Why downtime during app service scale up and down? RRS feed

  • Question

  • Our sites are running on a Standard App Service Plan with 2 instances.

    When we scale up or down, we experience about 5 minutes of downtime.

    Why is this?

    I thought azure is supposed to scale for me with no down time if I'm running more than 1 instance.

    Thursday, June 16, 2016 9:47 PM

All replies

  • Hello Jacques,

    Thank you for posting here!

    Do you receive any error messages?

    If the “Always On” is not turned On, I would suggest you to turn On the “Always On” feature and see if that helps.  

    Always On. By default, web apps are unloaded if they are idle for some period of time. This lets the system conserve resources. In Basic or Standard mode, you can enable Always On to keep the app loaded all the time. If your app runs continuous web jobs, you should enable Always On, or the web jobs may not run reliably. Please check the documentation for more information: Application settings

    Moving an ASP from one worker pool will cause cold starts of the apps in that ASP. This can cause requests to run slowly as your app is cold started on the new compute resources. The cold start can be avoided by using the application warm up capability in Azure App Service. The Application Initialization module described in the article also works for cold starts because the initialization process is also invoked when apps are cold started on new compute resources. Reference


    Friday, June 17, 2016 3:28 PM
  • Hi there 

    When Azure WebApp do a scale up or down it warm up an instances that its about to scale to

    so it supposed to be 99.9 available .

    can you please give more details about cpu usage and memory 


    Sunday, June 19, 2016 9:44 AM
  • Hello,

    I seem to have the same issue. When I scale up my app service through the portal, I can see the new instance being started in the live stream view of application insights. I also see the Application Initialization starting and warming up my site.

    But the traffic is immediately redirected to the new node, with very slow requests 'downtime' as result.

    I though the traffic is only redirected when the startup task specified in he Application Initialization is finished. My warmup task takes +/- 5 min.

    What do I miss?

    Kind regards,


    • Edited by euhm Thursday, September 14, 2017 8:13 PM Added timing warmup task.
    Thursday, September 14, 2017 8:05 PM
  • I believe the Always On feature is turned On/enabled, do confirm on this.


    To highlight, the home directory contains an app's content, and application code can write to it. If an app runs on multiple instances, the home directory is shared among all instances so that all instances see the same directory. So, for example, if an app saves uploaded files to the home directory, those files are immediately available to all instances.


    There is a discussion on a similar query here, you may look into that for details.


    Do click on "Mark as Answer" on the post that helps you, this can be beneficial to other community members.

    Tuesday, September 19, 2017 9:30 PM
  • I have the same issue when scale up or down of app service plan, result in server outage. However, scale out and in work well.
    Wednesday, November 15, 2017 1:00 AM
  • We seamless reroute requests to a different set of instances. A few first requests may run a bit slower. Ensure that Always On is enabled.
    You could refer the documentation Enable diagnostics logging for web apps in Azure App Service for reviewing the logs and also the application code.


    Do click on "Mark as Answer" on the post that helps you, this can be beneficial to other community members.

    Friday, November 17, 2017 6:22 PM
  • For anyone coming across this issue in the future, web apps aren't warmed up during scale up/down, only during scaling in out.

    Here's a link confirming this: https://social.msdn.microsoft.com/Forums/azure/en-US/4d5f9dd1-6459-40d2-8fc3-9bfe6b8efd92/scaling-updown-doesnt-wait-for-application-initialization-to-finish?forum=windowsazurewebsitespreview

    Here's a feedback link to request this feature be added to azure web app: https://feedback.azure.com/forums/169385-web-apps/suggestions/33580975-add-application-initialization-support-for-scale-u

    Saturday, March 17, 2018 3:54 PM