Running Aspire and Seeing Connection failure from the App Host when starting up.

Question

Running Aspire and Seeing Connection failure from the App Host when starting up.

Trent Whiteley 0

My program.cs is below. When I start the app up with the container creation commented out everything comes up fine. Uncomment it and I get the error, Failed to connect to 127.0.0.1:9712. I know that is the Cosmos DB. The dashboard that comes up with Aspire shows that both the database and the container are healthy (see below). However, if I select the Data Explorer, it shows no container has been created and fails in the same way if I try to create a container manually.

User's image

using Projects;

var builder = DistributedApplication.CreateBuilder(args);

var secrets = builder.ExecutionContext.IsPublishMode

? builder.AddAzureKeyVault("secrets")

: builder.AddConnectionString("key-vault");

var insights = builder.AddConnectionString("myInsightsResource");

var storage = builder.AddAzureStorage("storage").RunAsEmulator()

.AddBlobs("blobs");

var cache = builder.AddAzureRedis("cache").RunAsContainer();

var sql = builder.AddSqlServer("sql")

.WithDataVolume()

.WithLifetime(ContainerLifetime.Persistent);

var sqlDb = sql.AddDatabase("sqldatabase");

var sqlDbManager = builder.AddProject<F22_Core_Data_SqlDatabaseManager>("sqldbmanager")

.WithReference(sqlDb)

.WaitFor(sqlDb);

#pragma warning disable ASPIRECOSMOSDB001

var cosmos = builder.AddAzureCosmosDB("cosmos-db")

.RunAsPreviewEmulator(p => p.WithDataExplorer().WithDataVolume().WithLifetime(ContainerLifetime.Persistent));

var tenantData = cosmos.AddCosmosDatabase("tenant")

.AddContainer("tenantData", "/id");

//var userData = cosmos.AddCosmosDatabase("user")

// .AddContainer("userData", "/id");

#pragma warning restore ASPIRECOSMOSDB001

var prototype = builder.AddProject<F22_Services_PrototypeService>("prototype")

.WithReference(insights)

.WithReference(cache)

.WaitFor(cache)

.WithReference(sqlDb)

.WaitFor(sqlDbManager)

.WaitFor(cosmos)

.WithReference(cosmos);

builder.AddProject<F22_Client_Web>("webfrontend")

.WithExternalHttpEndpoints()

.WithReference(storage)

.WithReference(secrets)

.WithReference(cache)

.WaitFor(cache)

.WithReference(prototype)

.WaitFor(prototype)

.WithReference(insights);

builder.Build().Run();

Anonymous

2025-06-26T18:39:35.95+00:00
Hi Trent Whiteley

Greeting!

One common cause of connection failures from the App Host during startup is related to misconfigured or missing dependencies, especially when working with services like Azure Cosmos DB.

In one reported case, the failure occurred when a Cosmos DB container was being created at startup. The error was: "Failed to connect to 127.0.0.1:9712", which pointed to the local Cosmos DB emulator. Interestingly, the Aspire dashboard showed the database and container as healthy, but the Data Explorer revealed that no container had actually been created.

Here are a few things you can try:

Comment out container creation: If you’re programmatically creating containers in your Program.cs, try commenting out those lines to see if the app starts cleanly.

Check port conflicts: Ensure that nothing else is using the same port (like 9712 for Cosmos DB). Tools like Fiddler or other proxies can sometimes interfere.

Verify service readiness: Use .WaitFor(...) on dependent services to make sure they’re fully initialized before your app tries to connect.

Inspect connection strings: A missing or malformed connection string (like ConnectionStrings__Messaging) can also cause failures. I hope this helps, please let me know if need any further assistance we are happy to help.

1 answer

Your answer

Anonymous

2025-06-26T18:39:35.95+00:00

Hi Trent Whiteley

Greeting!

One common cause of connection failures from the App Host during startup is related to misconfigured or missing dependencies, especially when working with services like Azure Cosmos DB.

In one reported case, the failure occurred when a Cosmos DB container was being created at startup. The error was: "Failed to connect to 127.0.0.1:9712", which pointed to the local Cosmos DB emulator. Interestingly, the Aspire dashboard showed the database and container as healthy, but the Data Explorer revealed that no container had actually been created.

