Merge pull request #299889 from jaesoni/sse-preview

Sse preview

Author: PMEds28

articles/application-gateway/toc.yml

@@ -289,6 +289,8 @@
       href: ../reliability/migrate-app-gateway-v2.md?toc=/azure/application-gateway/toc.json
   - name: Proxy buffer configuration
     href: proxy-buffers.md
+  - name: Using Server-sent events
+    href: use-server-sent-events.md    
   - name: Custom error pages
     href: custom-error.md
   - name: Migrate to v2 SKU

articles/application-gateway/use-server-sent-events.md

@@ -0,0 +1,48 @@
+---
+title: Using Server-sent events with Application Gateway (Preview)
+description: This article provides guidance for using Server-sent events with your Azure Application Gateway.
+services: application-gateway
+author: jaesoni
+ms.service: azure-application-gateway
+ms.topic: concept-article
+ms.date: 05/15/2025
+ms.author: jaysoni
+---
+
+# Using Server-sent events with Application Gateway (Preview)
+
+Azure Application Gateway offers support for Server-sent events (SSE). This document provides guidelines to ensure the seamless operation of this feature when used with Azure Application Gateway.
+
+> [!Note]
+> - The SSE support with Azure Application Gateway is currently in Preview phase.
+> - The response time recorded in [Access logs](monitor-application-gateway-reference.md#resource-logs) represents the total duration for which the connection was established. Multiple event streams may pass through during this connection period. 
+
+## Configuring Application Gateway
+
+### Response Buffer 
+
+The [Response Buffers on Application Gateway resource](proxy-buffers.md) should be disabled. This configuration enables your application gateway resource to send responses to clients as streams from the backend server are received.
+
+### Backend Setting - Request timeout 
+
+Ensure that the [Request time-out (seconds) in Backend Settings](configuration-http-settings.md?tabs=backendhttpsettings#request-timeout) is configured to exceed the idle time between events. Otherwise, your application gateway resource terminates the connection prematurely.
+
+
+## Configuring backend server
+
+The backend server should send the following Response headers to ensure proper stream handling by the clients and any intermediaries. Note, these headers aren't a requirement for Application Gateway as a proxy.
+
+### Content-Type: text/event-stream
+This header notifies the client that the response will be a stream of events, enabling them to correctly handle the Server-sent events (SSE) protocol.
+
+### Connection: keep-alive
+This header maintains the TCP connection, allowing the backend server to send events to the client continuously.
+
+### Transfer-Encoding: chunked 
+This header allows the server to send responses in chunks without requiring a Content-Length header. It's necessary for streaming responses through SSE when the total size can't be determined in advance.
+
+### Cache-Control: no-cache 
+It's advisable to use this header to prevent intermediaries like CDNs from caching the SSE response.
+
+## Next steps
+Learn about [Request and Response Proxy Buffers](proxy-buffers.md) in Application Gateway.