Can I do a swap on a Blazor Server App service without disconnecting users?

Sean Wauchope 30

I run an App Service hosting a Blazor Server Application. The App service has a staging and production slot. When I do a swap from staging to production I would like users to remain connected and have 0 downtime. Is this possible? The application also connects to Azure SignalR Service. Currently it takes around 2-4 minutes to complete the swap during which time the application is offline.

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ajkuma 22,241 Reputation points Microsoft Employee

2023-05-23T18:09:20.67+00:00

@Sean Wauchope ,

We have noticed that you recently had a negative experience on our Q&A community platform. We appreciate you taking the time to provide feedback and we would like to learn how we can improve to make your experience more positive.

Based on the detailed insights and suggestions shared by Bruce and Michael in their responses, we understand that you have created a support ticket for further investigation. We are here to assist you and are committed to enhancing your experience on our platform. Your feedback is highly valued and we eagerly await your response.

Thank you once again for your valuable time and feedback.

4 answers

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Bruce (SqlWork.com) 55,196 Reputation points

2023-05-21T15:20:37.3433333+00:00

No it’s not possible. All the state information is memory, and tied to the circuit (websocket connection). this is lost when you switch server process.

you can have the server persist state, and add reconnection logic to client app via JavaScript. See modify the reconnection handler

https://learn.microsoft.com/en-us/aspnet/core/blazor/fundamentals/signalr?view=aspnetcore-7.0
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Sean Wauchope 30 Reputation points

2023-05-22T17:19:02.33+00:00

That's a bummer. My goal is to prevent downtime.

Sean Wauchope 30 Reputation points

2023-05-22T18:00:24.8733333+00:00

From your link I see some configuration options that I could try.

Sean Wauchope 30 Reputation points

2023-05-22T18:05:13.4833333+00:00

Never mind it looks like these configurations are considered after a disconnect.
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Michael Washington 891 Reputation points MVP

2023-05-22T17:07:16.41+00:00

I do Azure swaps all the time. It does not take 2 minutes. It usually takes 30 seconds on a busy site. All my users stay connected ~~because an Azure swap is on the same process~~.
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Sean Wauchope 30 Reputation points

2023-05-22T17:16:20.6433333+00:00

Nice to know it works great for you. Are you also hosting a Blazor Server application? I do not have the same results as you running my Blazor Server Application. Initially when I deployed the application it would take less than 30 seconds as well but as of late it takes 2-4mins. Also swapping has always disconnected users on our Blazor Server Applicaton.

Michael Washington 891 Reputation points MVP

2023-05-22T17:18:52.0066667+00:00

Yes I am hosting Blazor server apps. Perhaps make your Azure app service plan bigger?

Sean Wauchope 30 Reputation points

2023-05-22T17:22:09.47+00:00

I think my plan should be good enough, see below. What plan are you on?

Michael Washington 891 Reputation points MVP

2023-05-22T17:27:28.8733333+00:00

Michael Washington 891 Reputation points MVP

2023-05-22T17:28:54.14+00:00

However, I am not using a SignalR service, I have simply enabled web sockets in General settings. Perhaps that is the difference.

Sean Wauchope 30 Reputation points

2023-05-22T17:33:09.3833333+00:00

Therefore the premium plan should be good enough to get those same swap results as a standard plan. Could be the SignalR Service that's causing it. It is recommended to use Azure SignalR Service when hosting Blazor Server Applications.

Michael Washington 891 Reputation points MVP

2023-05-22T17:50:48.6033333+00:00

Yes, it is recommended to use Azure SignalR Service when hosting Blazor Server Applications, however I have several large sites and have not needed it.

Michael Washington 891 Reputation points MVP

2023-05-22T17:52:23.84+00:00

Sean Wauchope 30 Reputation points

2023-05-22T18:05:56.5333333+00:00

Yes those are the settings that I use as well. They do not prevent the disconnect.

Michael Washington 891 Reputation points MVP

2023-05-22T18:11:40.8366667+00:00

Your users do stay logged in, correct? It is only the disconnect because the swap is taking so long?

