Endpoint Policies

An endpoint policy is a reusable rule that Wolverine applies to every messaging endpoint (listeners, senders, and local queues) as your application bootstraps. Rather than configuring each Rabbit MQ queue or Azure Service Bus subscription one at a time, you can write a single policy that's evaluated against all endpoints — including ones created by conventional routing that you never explicitly declared.

Endpoint policies are how a lot of Wolverine's own behavior is implemented. For example, the Rabbit MQ UseQuorumQueues() helper is just an endpoint policy that flips every application-owned queue to a quorum queue.

Writing an Endpoint Policy

Implement the Wolverine.Configuration.IEndpointPolicy interface. It has a single method that receives each Endpoint together with the live IWolverineRuntime:

// Force every application listening endpoint to process messages inline.
// Wolverine's own internal/system endpoints are left alone.
public class InlineListenersPolicy : IEndpointPolicy
{
    public void Apply(Endpoint endpoint, IWolverineRuntime runtime)
    {
        // Don't touch endpoints that Wolverine itself owns
        if (endpoint.Role == EndpointRole.System) return;

        if (endpoint.IsListener)
        {
            endpoint.Mode = EndpointMode.Inline;
        }
    }
}

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A few things worth knowing about the Endpoint you're handed:

• endpoint.Role is either EndpointRole.System (something Wolverine created and owns internally) or EndpointRole.Application (something you declared, or that conventional routing created on your behalf). It's almost always correct to skip System endpoints in your policy.
• endpoint.Mode controls the durability/buffering behavior and is one of EndpointMode.Durable, EndpointMode.BufferedInMemory, or EndpointMode.Inline.
• endpoint.IsListener tells you whether the endpoint is receiving messages.
• endpoint.Uri is the unique address of the endpoint, which is handy for matching by transport scheme or queue name.

Your policy runs once per endpoint during bootstrapping, so it's the right place for cross-cutting configuration but not for per-message logic.

Registering a Policy

Policies are registered through opts.Policies inside UseWolverine(). You can add a policy type that will be created by Wolverine, or add a pre-built instance:

using var host = await Host.CreateDefaultBuilder()
    .UseWolverine(opts =>
    {
        // Register a custom IEndpointPolicy type
        opts.Policies.Add<InlineListenersPolicy>();

        // Or register a policy inline with LambdaEndpointPolicy<T>.
        // The lambda only runs for endpoints assignable to T (here, every Endpoint)
        opts.Policies.Add(new LambdaEndpointPolicy<Endpoint>((endpoint, runtime) =>
        {
            if (endpoint.Role == EndpointRole.System) return;
            endpoint.Mode = EndpointMode.BufferedInMemory;
        }));
    }).StartAsync();

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When you only need to react to one kind of endpoint (say, just Rabbit MQ queues), the generic LambdaEndpointPolicy<T> is the most convenient option. The supplied lambda is only invoked for endpoints that are assignable to T. This is exactly how Wolverine's Rabbit MQ integration implements UseQuorumQueues():

// From Wolverine's own Rabbit MQ integration
Options.Policies.Add(new LambdaEndpointPolicy<RabbitMqQueue>((queue, _) =>
{
    if (queue.Role == EndpointRole.Application)
    {
        queue.QueueType = QueueType.quorum;
    }
}));

Built-in Endpoint Policy Helpers

You don't always need to write your own IEndpointPolicy. For the most common cases, opts.Policies already exposes convenience methods that build endpoint policies for you. Each one automatically skips Wolverine's internal System endpoints:

• opts.Policies.AllListeners(x => ...) — apply a ListenerConfiguration action to every (non-local) listening endpoint.
• opts.Policies.AllSenders(x => ...) — apply an ISubscriberConfiguration action to every sending endpoint.
• opts.Policies.AllLocalQueues(x => ...) — apply configuration to every local queue.

For example:

opts.Policies.AllListeners(x => x.MaximumParallelMessages(5));

See Configuring Listeners for more on what's available through these helpers.