Basics

One way or another, Wolverine is all about messages within your system or between systems. Staying inside a single Wolverine system, a message is typically just a .NET class (or struct, but reserve that for strictly local usage) or C# record. A message generally represents either a "command" that should trigger an operation or an "event" that just lets another part of your system know that something happened. Here's a couple simple samples:

// A "command" message
public record DebitAccount(long AccountId, decimal Amount);

// An "event" message
public record AccountOverdrawn(long AccountId);

^{snippet source | anchor}

The next concept in Wolverine is a message handler, which is just a method that "knows" how to process an incoming message. Here's an extremely simple example:

public static class DebitAccountHandler
{
    public static void Handle(DebitAccount account)
    {
        Console.WriteLine($"I'm supposed to debit {account.Amount} from account {account.AccountId}");
    }
}

^{snippet source | anchor}

Wolverine can act as a completely local mediator tool that allows your code to invoke the handler for a message at any time without having to know anything about exactly how that message is processed with this usage:

public async Task invoke_debit_account(IMessageBus bus)
{
    // Debit $250 from the account #2222
    await bus.InvokeAsync(new DebitAccount(2222, 250));
}

^{snippet source | anchor}

There's certainly some value in Wolverine just being a command bus running inside of a single process, Wolverine also allows you to both publish and process messages received through external infrastructure like Rabbit MQ or Pulsar.

To put this into perspective, here's how a Wolverine application could be connected to the outside world:

TIP

The diagram above should just say "Message Handler" as Wolverine makes no structural differentiation between commands or events, but Jeremy is being too lazy to fix the diagram.

Terminology

• Message -- Typically just a .NET class or C# record that can be easily serialized. See messages and serialization for more information
• Envelope -- Wolverine's Envelope Wrapper model that wraps the raw messages with metadata
• Message Handler -- A method or function that "knows" how to process an incoming message. See Message Handlers for more information
• Transport -- This refers to the support within Wolverine for external messaging infrastructure tools like Rabbit MQ, Amazon SQS, Azure Service Bus, or Wolverine's built in TCP transport
• Endpoint -- The configuration for a Wolverine connection to some sort of external resource like a Rabbit MQ exchange or an Amazon SQS queue. The Async API specification refers to this as a channel, and Wolverine may very well change its nomenclature in the future to be consistent with Async API.
• Sending Agent -- You won't use this directly in your own code, but Wolverine's internal adapters to publish outgoing messages to transport endpoints
• Listening Agent -- Again, an internal detail of Wolverine that receives messages from external transport endpoints, and mediates between the transports and executing the message handlers
• Message Store -- Database storage for Wolverine's inbox/outbox persistent messaging
• Durability Agent -- An internal subsystem in Wolverine that runs in a background service to interact with the message store for Wolverine's transactional inbox/outbox functionality

For more context, see