CancellationToken ct = default);
}
```

### Example Usage

```csharp
// Works with both PostgreSQL and in-memory
public class OrderService
{
    private readonly IEventStreamStore _eventStore;

    public async Task PublishOrderPlacedAsync(int orderId, string customer, decimal amount)
    {
        await _eventStore.AppendAsync("orders", new[]
        {
            new OrderPlacedEvent
            {
                OrderId = orderId,
                CustomerName = customer,
                TotalAmount = amount
            }
        });
    }

    public async Task ProcessOrdersAsync()
    {
        await foreach (var @event in _eventStore.ReadStreamAsync("orders", fromOffset: 0))
        {
            Console.WriteLine($"Order event: {@event.EventType}");
        }
    }
}
```

## Database Setup

### PostgreSQL (Docker)

```bash
# Start PostgreSQL
docker run -d --name postgres \
  -e POSTGRES_PASSWORD=postgres \
  -e POSTGRES_DB=eventstore \
  -p 5432:5432 \
  postgres:16

# Tables created automatically on first run
dotnet run
```

### PostgreSQL (Production)

```bash
# Create database
createdb eventstore

# Run migrations (automatic)
# Tables created when application starts

# Or run migrations manually
dotnet ef database update
```

## Performance

### PostgreSQL Optimizations

1. **Connection pooling** (see [Connection Pooling](connection-pooling.md))
2. **Batch operations** for bulk inserts
3. **SKIP LOCKED** for concurrent dequeue
4. **Indexes** on stream_name, offset, timestamp
5. **Partitioning** for large streams (optional)

### In-Memory Optimizations

1. **Thread-safe collections** (ConcurrentQueue, ConcurrentDictionary)
2. **No I/O overhead**
3. **Direct memory access**

## Migration

### Development to Production

Switch from in-memory to PostgreSQL:

**Before (Development):**
```csharp
builder.Services.AddInMemoryEventStreaming();
```

**After (Production):**
```csharp
builder.Services.AddPostgresEventStreaming(
    builder.Configuration.GetConnectionString("EventStore"));
```

No code changes needed - same `IEventStreamStore` interface!

## Monitoring

### PostgreSQL

- Query stream sizes: `SELECT stream_name, COUNT(*) FROM events GROUP BY stream_name`
- Monitor consumer lag via consumer_offsets table
- Use pg_stat_statements for query performance

### In-Memory

- Check memory usage
- Monitor GC pressure
- Use memory profiler for large streams

## Learn More

- [In-Memory Storage](in-memory-storage.md) - Development setup
- [PostgreSQL Storage](postgresql-storage.md) - Production deployment
- [Database Schema](database-schema.md) - PostgreSQL schema details
- [Connection Pooling](connection-pooling.md) - Performance tuning

## See Also

- [Event Streaming Overview](../README.md)
- [Getting Started](../fundamentals/getting-started.md)
- [Consumer Groups](../consumer-groups/README.md)
- [Retention Policies](../retention-policies/README.md)