AppHost eventing APIs

In Aspire, eventing allows you to publish and subscribe to events during various AppHost life cycles. Eventing is more flexible than life cycle events. Both let you run arbitrary code during event callbacks, but eventing offers finer control of event timing, publishing, and provides supports for custom events.

AppHost eventing

The following events are available in the AppHost and occur in the following order:

1. BeforeStartEvent: This event is raised before the AppHost starts.
2. ResourceEndpointsAllocatedEvent: This event is raised per resource after its endpoints are allocated.
3. AfterResourcesCreatedEvent: This event is raised after resources are created.

Subscribe to AppHost events

To subscribe to built-in AppHost events, use the typed API available for each event. C# provides builder extension methods for selected events and the lower-level Eventing.Subscribe<T>() API for others. TypeScript provides named subscription methods:

TypeScript

apphost.mts

import { createBuilder } from './.aspire/modules/aspire.mjs';

const builder = await createBuilder();

await builder.subscribeBeforeStart(async () => {
  console.log('BeforeStartEvent');
});

await builder.subscribeAfterResourcesCreated(async () => {
  console.log('AfterResourcesCreatedEvent');
});

await builder.build().run();

C#

AppHost.cs

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

var builder = DistributedApplication.CreateBuilder(args);

builder.OnBeforeStart(static (@event, cancellationToken) =>
{
    var logger = @event.Services.GetRequiredService<ILogger<Program>>();
    logger.LogInformation("BeforeStartEvent");
    return Task.CompletedTask;
});

builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(
    static (@event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("AfterResourcesCreatedEvent");
        return Task.CompletedTask;
    });

builder.Build().Run();

The following C# builder-level helper methods are available for AppHost events:

Method	Event
OnBeforeStart	BeforeStartEvent — raised before the AppHost starts
OnBeforePublish	BeforePublishEvent — raised before manifest publishing begins
OnAfterPublish	AfterPublishEvent — raised after manifest publishing completes

For the full API surface, see the C# DistributedApplicationEventingExtensions API reference and the TypeScript Aspire.Hosting API reference.

If you need to subscribe via IDistributedApplicationEventing directly (for example, inside an IDistributedApplicationEventingSubscriber), you can use the lower-level Eventing.Subscribe<T>() API:

AppHost.cs

builder.Eventing.Subscribe<BeforeStartEvent>(
    static (@event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("BeforeStartEvent");
        return Task.CompletedTask;
    });

builder.Eventing.Subscribe<AfterResourcesCreatedEvent>(
    static (@event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("AfterResourcesCreatedEvent");
        return Task.CompletedTask;
    });

टिप्पणी

TypeScript AppHosts don’t expose the generic builder.Eventing.Subscribe<TEvent>() API. Use the named callback APIs generated for each supported event, such as subscribeBeforeStart, subscribeAfterResourcesCreated, addEventingSubscriber, and resource callbacks like OnResourceReadyonResourceReady.

सावधानी

Eventing.Subscribe<T>() requires T to be a concrete event type. Subscribing to an interface or abstract class throws an ArgumentException. For IDistributedApplicationEvent, the exception message is Cannot subscribe to interface or abstract type 'IDistributedApplicationEvent'. Subscribe to a concrete event type instead. Use a concrete event type such as BeforeStartEvent or AfterResourcesCreatedEvent.

When the AppHost is run, by the time the Aspire dashboard is displayed, you should see the following log output in the console:

info: Program[0]
      BeforeStartEvent
info: Aspire.Hosting.DistributedApplication[0]
      Aspire version: 13.6.0
info: Aspire.Hosting.DistributedApplication[0]
      Distributed application starting.
info: Aspire.Hosting.DistributedApplication[0]
      Application host directory is: ../AspireApp/AspireApp.AppHost
info: Aspire.Hosting.DistributedApplication[0]
      Now listening on: https://localhost:17178
info: Aspire.Hosting.DistributedApplication[0]
      Login to the dashboard at https://localhost:17178/login?t=<YOUR_TOKEN>
info: Program[0]
      AfterResourcesCreatedEvent
info: Aspire.Hosting.DistributedApplication[0]
      Distributed application started. Press Ctrl+C to shut down.

The log output confirms that event handlers are executed in the order of the AppHost life cycle events. The subscription order doesn’t affect execution order. The BeforeStartEvent is triggered before the AppHost starts, and AfterResourcesCreatedEvent is triggered after resources are created.

