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I have a "single page" web application which requires bi-directional communication between client and server.

The application was originally designed to rely on WebSockets, but has been modified to use SockJS, with WebSocket support disabled.

Clients, using the SockJS library, tend to use "XHR Streaming" as a means of bi-directional communication.

The server is a Java web application, using Spring's implementation of a SockJS server (from Spring's WebSocket module), hosted in Tomcat.

NGINX is used as a reverse proxy to Tomcat.

The default configuration of NGINX is for proxy_buffering to be enabled.

NGINX's documentation does not adequately explain the behaviour of the buffer: it is not obvious under what circumstances the buffer would be flushed (i.e. when data would actually be pushed to the client) if data is being streamed (from Tomcat) over a long-lived HTTP connection.

My observation is that data pushed from the server (a response to a client's request) might sit in NGINX's buffer until the next server-generated SockJS heartbeat occurs for that client. The effect of this is a delay of 25 seconds transmitting the response to the client!

I can very reliably reproduce this issue through experimentation - the behaviour is deterministic, but I can't explain the relationship between the configured buffer size(s), the size of data being transmitted, and NGINX's behaviour.

My server's responsibility is to generate responses to clients' command invocations; each response will vary in size (from a few bytes to tens of kilobytes), but is self-contained.

The primary goal is to reduce the latency of responses to client commands.

NGINX only sees a long-lived HTTP stream; dividing the stream's contents into individual command responses (for immediate despatch) would require NGINX to understand SockJS's proprietary protocol, which it can't.

Therefore, I believe that NGINX's buffering policy is fundamentally incompatible with my use case, and plan to disable proxy_buffering; is this wise?

NGINX's documentation suggests that if proxy_buffering is disabled, the upstream server (Tomcat) will be forced to keep the HTTP response open until all data has been received by the client (which seems a reasonable definition of buffering!).

Accordingly, NGINX's documentation advises against disabling proxy_buffering, as it will potentially waste upstream server resources.

However, because my clients use XHR Streaming, my server is already obligated to hold an HTTP connection open for each active client (right?). Therefore, disabling proxy_buffering shouldn't negatively impact my Tomcat server; is this correct?

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I think you are correct in your reasoning here. I have an application with the same setup, sockjs behind an nginx proxy. We were seeing a lot of dropped connections from a location with high latency. After finding this post and turning off proxy_buffering our dropped connections issues cleared up.

Based on the logs I was seeing, I believe the nginx buffering was preventing some of the messages from being properly sent between the client/server.

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