App Service Zone Redundancy Support And Possible Zone Redundancy Related Issues

• Current Requirements/Limitations for Enabling Zone Redundancy On Azure App Service
  

App Service Support for Availability Zones

• App Service Possible Issues and Solutions Related with Zone Redundancy
  

Scenario 1

Symptom: Web sockets error when accessing an App service web app in an app service plan which has enabled Zone Redundancy.

Error: Failed to start the transport 'WebSockets': Error: There was an error with the transport.

Root Cause: This issue happened when these 3 conditions are met.

• Zone Redundancy enabled.
  
• Web Sockets set to on (in App service Settings -> Configuration -> General Settings)
  
• ARR affinity set to off (in App service Settings -> Configuration -> General Settings)
  

Solution: Enable ARR affinity for the app service

Scenario 2

Symptom: Failed to scale down a Linux App service plan --e.g. from P1V3 to P1V2, it failed with message ---You cannot change the OS hosting your app at this time. Please recreate your app with the desired OS

Network trace shows--- http 400 bad request error when sending the PUT request to https://management.azure.com/

Root Cause: App Service Plan with fewer than 3 workers are not allowed for zone redundancy. Requested number of workers: 2

App service plan has Zone redundancy enabled with autoscale configured with an Auto Scale out rule, setting the minimum and default instance count to be less than 3.

Solution: Update minimum worker rule in Auto Scale to 3 or use Manual Scale

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