# Event Replay

Rebuild projections and reprocess historical events with rate limiting and progress tracking.

## Overview

Event replay enables you to rebuild projections, reprocess events after bug fixes, create new read models from historical data, and perform time-travel debugging.

**Key Features:**

- ✅ **Offset-Based Replay** - Replay from specific sequence numbers
- ✅ **Time-Based Replay** - Replay from timestamps
- ✅ **Rate Limiting** - Control replay speed with token bucket
- ✅ **Progress Tracking** - Monitor progress with callbacks
- ✅ **Event Filtering** - Replay only specific event types
- ✅ **Batch Processing** - Efficient streaming with configurable batches

## Quick Start

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

var builder = WebApplication.CreateBuilder(args);

// Enable event replay
builder.Services.AddPostgresEventReplay();

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

## Usage

### Replay from Offset

```csharp
var replayService = serviceProvider.GetRequiredService<IEventReplayService>();

await foreach (var @event in replayService.ReplayFromOffsetAsync(
    streamName: "orders",
    startOffset: 1000,
    options: new ReplayOptions
    {
        BatchSize = 100,
        MaxEventsPerSecond = 1000,
        ProgressCallback = progress =>
        {
            Console.WriteLine($"{progress.EventsProcessed} events @ {progress.EventsPerSecond:F0} events/sec");
        }
    }))
{
    await ProcessEventAsync(@event);
}
```

### Replay from Time

```csharp
await foreach (var @event in replayService.ReplayFromTimeAsync(
    streamName: "orders",
    startTime: DateTimeOffset.UtcNow.AddDays(-7)))
{
    await RebuildProjectionAsync(@event);
}
```

## Features

### [Replay from Offset](replay-from-offset.md)
Replay events from specific sequence numbers for rebuilding projections.

### [Replay from Time](replay-from-time.md)
Replay events from specific timestamps for time-travel debugging.

### [Rate Limiting](rate-limiting.md)
Control replay speed with token bucket rate limiting.

### [Progress Tracking](progress-tracking.md)
Monitor progress with metrics and estimated completion times.

## Common Use Cases

**Rebuild Read Models:**
```csharp
// Reset projection checkpoint
await _checkpointStore.ResetCheckpointAsync("order-summary");

// Replay all events
await foreach (var @event in replayService.ReplayFromOffsetAsync("orders", 0))
{
    await _projection.HandleAsync(@event);
}
```

**Reprocess After Bug Fix:**
```csharp
// Fix deployed, reprocess last 7 days
var sevenDaysAgo = DateTimeOffset.UtcNow.AddDays(-7);

await foreach (var @event in replayService.ReplayFromTimeAsync("orders", sevenDaysAgo))
{
    await ReprocessEventAsync(@event);
}
```

**Create New Projection:**
```csharp
// New projection needs historical data
await foreach (var @event in replayService.ReplayFromOffsetAsync("orders", 0, new ReplayOptions
{
    EventTypeFilter = new[] { "OrderPlacedEvent", "OrderShippedEvent" },
    MaxEventsPerSecond = 5000
}))
{
    await _newProjection.HandleAsync(@event);
}
```

## See Also

- [Event Streaming Overview](../README.md)
- [Projections](../projections/README.md)
- [Persistent Streams](../fundamentals/persistent-streams.md)