Azure App Service
Azure App Service is a service used to create and deploy scalable, mission-critical web apps.
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The site warm-up request failed with response code: 'ServiceUnavailable'.
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細井勉
0
Reputation points
When deploying a project from Visual Studio Professional 2022 (64-bit) version 17.13.3 to a deployment slot with the following settings:
Configuration: Release
Target Framework: .NET 9.0
Deployment Mode: Framework
Target Runtime: win-x64
The message "Web App has been successfully published" appears, but shortly after, the message "The site warm-up request failed with response code: 'ServiceUnavailable'" almost always appears.
If I attempt the deployment again without giving up, this message does not appear the second time. In other words, every time I deploy, I need to attempt the deployment twice for it to be successfully completed.
It seems there might be a timing issue with the site's warm-up process. If there are any solutions for improvement, I would appreciate your guidance.
Azure App Service
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Bhargavi Naragani 5,270 Reputation points Microsoft External Staff Moderator2025-06-09T08:16:06.6033333+00:00 Hi @細井勉,
Can you please confirm whether you're deploying in Azure App Service?
If you are using Azure App Service, please check the private message and provide necessary details over there - To conduct a thorough investigation of this issue.
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Bhargavi Naragani 5,270 Reputation points Microsoft External Staff Moderator
2025-06-09T11:47:34.3033333+00:00 Hi @細井勉,
It seems like after deploying your .NET 9.0 app (Release, win-x64, Framework-dependent) to an Azure App Service deployment slot from Visual Studio 2022 (v17.13.3), you see "Web App has been successfully published", but then quickly get: "The site warm-up request failed with response code: 'ServiceUnavailable'". This usually happens only on the first deployment attempt. A second attempt usually completes without the warm-up error.
This behavior typically points to a race condition during the app warm-up process, where the app service slot isn’t quite ready to respond to HTTP requests right after deployment. This can be triggered by Cold start delay (especially in deployment slots not always running), Application initialization time being longer than expected, AppSettings or startup tasks that delay the site from becoming available.
In the case of .NET 9.0, being a newer target framework, it may also introduce slightly longer cold starts especially when framework-dependent deployment is used (i.e., the runtime is expected to be on the server).
You can explicitly configure the app to wait for the application to fully initialize before Azure considers it “ready”.
Add the following to your
web.config:<system.webServer> <applicationInitialization doAppInitAfterRestart="true" /> </system.webServer>This ensures Azure waits until the app is initialized before accepting traffic, useful during warm-up. https://learn.microsoft.com/en-us/iis/configuration/system.webserver/applicationinitialization/#configuration-sample
If you're deploying to a staging slot or your app has idle periods, enabling “Always On” will prevent cold starts and help reduce warm-up delays. In the Azure Portal, go to your App Service => Configuration => General settings. Set “Always On” = On. https://learn.microsoft.com/en-us/azure/app-service/configure-common?tabs=portal#configure-general-settings
You can also define a warm-up endpoint (e.g.,
/warmup) in your app that Azure pings to validate the service is live. This is particularly useful when combined with App Init or custom health checks. https://learn.microsoft.com/en-us/azure/app-service/deploy-staging-slots?tabs=portal#specify-custom-warm-upIf you are deploying to slots, ensure that app settings like connection strings or keys marked as “slot settings” do not delay startup by forcing resets or re-authentication. https://learn.microsoft.com/en-us/azure/app-service/deploy-staging-slots?tabs=portal#add-a-slot
Hope this information is helpful, if you have any further concerns or queries, please feel free to reach out to us.
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Sampath 3,750 Reputation points Microsoft External Staff Moderator2025-06-10T16:54:41.59+00:00 Hello @細井勉 ,
Thank you for the update, and I'm glad to hear the issue has been resolved!
To summarize the solution for others who might face the same problem:
Problem:
After deploying a .NET 9.0 app to an Azure App Service slot via Visual Studio 2022, the site reported:
"The site warm-up request failed with response code: 'ServiceUnavailable'"
This occurred consistently during the first deployment attempt, though the second usually succeeded.
Root Cause:
This behavior is often caused by a race condition during the app warm-up phase — the app may not be fully ready to accept HTTP requests when Azure considers the slot "live." This is especially likely in:
- Cold start scenarios
- Slots with longer app initialization times
- Framework-dependent deployments (like .NET 9.0)
Resolution:
To ensure the app is fully warmed up before the slot is considered live, the following App Settings were added to the deployment slot:
| Name | Value |
| ----------------------------------- | -------------- |
|
WEBSITE_SWAP_WARMUP_PING_PATH|/statuscheck||
WEBSITE_SWAP_WARMUP_PING_STATUSES|200|These settings configure Azure to ping the
/statuscheckendpoint and only consider the app ready if the response status code is200.For more info, see:
A Subtle Gotcha with Azure Deployment Slots and ASP.NET Core
If anyone else is experiencing a similar issue, you may also want to ensure:
- Always On is enabled in App Service settings
- Application Initialization is configured in
web.config(for IIS-based hosting)
Hope it helps!
Please remember to click "Accept the answer” and
Yesso this can benefit other community members. -
細井勉 0 Reputation points
2025-06-11T23:57:31.38+00:00 I followed Naragani's advice and created the web.config, but the issue was not resolved. AlwaysOn has been enabled from the beginning. I will now try Sampath's advice.
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Sampath 3,750 Reputation points Microsoft External Staff Moderator
2025-06-12T04:31:35.4133333+00:00 Hi @細井勉, Thank you for your follow-up and patience. Please try to get back to us. Thank you.
WEBSITE_SWAP_WARMUP_PING_STATUSES: Valid HTTP response codes for the warm-up operation. Add this app setting with a comma-separated list of HTTP codes. An example is200,202. If the returned status code isn’t in the list, the warmup and swap operations are stopped. By default, all response codes are valid.Seems straightforward. So I set something up like this:
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WEBSITE_SWAP_WARMUP_PING_PATH:/statuscheck -
WEBSITE_SWAP_WARMUP_PING_STATUSES:200
Refer to this DOC for A Subtle Gotcha with Azure Deployment Slots and ASP .NET Core .
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細井勉 0 Reputation points
2025-06-13T00:44:48.14+00:00 Following Sampath's advice, I added the environment variables
WEBSITE_SWAP_WARMUP_PING_PATH: /statuscheckandWEBSITE_SWAP_WARMUP_PING_STATUSES: 200to the Deploy slot settings, and it seems to have fixed the issue. Thank you, Sampath and Naragani! -
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細井勉 0 Reputation points
2025-06-16T07:09:10.3833333+00:00 I wrote, "It seems the issue has been resolved," but it turns out the problem still persists.
Currently, I have:
Created the web.config file
Enabled AlwaysOn
Added the environment variables WEBSITE_SWAP_WARMUP_PING_PATH: /statuscheck and WEBSITE_SWAP_WARMUP_PING_STATUSES: 200 to the deployment slot settings
However, I still see the following message at first: "The site warm-up request failed with response code: 'ServiceUnavailable'."
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細井勉 0 Reputation points
2025-06-17T23:44:38.36+00:00 Yesterday, I wrote: "I wrote, 'it turns out the problem still persists." However, when I tried deploying this morning, the site warm-up request succeeded. I would like to monitor the situation for a little while longer.
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