How to enhance Azure Event Grid Tutorial to accommodate App Service Auto Scaler?

Siegfried Heintze 1,861

I've been studying https://learn.microsoft.com/en-us/training/modules/azure-event-grid/8-event-grid-custom-events and https://github.com/Azure-Samples/azure-event-grid-viewer and I'm wondering if this sample razor app would run in an App Service Scale Set (or auto scaler).

Do you think it would run correctly? Tell me why!

I'm thinking it would not run correctly because the when the event grid makes the HTTP/REST call to the razor app, the load balancer would route it to a single instance of the razor app (instead of all the instances of the razor app) and only those users being routed to the same instance would see the dynamically changing display.

So how would you fix this so all the users would see the updated display regardless of which instance of the web app they get routed to?

The reason I'm asking is because I want to implement an Azure Cosmos DB sql change feed that would call the event grid to (indirectly) dynamically update the HTML table (or better yet, quick grid) display (in real time) of my blazor server web app as other users enter data.

In response to VenkateshDodda-MSFT's Feb 9 comment:

The links point to a tutorial containing a razor app that includes a simple web page and a simple API. For the tutorial one sends an event to the event grid (containing some JSON) using bash and CURL and the event grid forwards this to the razor app's REST API which uses signalR (websockets, I think) to forward the event to the browser resident javascript which displays the resulting data without requiring the user to refresh the page.

How would this work with a load balancer and several instances of the razor app (randomly being created and destroyed by the auto scaler) inside an app service scale set with a load balancer randomly selecting an instance of the razor app web server for each client's browsers HTTP request?

Does this help explain my question?

Thanks

Siegfried

VenkateshDodda-MSFT 18,441 Reputation points Microsoft Employee

2023-02-09T10:06:05.1366667+00:00

@Siegfried Heintze Thank you for reaching out to Microsoft Q&A.

From Webapp standpoint of view, Azure web app content is stored on Azure file system Storage and is surfaced up in a durable manner as a content share. For each instance, the sandbox implements a dynamic symbolic link in kernel mode which maps d:\home to the web app content home directory. No matter how many instances the web app is deployed on, each can access their home directory using 'd:\home'.

I have also added additional tags to receive insights from the targeted SMEs and also could you please help share more information about your requirement regarding the Azure Cosmos DB.
VenkateshDodda-MSFT 18,441 Reputation points Microsoft Employee

2023-02-13T10:42:15.1866667+00:00

@Siegfried Heintze Thanks for sharing more information. The number of instances in app service doesn't matters for in this scenario since the incoming requests will be served by any one of the instances.
These things happen at backend, since app services is platform as service even the request from the razor app or through multiple client the requests will be processed.
Siegfried Heintze 1,861 Reputation points

2023-02-14T03:46:04.7666667+00:00

VenkateshDoda:

I'm reading https://learn.microsoft.com/en-us/aspnet/signalr/overview/getting-started/introduction-to-signalr#what-is-signalr where it says:

SignalR handles connection management automatically, and lets you broadcast messages to all connected clients simultaneously, like a chat room.

So are you telling me that even though my browser is being serviced by web app instance #1 and instance #2 of the web app gets the REST call from the event grid, the signal R inside webapp instance #2 will know the IP address of my browser and create a web socket connection to the javascript in my browser and send the appropriate update so my HTML DOM is updated appropriately?

How would it do that if instance #2 had never serviced my browser before?

Siegfried