[Application Gateway] Downtime of Backend pool update

@김세형

Thank you for reaching out.

I understand you have questions about downtime for Application Gateway when the backend pool members are updated.

Based on your questions above.

Is there any effect if the client requests during updating backend pool.

There should be no disruption if Connection Draining feature is enabled on your application gateway.

Azure Application gateway supports a connection draining feature which can be enabled via HTTP settings. You can set up connection draining to change members within a backend pool without disruption.

Connection draining helps you achieve graceful removal of backend pool members during planned service updates. Once enabled, the application gateway ensures all deregistering instances of a backend pool don't receive any new requests while allowing existing requests to complete within a configured time limit. It applies to cases where backend instances are explicitly removed from the backend pool after a configuration change by a user. Connection draining is invoked for every single update to the gateway.

The only exception is when requests continue to be proxied to the deregistering instances because of gateway-managed session affinity.

For both your questions below.

First, Path based rules are created with backend pool on routing rule. How is the downtime when changing the backend target of the existing rule?

Second, How is the downtime when adding or removing the backend target on backend pool.

As Connection draining is invoked for every single update to the gateway then there should be no disruption/downtime to App Gateway.

Although it is highly recommended to make changes to Application gateway during a maintenance window to avoid any unforeseen issues/events.

Additional reference:
https://learn.microsoft.com/en-us/azure/application-gateway/application-gateway-faq#does-application-gateway-support-connection-draining

Please let me know if you have any additional questions/ concerns. Thank you!

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