Re: MPM event unstable behind AWS ALB

[Igor Cicimov <icicimov@xxxxxxxxx>]

In a proxy chains like this getting the timeouts in sync is the most important thing. Make sure that you have done that.

On Mon, 20 Dec 2021, 08:37 Dan Washusen, <dan@xxxxxxxxxxxx> wrote:

Hi All,

I've been experimenting with the MPM event module with Apache instances sitting behind an AWS Application Load Balancer (ALB) and it really doesn't seem to be working well. Response times shoot up (compared to MPM event worker) and we see a fair few 502 errors returned (by the AWS ALB.

The basic layout is: AWS Application Load Balancer -> Apache 2.4.x -> AWS Internal TCP Load Balancer (NLB) -> Jetty App Servers

Debugging the issue I think I traced it down to Jetty returning a 408 error because it can't read the request body in a timely manner. So it seems like for some reason MPM is sending the request body...?

We're running Ubuntu 20.04 with Apache 2.4.41-4ubuntu3.8 with the following worker configuration:

ServerLimit 250
StartServers 100
MinSpareThreads 75
MaxSpareThreads 250
ThreadLimit 64
ThreadsPerChild 64
MaxRequestWorkers 8000

I've come across several random posts mentioning that the MPM event module doesn't work behind an ALB but no-one seems to be into any detail. Anyone have some debugging/configuration suggestions?

Thanks,

Dan

p.s. I've created a serverfault post showing graphs etc: https://serverfault.com/questions/1087747/apache-2-4-mpm-event-module-causing-intermittent-502-errors-and-slow-response-ti