Here are a few things you can try:

Comment out container creation: If you’re programmatically creating containers in your Program.cs, try commenting out those lines to see if the app starts cleanly.

Check port conflicts: Ensure that nothing else is using the same port (like 9712 for Cosmos DB). Tools like Fiddler or other proxies can sometimes interfere.

Verify service readiness: Use .WaitFor(...) on dependent services to make sure they’re fully initialized before your app tries to connect.

Inspect connection strings: A missing or malformed connection string (like ConnectionStrings__Messaging) can also cause failures. I hope this helps, please let me know if need any further assistance we are happy to help.

Answer 1

Trent Whiteley 0

As my question stated, everything comes up fine if I don't create a container at startup. However, bringing up the Cosmos dashboard and attempting to manually create a container fails with the same message. This was all running fine before the 9.3.1 Aspire update after which it started failing. I've already checked and there are no other processes using that port and the AppHost is already waiting on the Cosmos DB to start up.

Anonymous

2025-06-30T22:18:00.21+00:00
Hi Trent Whiteley

Thanks for the detailed context — it sounds like you're encountering a regression or compatibility issue introduced with the Aspire 9.3.1 update that's affecting Cosmos DB container creation at startup or via the dashboard.

Here are a few focused steps and considerations to help troubleshoot and potentially resolve the issue:

🧪 1. Check for Breaking Changes in Aspire 9.3.1

Review the Aspire 9.3.1 release notes or changelog for any breaking changes related to:

Cosmos DB integration

Container lifecycle management Startup sequencing or dependency injection

⚙️ 2. Verify Cosmos DB Emulator or Local Instance

If you're using the Cosmos DB Emulator locally:

Ensure it's running with admin privileges

Confirm it's listening on the correct port (usually https://localhost:8081)

Check for certificate issues — the emulator uses a self-signed cert that might need to be re-trusted after updates

You can test connectivity with:

bash

curl -k https://localhost:8081/_explorer/emulator.pem

🧵 3. Inspect AppHost and Service Dependencies

Since you mentioned AppHost is waiting on Cosmos DB:

Double-check the apphost.appsettings.json or aspire.app.json (depending on your setup) for correct service bindings

Ensure Cosmos DB is marked as a required dependency and that the container creation logic is not racing ahead of the emulator or service being ready

🛠️ 4. Enable Verbose Logging

Add detailed logging to your startup logic:

csharp

builder.Logging.SetMinimumLevel(LogLevel.Debug);

Then inspect logs for:

Timeout errors

Unauthorized access

Port binding issues

Any HttpRequestException or SocketException

🧼 5. Clear Aspire Dev Environment Cache

Sometimes Aspire caches stale service state:

bash

dotnet aspire clean

Then rebuild and restart:

bash

dotnet aspire run

🧪 6. Try Manual Container Creation via SDK

As a workaround, try creating the container manually using the Cosmos DB SDK:

csharp

await cosmosClient.GetDatabase("MyDb") .CreateContainerIfNotExistsAsync("MyContainer", "/partitionKey");

This can help isolate whether the issue is with the dashboard UI or the underlying SDK/API.

🧭 7. Check for Port Conflicts or Firewall Rules

Even if no process is using the port, Aspire might be blocked by:

Windows Defender or third-party firewalls

Conflicting Docker containers (if using containerized Aspire services)Thanks for the detailed context — it sounds like you're encountering a regression or compatibility issue introduced with the Aspire 9.3.1 update that's affecting Cosmos DB container creation at startup or via the dashboard. I hope this helps, please let me know if you need any further assistance we are happy to help.
Anonymous

2025-07-01T09:07:48.17+00:00

Hi Trent Whiteley

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
Anonymous

2025-07-02T04:16:28.5766667+00:00

Hi Trent Whiteley

We haven’t heard from you on the last response and was just checking back to see if you have a resolution yet. In case if you have any resolution please do share that same with the community as it can be helpful to others. Otherwise, will respond with more details and we will try to help.
Daniel Saner 45 Reputation points

2026-01-14T16:46:24.9866667+00:00

I'm sorry, but these LLM answers are less than useless… we can all copy and paste our problems into ChatGPT. We come to these forums because the obvious approaches didn't solve our problem.

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Running Aspire and Seeing Connection failure from the App Host when starting up.

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