Create using-sse.md

Author: jaesoni

articles/application-gateway/using-sse.md

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+---
+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 will terminate 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 are not 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 is necessary for streaming responses through SSE when the total size cannot be determined in advance.
+
+### Cache-Control: no-cache 
+It is generally 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.