[Glass] load balancer configuration

[Johan Brichau]

Hi Otto,

Mind that with Seaside, requests for the same session cannot be processed in parallel. 
This is why at Yesplan, I use sticky sessions to route all requests for a single session. An old write-up about that approach is still online (and we still do it this way): [1].

We use the Seaside session url query parameter (‘_s’) to hash requests to an upstream. Depending on the hash distribution, this may have the downside that load is not evenly distributed across all upstreams.
To possibly solve that, we have already been thinking to let the Seaside application add another parameter to the generated urls based on how many sessions exist and, as such, let Seaside control the Nginx load balancing distribution.

Also, in our experience, when one upstream times out, Nginx will re-route the request to another upstream. This, of course, still means the end user is waiting for the request longer than necessary.
Having an nginx configuration that does sticky sessions unless a request is not accepted after a specified amount of time would be the ideal situation imho. 

Johan

[1] https://jbrichau.github.io/blog/when-to-use-http-session-affinity-in-glass

> On 20 Dec 2023, at 13:04, Otto Behrens via Glass <glass at lists.gemtalksystems.com> wrote:
> 
> Hi,
> 
> We are using nginx to load balance in front of GemStone that runs a Seaside application. Some of our requests run too long (we are working hard to cut them down) and in general, the time it takes to service a request in our application varies between 0.1 and about 4 seconds. We are improving and getting more towards the lower end of that. 
> 
> Because of this, we use the least_conn directive and we persist session state so that we could use any of our GemStone upstream sessions to service a request. Requests are generally load balanced to idle sessions and there are theoretically no requests that wait for another to get serviced. Perhaps this is not optimal and you have better suggestions. It has worked ok for a long time, but should we consider another approach?
> 
> When our code misbehaves and a request takes let's say 60 seconds to handle, things go pear shaped (yes we want to eliminate them). The user clicks "back" on the browser or closes the browser and nginx picks it up with: 
> "epoll_wait() reported that client prematurely closed connection, so upstream connection is closed too while sending request to upstream"
> 
> We suspect our problem is: when this happens, it appears as if nginx then routes requests to that same upstream, which is unable to handle it because it is busy handling the previous request (which is taking too long), even with some upstream sessions sitting idle. Some users then end up with no response.
> 
> Ideally, we would like to catch the situation in the GemStone session and stop processing the request (when nginx closes the upstream connection). Alternatively, we could set timeouts long enough so that if the browser prematurely closes the connection, nginx does not close the upstream connection. 
> 
> Do you have a suggestion to handle this? Does it make sense to get timeouts (which ones?) to align so that this does not happen?
> 
> Thanks a lot
> Otto Behrens
> +27 82 809 2375
> 
>  <http://za.linkedin.com/in/waltherbehrens>www.finworks.biz <http://www.finworks.biz/>
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