Why the Critter Stack
Why the Critter Stack
Event sourcing, documents, and messaging that fit in your head
Most .NET teams building event-driven systems end up with a mediator from one project, a message bus from another, an event store from a third, and a test harness that understands none of them. The Critter Stack is one family of open source tools, built by one team, designed to work together: Marten, Polecat, or Fisher for the event store and documents on the database you already run, Wolverine for commands, messages, and HTTP, Alba and Bobcat for the tests that prove it works, and CritterWatch to run it in production.
Proven in Production
Open source since 2015 and used by teams in finance, healthcare, logistics, IoT, and more. Numbers as of November 2026.
Why Teams Choose the Critter Stack
Low ceremony, by design
Handlers are plain methods. No base classes, no interfaces, no marker attributes you forget. Wolverine generates the pipeline around your code at build time, so what you read in the handler is what runs, and the business logic stays a pure function.
Wolverine, the low-ceremony champion ➔Vertical slices with Wolverine.HTTP ➔
The most complete event sourcing on .NET
Streams, snapshots, inline and async projections, blue/green rebuilds, strong consistency when you need it, and the same model on PostgreSQL, SQL Server 2025, and SQLite. Marten has a decade of production lessons behind it, and Polecat and Fisher inherit all of them.
A practical introduction to event sourcing ➔Scaling event sourcing with Marten ➔
Messaging you can trust
A transactional inbox and outbox, retries, circuit breakers, dead letters, sagas, and scheduled and recurring messages behave the same way on RabbitMQ, Azure Service Bus, AWS, Kafka, NATS, and the rest. We try to break it on purpose so you do not find out in production.
Ordered messaging across a cluster ➔Wolverine best practices, revisited ➔
Testable from day one
Alba runs your real ASP.NET Core application in memory and asserts on what it did. Bobcat turns ordinary test methods into Given/When/Then specifications and runs big suites across workers with flakiness reported honestly. Handlers are pure functions, so unit tests need no mocks.
Observable and operable
OpenTelemetry tracing and metrics are built in. CritterWatch reads them and turns them into a console that shows which listener is stuck, why a projection is behind, and which dead letters share a cause, then lets you fix it, with an audit trail, from the console or from an AI agent.
Built for AI-assisted development
Convention-based, low-ceremony code is exactly what coding agents produce well. The Critter Stack AI Skills teach agents the traps and the idioms, Bobcat specs give them an honest definition of done, and CritterWatch exposes operations over MCP with the guard rails an agent needs.
Introducing the Critter Stack AI Skills ➔The codebase is the prompt ➔
In Their Words
Critter Stack libraries are some of the most mature libraries I've ever used and their feature set and integration is perfectly tailored for real world problems to solve.
The ease of implementing event driven and nosql support is what I value the most. Not having the struggle of making different frameworks play well together to get the same functionality is both time, and therefore, cost saving.
What Is in the Stack
Each tool stands on its own. Together they cover the whole lifecycle, from the first command handler to the production console.
MartenDocument database and event store on PostgreSQL
WolverineCommand bus, message bus, and HTTP endpoints
CritterWatchMonitoring and operations for the Critter Stack
PolecatEvent store and document database on SQL Server 2025
FisherEmbedded event store and document database on SQLite
BobcatExecutable specifications and test supervision
AlbaIntegration testing for ASP.NET Core
WeaselSchema management for PostgreSQL, SQL Server, and SQLite
AI SkillsDocumentation written for coding agents
StoatPreviewThe design-time surface for Critter Stack development
Getting Started
Pick your database, then start with Marten or Wolverine
PostgreSQL shops start with Marten, SQL Server shops with Polecat, and embedded or edge scenarios with Fisher. Add Wolverine when you want command handling, messaging, or HTTP endpoints without the ceremony.
Choose your tools ➔Try the sample projects
Working examples that demonstrate Event Sourcing, CQRS, and messaging patterns in action, including the aggregate handler workflow and integration testing.
View on GitHub ➔Read how it works in practice
The blog publishes release notes, deep dives, and lessons from real systems several times a week, filtered by product so you can follow just the tools you use.
Read the blog ➔Talk to us
Evaluating whether the Critter Stack fits your project? Book a free call, join the Discord, or ask about a workshop for your team.
Get in touch ➔Need Help Getting Started?
JasperFx Software builds the Critter Stack and offers consulting, support plans, and workshops to help your team succeed with it.