Add TrySubscribeOnce API in IDistributedApplicationEventing #9461

Copilot · 2025-05-22T03:28:55Z

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Updated Design for TrySubscribeOnce API

This issue is to introduce idempotent subscription support to eventing, similar to TryAddLifecycleHook<T>, but for distributed application events.

Proposed API: TrySubscribeOnce

This API will provide both resource and non-resource variants, mirroring the existing Subscribe methods. The callback uses Func<T, CancellationToken, Task>, and the API is generic for event type consistency.

Non-resource event subscription
bool TrySubscribeOnce<T>(
    object key,
    Func<T, CancellationToken, Task> callback,
    [NotNullWhen(true)] out DistributedApplicationEventSubscription? subscription
) where T : IDistributedApplicationEvent;

// Overload: uses the IDistributedApplicationEventing instance as the key
bool TrySubscribeOnce<T>(
    Func<T, CancellationToken, Task> callback,
    [NotNullWhen(true)] out DistributedApplicationEventSubscription? subscription
) where T : IDistributedApplicationEvent;
Resource-scoped event subscription
bool TrySubscribeOnce<T>(
    IResource resource,
    object key,
    Func<T, CancellationToken, Task> callback,
    [NotNullWhen(true)] out DistributedApplicationEventSubscription? subscription
) where T : IDistributedApplicationResourceEvent;

// Overload: uses the IDistributedApplicationEventing instance as the key
bool TrySubscribeOnce<T>(
    IResource resource,
    Func<T, CancellationToken, Task> callback,
    [NotNullWhen(true)] out DistributedApplicationEventSubscription? subscription
) where T : IDistributedApplicationResourceEvent;
Behavior

The key can be any object. This enables idempotency per logical subscription and supports both arbitrary keys and the "type as key" lifecycle hook pattern.

If a subscription with the key already exists, the call is a no-op, returns false, and the out var is null.

If a subscription is added, the call returns true and the out var contains the subscription.

The [NotNullWhen(true)] attribute is used on the out parameter for better nullability analysis.

Reusing a key is expected and safe; this is the purpose of the API.

No migration or removal of lifecycle hook usage will be performed in this PR.

Rationale

Mirrors the existing Subscribe API (generic, resource/non-resource, async callback).

Makes it easy to enforce "subscribe once" semantics.

Flexible: supports both "type as key" and arbitrary key scenarios.

This issue description was updated to include the detailed API design and the use of [NotNullWhen(true)] for the out subscription parameter.

Fixes #9042.

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mitchdenny · 2025-05-22T03:38:03Z

@copilot try again, it looks like you may have had a transient error.

Co-authored-by: mitchdenny <513398+mitchdenny@users.noreply.github.com>

Copilot · 2025-05-22T04:01:32Z

@copilot try again, it looks like you may have had a transient error.

I've implemented the TrySubscribeOnce API in the DistributedApplicationEventing interface and class as requested. The implementation adds support for both resource and non-resource variants with appropriate key-based idempotency checks. I also added tests to verify the functionality works correctly. See commit eb4c4e8.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

dnceng.pkgs.visualstudio.com
- Triggering command: /home/REDACTED/work/aspire/aspire/.dotnet/dotnet test tests/Aspire.Hosting.Tests/Aspire.Hosting.Tests.csproj --filter FullyQualifiedName~TrySubscribeOnce -v n (dns block)
pkgs.dev.azure.com
- Triggering command: /home/REDACTED/work/aspire/aspire/.dotnet/dotnet test tests/Aspire.Hosting.Tests/Aspire.Hosting.Tests.csproj --filter FullyQualifiedName~TrySubscribeOnce -v n (dns block)
- Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

mitchdenny · 2025-05-22T04:50:58Z

@copilot add default implementations on the interfaces which throw NotImplementedException.

Copilot

Pull Request Overview

Add idempotent subscribe-once support for distributed application events via new TrySubscribeOnce API overloads and verify their behavior through unit tests.

Introduce four new TrySubscribeOnce overloads in IDistributedApplicationEventing for non-resource and resource events, with and without custom keys.
Implement subscription deduplication in DistributedApplicationEventing using internal dictionaries keyed by event type, resource, and user-provided key.
Add unit tests in DistributedApplicationBuilderEventingTests.cs to confirm that only the first subscription attempt succeeds and callbacks fire as expected.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Aspire.Hosting/Eventing/IDistributedApplicationEventing.cs	Added `TrySubscribeOnce` method signatures with `[NotNullWhen(true)]` out parameters and overloads for resources
src/Aspire.Hosting/Eventing/DistributedApplicationEventing.cs	Implemented each `TrySubscribeOnce` overload, managing subscription dictionaries to enforce idempotency
tests/Aspire.Hosting.Tests/Eventing/DistributedApplicationBuilderEventingTests.cs	Added tests to assert that `TrySubscribeOnce` only succeeds once per key (and fires callbacks correctly)

Comments suppressed due to low confidence (2)

src/Aspire.Hosting/Eventing/DistributedApplicationEventing.cs:129

The Unsubscribe method removes the subscription from the event lists but does not remove it from _subscriptionKeyLookup or _resourceSubscriptionKeyLookup, preventing future TrySubscribeOnce calls with the same key from succeeding after an unsubscribe. Consider removing these entries when unsubscribing.

public void Unsubscribe(DistributedApplicationEventSubscription subscription)

tests/Aspire.Hosting.Tests/Eventing/DistributedApplicationBuilderEventingTests.cs:423

There are no tests covering the behavior of resubscribing after calling Unsubscribe. Consider adding test(s) to verify that TrySubscribeOnce returns true again once a previous subscription is unsubscribed.

