Bug: concourse-summary leaves ~1k network connections open

[klakin-pivotal]

Versions Involved

• Concourse: 4.2.1 and 5.1.0
• concourse-summary: 6b447f0

Problem Description

concourse-summary will occasionally overwhelm our Concourse server with network connections to its atc processes. This causes the Concourse server's web workers to run out of file handles and be unable to function correctly.
Investigation reveals that our concourse-summary instance has ~1k ESTABLISHED connections to our Concourse server and each of our two web instances have ~1k ESTABLISHED connections between the atc process running on the web instance and the concourse-summary instance. (Yes, this mismatch seems a little strange.)

As one would expect, the following procedure shuts down these ESTABLISHED connections and gets us back in working order:

• Stop concourse-summary
• Restart the atc service on each Concourse web instance
• Wait fo the atc services to come back up
• Start concourse-summary

Expected Behavior

concourse-summary should only have enough network connections open to get its job done. Given that there are less than 200 connections open when we restart concourse-summary, ~1k connections seems to be too many connections.

More Details

We have seen this issue happen twice in the past ~four months. We do not currently know if this is a gradual increase in the number of ESTABLISHED connections, or if this happens suddenly.
Our web instances are behind a GCP TCP Regional Load Balancer.
Our concourse-summary instance is providing a summary of both our Concourse server (version 4.2.1) and the Wings Concourse server (version 5.1.0).
concourse-summary is deployed in a 2.4 PCF running on top of vSphere.
Unfortunately, we don't know what software (concourse-summary, Concourse, GCP Load Balancer) is at fault.

[dgodd]

Hi Kenneth, I'm very interested in this problem. I will play around with my own instances, but if you are able to start logging ESTABLISHED connections over time, it would be interesting to know if it is a slow burn or quickly problematic

[klakin-pivotal]

We have some work in our backlog that will track this over time... we just need to get it prioritized. If this happens again (and we have the tracking in place), I'll make sure that we put details in this GH Issue.

[dgodd]

@klakin-pivotal I suspect that this may be related to crystal-lang/crystal#8025. Unfortunately that has not yet landed on a release, but it would be interesting to check if the problem exists after the next release of crystal