Installation

Prerequisites

• .NET 10 — Opossum targets net10.0.
• A writable directory on a local or network drive for event storage.

Install via NuGet

.NET CLI

dotnet add package Opossum

Package Manager Console (Visual Studio)

Install-Package Opossum

PackageReference (.csproj)

<PackageReference Include="Opossum" Version="*" />

Check NuGet for the latest stable version.

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Register with Dependency Injection

Opossum integrates with Microsoft's standard IServiceCollection. All services chain from AddOpossum():

using Opossum.DependencyInjection;
using Opossum.Mediator;
using Opossum.Projections;

var builder = WebApplication.CreateBuilder(args);

builder.Services
    // Core event store
    .AddOpossum(options =>
    {
        options.RootPath = @"D:\MyAppData\EventStore"; // where events are stored
        options.UseStore("MyApp");                      // store name
        options.FlushEventsImmediately = true;          // durability (recommended for production)
    })
    // Mediator (optional — for command/query handling)
    .AddMediator()
    // Projection system (optional — for read models)
    .AddProjections(options =>
    {
        options.ScanAssembly(typeof(Program).Assembly); // auto-discover IProjectionDefinition<T>
    });

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

What gets registered

AddOpossum()

Service	Lifetime	Description
IEventStore	Singleton	Core append/read interface
IEventStoreAdmin	Singleton	Destructive admin operations (DeleteStoreAsync)
IEventStoreMaintenance	Singleton	Additive maintenance operations (retroactive tag migration)

AddProjections()

Service	Lifetime	Description
IProjectionManager	Singleton	Manages projection state and updates
IProjectionRebuilder	Singleton	Orchestrates projection rebuilds
IProjectionStore<TState>	Singleton	Reads projection state — one registration per projection type, created automatically during assembly scan

AddMediator()

Service	Lifetime	Description
IMediator	Singleton	Message dispatch

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Minimum Configuration

The only required configuration is a RootPath and a store name:

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

Opossum creates the directory structure automatically on first startup.

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Next Steps

→ Quick Start — build your first event-sourced feature in 5 minutes
→ Configuration — full reference for all options