Copilot · 2025-05-22T04:53:23Z

src/Aspire.Hosting/Eventing/DistributedApplicationEventing.cs

+
+        if (_eventSubscriptionListLookup.TryGetValue(typeof(T), out var subscriptions))
+        {
+            subscriptions.Add(subscription);


Adding to the List without synchronization can cause race conditions under concurrent subscriptions. Consider using a thread-safe collection or locking around list modifications.

@copilot can you look at this.

…stributedApplicationEventing interface Co-authored-by: mitchdenny <513398+mitchdenny@users.noreply.github.com>

Copilot · 2025-05-22T05:05:06Z

@copilot add default implementations on the interfaces which throw NotImplementedException.

I've added default implementations that throw NotImplementedException to all methods in the IDistributedApplicationEventing interface as requested. See commit cafe9c7.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

dnceng.pkgs.visualstudio.com
- Triggering command: ./.dotnet/dotnet test tests/Aspire.Hosting.Tests/Aspire.Hosting.Tests.csproj --filter FullyQualifiedName~TrySubscribeOnce -v n (dns block)
pkgs.dev.azure.com
- Triggering command: ./.dotnet/dotnet test tests/Aspire.Hosting.Tests/Aspire.Hosting.Tests.csproj --filter FullyQualifiedName~TrySubscribeOnce -v n (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

mitchdenny · 2025-05-23T07:02:10Z

src/Aspire.Hosting/DistributedApplicationBuilder.cs

+                _innerBuilder.Services.AddSingleton<DashboardInjectionService>();
+                Eventing.Subscribe<BeforeStartEvent>((e, ct) =>
+                {
+                    var injectionService = e.Services.GetRequiredService<DashboardInjectionService>();
+                    injectionService.InjectDashboard(e.Model);
+                    return Task.CompletedTask;
+                });
+


@davidfowl looking at this change I'm not entirely certain we should get rid of IDistributedApplicationLifecycleHook completely. There are instances where that approach works really well (such as these global things like injecting the dashboard).

I think where TrySubscribeOnce comes in handy is more for hosting integrations where you may have multiple of a resource.

Copilot · 2025-05-23T07:08:51Z

@copilot add default implementations on the interfaces which throw NotImplementedException.

The IDistributedApplicationEventing interface already has default implementations that throw NotImplementedException for all methods. This was implemented in commit cafe9c7. All methods in the interface have bodies that throw NotImplementedException.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

dnceng.pkgs.visualstudio.com
- Triggering command: ./.dotnet/dotnet test tests/Aspire.Hosting.Tests/Aspire.Hosting.Tests.csproj --filter FullyQualifiedName~TrySubscribeOnce -v n (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to my firewall allow list

Initial plan for issue

d973406

Copilot AI assigned Copilot and mitchdenny May 22, 2025

Copilot started work on behalf of mitchdenny May 22, 2025 03:28 View session

github-actions bot added the needs-area-label label May 22, 2025

Copilot finished work on behalf of mitchdenny May 22, 2025 03:34

Copilot started work on behalf of mitchdenny May 22, 2025 03:38 View session

Implement TrySubscribeOnce API in DistributedApplicationEventing

eb4c4e8

Co-authored-by: mitchdenny <513398+mitchdenny@users.noreply.github.com>

Copilot finished work on behalf of mitchdenny May 22, 2025 04:02

Copilot started work on behalf of mitchdenny May 22, 2025 04:51 View session

mitchdenny marked this pull request as ready for review May 22, 2025 04:51

Copilot AI review requested due to automatic review settings May 22, 2025 04:51

mitchdenny self-requested a review as a code owner May 22, 2025 04:51

mitchdenny changed the title ~~[WIP] Add TrySubscribeOnce API in IDistributedApplicationEventing~~ Add TrySubscribeOnce API in IDistributedApplicationEventing May 22, 2025

Copilot AI reviewed May 22, 2025

View reviewed changes

Add default implementations that throw NotImplementedException in IDi…

cafe9c7

…stributedApplicationEventing interface Co-authored-by: mitchdenny <513398+mitchdenny@users.noreply.github.com>

Copilot finished work on behalf of mitchdenny May 22, 2025 05:05

Copilot started work on behalf of mitchdenny May 22, 2025 05:05 View session

Copilot finished work on behalf of mitchdenny May 22, 2025 05:11

Example of changing from a lifecycle hook to a event.

79a515a

mitchdenny reviewed May 23, 2025

View reviewed changes

Copilot started work on behalf of mitchdenny May 23, 2025 07:02 View session

Copilot finished work on behalf of mitchdenny May 23, 2025 07:18

danmoseley added area-meta and removed needs-area-label labels May 27, 2025

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Add TrySubscribeOnce API in IDistributedApplicationEventing #9461

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Add TrySubscribeOnce API in IDistributedApplicationEventing #9461

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Copilot AI commented May 22, 2025

Updated Design for TrySubscribeOnce API

Proposed API: TrySubscribeOnce

Non-resource event subscription

Resource-scoped event subscription

Behavior

Rationale

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Pull Request Overview

Reviewed Changes

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Proposed API: `TrySubscribeOnce`

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