Include session-affinity traffic in an Application Gateway draining test

Source facts

Application Gateway connection draining normally stops new requests to a deregistering backend while retaining existing connections until the configured timeout. Microsoft documents an exception: gateway-managed session affinity can continue sending requests to that backend. A gateway configuration update can also terminate ongoing connections when the draining timeout expires. Microsoft Learn.

Applicability

Use this review for an Application Gateway backend-removal or configuration-change plan. Identify whether the application uses the gateway’s affinity cookie, the affected backend setting, and the longest transfers that the application owner expects to preserve.

DSE recommendation

DSE recommends documenting separate expectations for a new client, an established transfer, and a returning affinity-bound client. Agree when the application team can shut down the removed backend. Choose the draining interval against the actual transfer requirement and current source limits, and retain a way to restore the approved backend membership if the rehearsal fails.

Verification

During an approved rehearsal, send those three traffic patterns while removing one test backend. Correlate request destinations and transfer completion with removal and timeout timestamps. Repeat the relevant configuration-update case. Record any request still reaching the old backend before declaring it ready for shutdown.

Official references

Microsoft Learn: Azure Application Gateway Backend Settings configuration.