Resource eventing

In addition to the AppHost events, you can also subscribe to resource events. Resource events are raised specific to an individual resource. Resource events are defined as implementations of the IDistributedApplicationResourceEvent interface. The following resource events are available in the listed order:

1. InitializeResourceEvent: Raised by orchestrators to signal to resources that they should initialize themselves.
2. ResourceEndpointsAllocatedEvent: Raised when the orchestrator allocates endpoints for a resource.
3. ConnectionStringAvailableEvent: Raised when a connection string becomes available for a resource.
4. BeforeResourceStartedEvent: Raised before the orchestrator starts a new resource.
5. ResourceReadyEvent: Raised when a resource initially transitions to a ready state.

Subscribe to resource events

To subscribe to resource events, use the convenience-based extension methods. After you have a distributed application builder instance, and a resource builder, walk up to the instance and chain a call to the desired event API:

TypeScript

apphost.mts

import { createBuilder } from './.aspire/modules/aspire.mjs';

const builder = await createBuilder();

const cache = await builder.addRedis('cache');

await cache.onInitializeResource(async () => {
  console.log('1. onInitializeResource');
});

await cache.onResourceEndpointsAllocated(async (event) => {
  const resource = await event.resource();
  console.log(`2. endpoints allocated for ${resource.getResourceName()}`);
});

await cache.onConnectionStringAvailable(async (event) => {
  const resource = await event.resource();
  console.log(
    `3. connection string available for ${resource.getResourceName()}`
  );
});

await cache.onBeforeResourceStarted(async () => {
  console.log('4. onBeforeResourceStarted');
});

await cache.onResourceReady(async () => {
  console.log('5. onResourceReady');
});

await builder.build().run();

C#

AppHost.cs

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

var builder = DistributedApplication.CreateBuilder(args);

var cache = builder.AddRedis("cache");

cache.OnResourceReady(static (resource, @event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("5. OnResourceReady");

        return Task.CompletedTask;
    });

cache.OnInitializeResource(
    static (resource, @event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("1. OnInitializeResource");

        return Task.CompletedTask;
    });

cache.OnBeforeResourceStarted(
    static (resource, @event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("4. OnBeforeResourceStarted");


        return Task.CompletedTask;
    });

cache.OnResourceEndpointsAllocated(
    static (resource, @event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("2. OnResourceEndpointsAllocated");

        return Task.CompletedTask;
    });

cache.OnConnectionStringAvailable(
    static (resource, @event, cancellationToken) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("3. OnConnectionStringAvailable");

        return Task.CompletedTask;
    });

var apiService = builder.AddProject<Projects.AspireApp_ApiService>("apiservice");

builder.AddProject<Projects.AspireApp_Web>("webfrontend")
    .WithExternalHttpEndpoints()
    .WithReference(cache)
    .WaitFor(cache)
    .WithReference(apiService)
    .WaitFor(apiService);

builder.Build().Run();

The preceding code subscribes to the InitializeResourceEvent, ResourceReadyEvent, ResourceEndpointsAllocatedEvent, ConnectionStringAvailableEvent, and BeforeResourceStartedEvent events on the cache resource. Chain calls to the event methods to subscribe to multiple events on the same resource.

टिप्पणी

TypeScript callbacks receive the event object. Use await event.resource() and await event.services() when you need the resource or service provider.

• OnInitializeResourceonInitializeResource: Subscribes to the InitializeResourceEvent.
• OnResourceEndpointsAllocatedonResourceEndpointsAllocated: Subscribes to the ResourceEndpointsAllocatedEvent event.
• OnConnectionStringAvailableonConnectionStringAvailable: Subscribes to the ConnectionStringAvailableEvent event.
• OnBeforeResourceStartedonBeforeResourceStarted: Subscribes to the BeforeResourceStartedEvent event.
• OnResourceReady / onResourceReady: Subscribes to the ResourceReadyEvent event.
• OnResourceStoppedonResourceStopped: Subscribes to the ResourceStoppedEvent event.

When the AppHost is run, by the time the Aspire dashboard is displayed, you should see the following log output in the console:

info: Aspire.Hosting.DistributedApplication[0]
      Aspire version: 13.6.0
info: Aspire.Hosting.DistributedApplication[0]
      Distributed application starting.
info: Aspire.Hosting.DistributedApplication[0]
      Application host directory is: ../AspireApp/AspireApp.AppHost
info: Program[0]
      1. OnInitializeResource
info: Program[0]
      2. OnResourceEndpointsAllocated
info: Program[0]
      3. OnConnectionStringAvailable
info: Program[0]
      4. OnBeforeResourceStarted
info: Aspire.Hosting.DistributedApplication[0]
      Now listening on: https://localhost:17222
info: Aspire.Hosting.DistributedApplication[0]
      Login to the dashboard at https://localhost:17222/login?t=<YOUR_TOKEN>
info: Program[0]
      5. OnResourceReady
info: Aspire.Hosting.DistributedApplication[0]
      Distributed application started. Press Ctrl+C to shut down.

