🦝 Opossum — File System Event Store

A .NET library that turns your file system into a fully functional event store.

Opossum implements the DCB (Dynamic Consistency Boundaries) specification — bringing event sourcing, optimistic concurrency, projections, and the mediator pattern to any .NET application without a database server.

NuGet .NET License Docs

Install

dotnet add package Opossum

30-Second Example

// Register
builder.Services.AddOpossum(options =>
{
    options.RootPath = @"D:\MyData\EventStore";
    options.UseStore("MyApp");
});

// Append an event
await eventStore.AppendAsync(
    [new NewEvent { Event = new DomainEvent { Event = new OrderPlacedEvent(orderId) } }],
    condition: null);

// Read events
var events = await eventStore.ReadAsync(Query.All(), readOptions: null);

No Docker. No database migrations. No cloud dependencies. Just files.

Why Opossum?

Scenario Why Opossum Fits
Automotive / POS / field-service 100% offline, complete audit trail, local compliance
SMB on-premises software No recurring cloud fees, IT staff manages files
Multi-workstation LAN deployments Cross-process safety via OS file locking
Development & testing Zero infrastructure — just run the app
Compliance-heavy industries Immutable event log, optional OS-level write protection

What's Included

  • ✅ Event Store — Append and read immutable events stored as JSON files
  • ✅ Optimistic Concurrency — DCB AppendCondition prevents lost updates
  • ✅ Tag-Based Indexing — Fast domain-scoped queries without full scans
  • ✅ Projections — Auto-maintained read models that rebuild from events
  • ✅ Mediator — Command/query dispatch with automatic handler discovery via DI
  • ✅ Cross-Process Safety — OS-level file lock serialises appends across machines
  • ✅ Durability Guarantees — Configurable flush-to-disk on every write
  • ✅ OpenTelemetry — Built-in activity source for distributed tracing

Getting Started

→ Installation
→ Quick Start
→ Configuration

Learn the Concepts

→ What is an Event Store?
→ DCB Specification
→ Projections
→ Mediator Pattern

Explore the API

→ API Reference

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