AlmostServiceBus

A local Azure Service Bus emulator compatible with the official Azure SDK (Azure.Messaging.ServiceBus), MassTransit, Wolverine, and NServiceBus.

AlmostServiceBus is flexible in how you run it:

• Embedded in your test suite — runs in-process with per-test namespace isolation, so your integration tests run in parallel without interfering with each other
• Standalone for local dev — run as a dotnet tool or via Aspire, and point your app at it, just like the real thing

Features

• No infrastructure dependencies — no Docker, no SQL Server, no port conflicts
• Namespace isolation — SharedAccessKeyName can be passed an arbitrary value which will create an isolated namespace.
• Full AMQP 1.0 protocol via AMQPNetLite — no HTTP polling or fakes
• Queues with PeekLock, dead-lettering, duplicate detection, and max delivery count
• Topics & Subscriptions with SQL and correlation filters, forwarding, fan-out
• Sessions (FIFO) with session locking, next-available-session, isolated delivery, and session state
• Scheduled messages with enqueue-time semantics
• AMQP transactions — System.Transactions.TransactionScope works end-to-end, including cross-entity transactions (EnableCrossEntityTransactions); commit applies all operations atomically, rollback applies none
• Batch message support — correctly decodes Azure SDK ServiceBusMessageBatch transfers
• Management API — Atom XML REST API for queue/topic/subscription CRUD
• Plaintext, MS-emulator compatible — clients connect with UseDevelopmentEmulator=true, the same flag used with Microsoft's official Service Bus emulator. No TLS, no dev cert, no privileged ports.
• Vue diagnostic dashboard on port 15672

Installation

Available as NuGet packages:

# CLI tool — quickest way to get started
dotnet tool install --global AlmostServiceBus.Tool

# Core emulator (standalone or embedded)
dotnet add package AlmostServiceBus

# Test host with per-test namespace isolation
dotnet add package AlmostServiceBus.TestHost

# Aspire integration
dotnet add package AlmostServiceBus.Aspire.Hosting

Quick Start

Run standalone

# If installed as a global tool:
almost-servicebus

Connection string:

Endpoint=sb://localhost:5672;SharedAccessKeyName=<my-namespace>;SharedAccessKey=emulator

Note: using "RootManageSharedAccessKey" as the SharedAccessKeyName will map to the 'default' namespace.

Integration tests (in-process)

Add AlmostServiceBus.TestHost to your test project and use ServiceBusEmulatorFixture:

var fixture = new ServiceBusEmulatorFixture();
await fixture.StartAsync();

var client = new ServiceBusClient(fixture.ConnectionString, new ServiceBusClientOptions
{
    TransportType = ServiceBusTransportType.AmqpTcp,
    CustomEndpointAddress = new Uri($"sb://localhost:{fixture.PublicPort}")
});

// Use client normally...
await fixture.DisposeAsync();

Each fixture gets a unique namespace, so tests run in parallel without interference.

Aspire integration

Add AlmostServiceBus.Aspire.Hosting to your Aspire host project:

var builder = DistributedApplication.CreateBuilder(args);
var serviceBus = builder.AddServiceBusEmulator("servicebus");

When to Use (and When Not To)

Good fit:

• Integration tests — per-test namespace isolation means your tests run in parallel without interfering. No Docker, no SQL Server, sub-second startup.
• Local development — almost-servicebus and go. Point your app at localhost:5672 and iterate without an Azure subscription.
• CI pipelines — no Docker-in-Docker, no container orchestration. Just dotnet tool install and run.
• Aspire apps — drop-in AddServiceBusEmulator() resource, works like the real thing.

Not a good fit:

• Performance/load testing — this is an in-memory emulator, not a distributed broker. Backpressure and throughput characteristics don't match real ASB.
• Production — this should go without saying, but: don't.

Framework coverage: MassTransit has the deepest testing (runs against MassTransit's own ASB test suite). Wolverine and NServiceBus have conformance-level coverage. Contributions for additional framework tests, fixes, and new framework support are welcome.

Architecture

Client (Azure SDK / MassTransit / Wolverine / NServiceBus)
UseDevelopmentEmulator=true in the connection string
    │
    ▼
TcpMultiplexer (port 5672, plaintext) ─── first-byte sniffing
    ├── 0x41 (AMQP)  → plain AMQP backend
    └── HTTP verb     → plain HTTP backend
    │
    ├── AMQPNetLite + EmulatorContainer (message send/receive)
    └── Kestrel (management API + dashboard)
    │
    ▼
NamespaceRegistry (shared in-memory broker)
    ├── QueueEntity (channels, pending locks, DLQ)
    ├── TopicEntity → SubscriptionEntity (filters, forwarding)
    └── SessionManager (per-queue session partitioning)

The emulator is plaintext-only and compatible with Microsoft's official Service Bus emulator: clients use the same UseDevelopmentEmulator=true connection-string flag, which makes Azure.Messaging.ServiceBus use plain AMQP for data and plain HTTP for admin.

The emulator uses AMQPNetLite as the AMQP server (Microsoft.Azure.Amqp's server API is internal). A custom EmulatorContainer (replacing AMQPNetLite's ContainerHost) handles delivery tag rewriting, batch message decoding, and transaction coordinator links (a server-side coordinator buffers transactional work and applies it on commit). Message delivery uses channel-based waiting for instant wake-up on enqueue.

Framework Compatibility

Framework	Status	Notes
Azure SDK (Azure.Messaging.ServiceBus)	Full	PeekLock, sessions, scheduled messages, processors, batch sends
MassTransit	Full	Tested against MassTransit's own ASB test suite
Wolverine	High	149/155 tests pass; tracking correlation edge cases excluded
NServiceBus	Partial	AMQP transactions now supported (no longer requires ReceiveOnly transport mode)

Test Results

Suite	Passed	Total
Internal unit + integration	207	207
Conformance (emulator)	34	34
MassTransit ASB test suite	26	27
Wolverine ASB test suite	149	155

Configuration

CLI argument	Default	Description
--Port	5672	Main public port (plain AMQP + plain HTTP, multiplexed)
--DashboardPort	15672	Vue dashboard port (0 to disable)

Additional ports bound automatically:

• 5300 — admin HTTP, the port Azure.Messaging.ServiceBus uses for management when UseDevelopmentEmulator=true

Known Limitations

• Wolverine tracking — tracking_correlation_id_on_everything compliance tests time out. Standalone tests confirm correct AMQP behavior; the timeout is caused by Wolverine's internal handler pipeline, not the emulator. See tests/ms-emulator-comparison/ for a harness to verify against Microsoft's official emulator.

Development

# Run all tests
dotnet test AlmostServiceBus.sln --filter "FullyQualifiedName!~RealAsbConformanceTests"

# Run conformance tests against real Azure Service Bus
ASB_CONNECTION_STRING="Endpoint=sb://..." dotnet test tests/AlmostServiceBus.Conformance.Tests \
  --filter "FullyQualifiedName~RealAsbConformanceTests"

# Run Wolverine tests (emulator must be running on port 5673)
dotnet run --project src/AlmostServiceBus.Host -- --Port 5673 --DashboardPort 0 &
dotnet test external/wolverine/src/Transports/Azure/Wolverine.AzureServiceBus.Tests -f net10.0

# Compare against Microsoft's official emulator
cd tests/ms-emulator-comparison && ./run-wolverine-against-ms-emulator.sh

License

See LICENSE for details.