Bruce (SqlWork.com) 55,196 Reputation points

2023-05-22T18:17:51.56+00:00

deployment swaps are not the same process. each slot is a separate instance of the site. during a swap, both slot sites are running, the load balancer, just changes which instance the requests are forwarded to. on a swap all websocket connections and all in-memory state info will be lost.

Sean Wauchope 30 Reputation points

2023-05-22T18:19:54.8+00:00

Correct, after the application is back online a reload/refresh keeps the user logged in. Actually there are times when the user even has to log back in.

Michael Washington 891 Reputation points MVP

2023-05-22T18:25:01.08+00:00

I guess because my swaps are fast and the users never have to log back in I never experinced them being 'disconnected'.

Sean Wauchope 30 Reputation points

2023-05-22T18:28:35.19+00:00

Your plan shows 4 cores so maybe that helps.

Sean Wauchope 30 Reputation points

2023-05-22T19:23:49.6566667+00:00

I've purchased the support plan and submitted an issue to Azure Support. I'll update here with the results.

ajkuma 22,241 Reputation points Microsoft Employee

2023-05-23T17:58:36.5766667+00:00

Sean, Apologies for my delayed response.

Thanks for sharing the updates. Yes, please let us know how it goes with the support.
If you need further assistance, please let me know the #SR number, I can track this internally.

We appreciate your patience and collaboration!

Sean Wauchope 30 Reputation points

2023-05-23T21:25:00.1133333+00:00

Hello ajkuma,

The SR# is is 2305220040008579. I was given a link to Best Practices https://docs.microsoft.com/en-us/azure/app-service/deploy-staging-slots#specify-custom-warm-up and that's about it. I asked to be called by phone so that I can speak to someone in person. I'm not sure these changes apply for Blazor Server. It mentions settings in web.config.

ajkuma 22,241 Reputation points Microsoft Employee

2023-05-24T15:21:57.89+00:00

Thanks for sharing the #SR info, I'll further follow-up on this internally.

Bruce (SqlWork.com) 55,196 Reputation points

2023-05-25T16:59:09.1333333+00:00

the support team can help with faster swap, but blazor server does not have support for connection recovery after the server connection is closed, either by timeout, app recycle, or server swap.

you can add javascript code on the client to detect the connection lost, and restart the blazor app - typically by a page reload. you can also add code to save state (typically using local storage) to code a more graceful recovery from lost connections.

Sean Wauchope 30 Reputation points

2023-05-30T18:08:30.6433333+00:00

Update from support:

"As part of the warm up process, the application will try to "preload" if it has preload enabled and there will be errors during swap if he has any mistakes in that configuration.

We do not recommend disabling preloading as it can make the application start slower, but you can check in your code if you have any preloading settings set as in the example below.

<?xml version="1.0"?>

<Configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">

<system.applicationHost>

<sites>

<site name="%XDT_SITENAME%" xdt:Locator="Match(name)">

<application xdt:Transform="SetAttributes(preloadEnabled)" preloadEnabled="false">

</application>

</site>

</sites>

</system.applicationHost>

</configuration> "

The only setting I know of that is close to that is serverprerender in _host file so I'll change that and see if it helps.

Sean Wauchope 30 Reputation points

2023-06-07T23:09:07.2433333+00:00

Update: I will be capturing a memory dump for Azure support team on my next swap.
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Bruce (SqlWork.com) 55,196 Reputation points

2023-06-02T17:29:03.8466667+00:00

You should code a re-connect solution if you don't want user downtime.

You will get the same issue if the user switches to another tab for a period of time. The browser will close the websocket (signal/r) connection. This after a server timeout will close the circuit. When the user switches back to the tab, you will have the same lost circuit error.
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Sean Wauchope 30 Reputation points

2023-06-02T18:52:00.4066667+00:00

I do not have a disconnection issue if users open another tab for any amount of time.

Bruce (SqlWork.com) 55,196 Reputation points

2023-06-03T03:41:18.6166667+00:00

It’s browser dependent. Mobile browsers are very aggressive, and chrome is getting more aggressive. Safari closes the connection at about 5 minutes as inactive tab.
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