टिप्पणी

Some events block execution. For example, when the BeforeResourceStartedEvent is published, the resource startup blocks until all subscriptions for that event on a given resource finish executing. Whether an event blocks or not depends on how you publish it (see the following section).

Publish events

When subscribing to any of the built-in events, you don’t need to publish the event yourself as the AppHost orchestrator manages to publish built-in events on your behalf. However, you can publish custom events with the eventing API. To publish an event, you have to first define an event as an implementation of either the IDistributedApplicationEvent or IDistributedApplicationResourceEvent interface. You need to determine which interface to implement based on whether the event is a global AppHost event or a resource-specific event.

Then, you can subscribe and publish the event by calling the either of the following APIs:

• PublishAsync<T>(T, CancellationToken): Publishes an event to all subscribes of the specific event type.
• PublishAsync<T>(T, EventDispatchBehavior, CancellationToken): Publishes an event to all subscribes of the specific event type with a specified dispatch behavior.

Provide an EventDispatchBehavior

When events are dispatched, you can control how the events are dispatched to subscribers. The event dispatch behavior is specified with the EventDispatchBehavior enum. The following behaviors are available:

• EventDispatchBehavior.BlockingSequential: Fires events sequentially and blocks until they’re all processed.
• EventDispatchBehavior.BlockingConcurrent: Fires events concurrently and blocks until they’re all processed.
• EventDispatchBehavior.NonBlockingSequential: Fires events sequentially but doesn’t block.
• EventDispatchBehavior.NonBlockingConcurrent: Fires events concurrently but doesn’t block.

The default behavior is EventDispatchBehavior.BlockingSequential. To override this behavior, when calling a publishing API such as PublishAsync, provide the desired behavior as an argument.

Eventing subscribers

In some cases, such as extension libraries, you may need to access lifecycle events from a service rather than directly from the Aspire application model. You can implement IDistributedApplicationEventingSubscriber and register the service with AddEventingSubscriber (or TryAddEventingSubscriber if you want to avoid duplicate registrations).

AppHost.cs

using Aspire.Hosting.Eventing;
using Aspire.Hosting.Lifecycle;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

var builder = DistributedApplication.CreateBuilder(args);

builder.Services.AddEventingSubscriber<LifecycleLoggerSubscriber>();

builder.Build().Run();

internal sealed class LifecycleLoggerSubscriber(ILogger<LifecycleLoggerSubscriber> logger)
    : IDistributedApplicationEventingSubscriber
{
    public Task SubscribeAsync(
        IDistributedApplicationEventing eventing,
        DistributedApplicationExecutionContext executionContext,
        CancellationToken cancellationToken)
    {
        eventing.Subscribe<BeforeStartEvent>((@event, ct) =>
        {
            logger.LogInformation("1. BeforeStartEvent");
            return Task.CompletedTask;
        });

        eventing.Subscribe<ResourceEndpointsAllocatedEvent>((@event, ct) =>
        {
            logger.LogInformation("2. {Resource} ResourceEndpointsAllocatedEvent", @event.Resource.Name);
            return Task.CompletedTask;
        });

        eventing.Subscribe<AfterResourcesCreatedEvent>((@event, ct) =>
        {
            logger.LogInformation("3. AfterResourcesCreatedEvent");
            return Task.CompletedTask;
        });

        return Task.CompletedTask;
    }
}

टिप्पणी

C# eventing subscriber service classes are specific to the C# AppHost hosting model. TypeScript AppHosts can register callback-based subscribers with addEventingSubscriber or tryAddEventingSubscriber.

apphost.mts

import { createBuilder } from './.aspire/modules/aspire.mjs';

const builder = await createBuilder();

builder.addEventingSubscriber(async (events) => {
  events.onBeforeStart(async () => {
    console.log('1. BeforeStartEvent');
  });

  events.onAfterResourcesCreated(async () => {
    console.log('3. AfterResourcesCreatedEvent');
  });
});

await builder.build().run();

The subscriber approach keeps builder code minimal while still letting you respond to the same lifecycle moments as inline subscriptions:

• AddEventingSubscriber<T>() (or TryAddEventingSubscriber()) ensures the subscriber participates whenever the AppHost starts.
• SubscribeAsync is called once per AppHost execution, giving you access to IDistributedApplicationEventing and the DistributedApplicationExecutionContext should you need model- or environment-specific data.
• You can register handlers for any built-in event (AppHost or resource) or for your own custom IDistributedApplicationEvent types.

