Supporting SSE with AsyncMiddleManServlet

[rushalias]

trafficstars

Opening a ticket per Simone’s suggestion.

Application details:

OS: Ubuntu 18.04.3 LTS JRE: OpenJDK 64-Bit Server VM Corretto-11.0.6.10.1 Jetty: 9.4.26.v20200117

Issue:

We are running a reverse proxy based on Jetty's AsyncMiddleManServlet. Lately, a new use case has emerged where a backend sends out Server Sent Events (SSE). In particular, we are trying to reverse proxy calls to spring-boot-admin, in particular calls returning a stream of SSE (eg.InstancesController.java)

There is this recommendation, https://github.com/eclipse/jetty.project/issues/3459, which is to add a flush call in ProxyServlet.onResponseContent(). However, this seems to be a bit of a problem for us since we are using/subclassing AsyncMiddleManServlet and can not find the proper place to cut in with a flush() call.

Currently, we have it implemented in newServerResponseContentTransformer with Identity as the transformer, but this does not seem the right place to hook in the call for a flush.

@Override
protected ContentTransformer newServerResponseContentTransformer(
    HttpServletRequest clientRequest, 
    HttpServletResponse proxyResponse, 
    Response serverResponse)
{
    return (input, finished, output) -> {
        output.add(input); // Identity content transformer
        if ( is this SSE ) {
            proxyResponse.getOutputStream().flush();
        }
    };
}

We have deployed this in our pre-production environment and spring boot admin UI seems to work fine and is responsive.

A couple of questions:

• Is this the right place to cut in for the flush call?
• If ^^ is not the correct place to cut in, then where would you suggest we add the proposed flush call to support SSE in AsyncMiddleManServlet?

Thank you in advance for your help. -Lu

[stale[bot]]

This issue has been automatically marked as stale because it has been a full year without activity. It will be closed if no further activity occurs. Thank you for your contributions.

[splatch]

I come across this issue already twice as my attempts to get AsyncMiddleManServlet working with SSE failed. I did revert to plain ProxyServlet with forced flush inside ProxyServlet.onResponseContent when x-accel-buffering is set to no. Apparently content transformer is not a valid place to force flush as in my case. Fix suggested in this issue leaves my browser without events (result is either HTTP 504 or HTTP 200 without data).

[github-actions[bot]]

This issue has been automatically marked as stale because it has been a full year without activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions[bot]]

This issue has been automatically marked as stale because it has been a full year without activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions[bot]]

This issue has been automatically marked as stale because it has been a full year without activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions[bot]]

This issue has been closed due to it having no activity.