Sharing message contracts

To minimize the amount of information and dependencies shared between endpoints, it's recommended to use separate assemblies for message contracts. It's also recommended to use a separate message contract assembly for each service. This allows a service to evolve its contracts without impacting other services in the system. A message contract should be declared in the message contracts assembly of the service which owns the message contract.

Sharing contracts

A sender and receiver of a given message must use the same message contract. Message contracts may be shared in various ways:

• When all endpoints are located in a single solution, message contract assemblies may be directly referenced as project dependencies.
• When endpoints are located in multiple solutions, message contracts may be shared as NuGet packages. NuGet packages may be published on the file system or on a NuGet server. A custom NuGet config file may be used to configure additional NuGet package sources.
• Messages contracts may be shared as C# source files.

NServiceBus dependency

When marker interfaces are used to define messages, message contract projects have a dependency on the NServiceBus package. This may cause version conflicts when message contracts are updated in endpoints targeting older major versions of NServiceBus. This may be avoided by either:

• Referencing the oldest used NServiceBus major version from message contracts projects. NuGet dependency resolution allows endpoints on newer major versions of NServiceBus to reference assemblies that target an older version of NServiceBus, but not vice versa.
• Switching to unobtrusive mode. Unobtrusive mode allows an assembly to define message contracts without a dependency on the NServiceBus package, making it easy to share message contracts with endpoints targeting multiple versions of NServiceBus and running on various frameworks and platforms.

Versioning

When endpoints are updated independently from each other, it is important to take into account the message contracts shared between them (or even between endpoint instances).

After updating message contracts in an endpoint:

• Other endpoints must be able to process messages sent or published by the updated endpoint.
• The updated endpoint must able to process messages sent or published by other endpoints.

NServiceBus includes a message header containing the message type's fully qualified name, which includes the assembly name and version number. Therefore, for best compatibility, it is recommended to maintain a stable assembly version. The version of the contracts may be stated in the assembly file version, assembly informational version, or NuGet package version.

The evolving message contracts guidance has more details on safely updating message contracts.

Breaking down large contract assemblies

In the early stages of developing a system, combining all events, commands, and messages in a single contracts assembly is often a good way to start. As the system grows, breaking down the contracts into smaller parts makes it easier to evolve a system safely.

For example, an endpoint may have many subscribers to its events, but only one or two endpoints sending it commands. A good way to evolve the command senders without requiring the subscribers to update their contract assemblies would be to separate the command and events into two assemblies:

• EndpointName.Commands
• EndpointName.Events

Or when specific events are subscribed to by endpoints managed by other teams, it may make sense to extract those contracts into a separate assembly or NuGet package.