Use this pattern whenever you previously relied on IDistributedApplicationLifecycleHook. The lifecycle hook APIs remain only for backward compatibility and will be removed in a future release.

Migrating from lifecycle hooks

If you’re migrating from the deprecated IDistributedApplicationLifecycleHook interface, use the following mapping:

Old pattern (deprecated)	New pattern
BeforeStartAsync()	Subscribe to BeforeStartEvent
AfterEndpointsAllocatedAsync()	Subscribe to ResourceEndpointsAllocatedEvent
AfterResourcesCreatedAsync()	Subscribe to AfterResourcesCreatedEvent
TryAddLifecycleHook<T>()	TryAddEventingSubscriber<T>()

Before (deprecated):

OldLifecycleHook.cs

public class MyHook : IDistributedApplicationLifecycleHook
{
    public Task AfterResourcesCreatedAsync(
        DistributedApplicationModel model,
        CancellationToken cancellationToken)
    {
        // Handle event
        return Task.CompletedTask;
    }
}

// Registration
builder.Services.TryAddLifecycleHook<MyHook>();

After (recommended):

NewEventingSubscriber.cs

public class MySubscriber : IDistributedApplicationEventingSubscriber
{
    public Task SubscribeAsync(
        IDistributedApplicationEventing eventing,
        DistributedApplicationExecutionContext context,
        CancellationToken cancellationToken)
    {
        eventing.Subscribe<AfterResourcesCreatedEvent>((@event, ct) =>
        {
            // Handle event using context.Model
            return Task.CompletedTask;
        });

        return Task.CompletedTask;
    }
}

// Registration
builder.Services.TryAddEventingSubscriber<MySubscriber>();

सावधानी

The IDistributedApplicationLifecycleHook interface is deprecated as of Aspire 9.0 and will be removed in a future release. Migrate to IDistributedApplicationEventingSubscriber for new code.

Additional events

Beyond the core lifecycle events, Aspire provides additional events for specific scenarios:

Publishing events

When publishing your application (generating deployment manifests), these events are raised:

Event	When raised	Purpose
BeforePublishEvent	Before publishing begins	Validate or modify resources before manifest generation
AfterPublishEvent	After publishing completes	Perform cleanup or post-publish actions

TypeScript

apphost.mts

import { createBuilder } from './.aspire/modules/aspire.mjs';

const builder = await createBuilder();

await builder.subscribeBeforePublish(async (event) => {
  const model = await event.model();
  console.log(`Publishing ${model.getResources().length} resources`);
});

await builder.subscribeAfterPublish(async (event) => {
  const services = await event.services();
  console.log('Publish completed', services);
});

C#

AppHost.cs

builder.OnBeforePublish((@event, ct) =>
{
    // Validate resources before publishing
    return Task.CompletedTask;
});

builder.OnAfterPublish((@event, ct) =>
{
    // Post-publish actions
    return Task.CompletedTask;
});

For details on what happens during publishing, see Publishing and deployment overview.

Resource stopped event

The ResourceStoppedEvent is raised once each time a resource stops execution. If a handler restarts the resource, the restart doesn’t raise another event for the same stop. The event is raised again only when the resource subsequently stops:

TypeScript

apphost.mts

import { createBuilder } from './.aspire/modules/aspire.mjs';

const builder = await createBuilder();

const cache = await builder.addRedis('cache');

await cache.onResourceStopped(async (event) => {
  const resource = await event.resource();
  console.log(`Resource ${resource.getResourceName()} stopped`);
});

C#

AppHost.cs

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

var builder = DistributedApplication.CreateBuilder(args);

var cache = builder.AddRedis("cache");

cache.OnResourceStopped(
    static (resource, @event, ct) =>
    {
        var logger = @event.Services.GetRequiredService<ILogger<Program>>();
        logger.LogInformation("Resource {Name} stopped", resource.Name);
        return Task.CompletedTask;
    });

builder.Build().Run();

टिप्पणी

Event publishing is synchronous and blocking — event handlers can delay further execution. Keep handlers lightweight and avoid long-running operations.

See also

• Custom resources
• Resource annotations