Blog :: Postmortem: 502s During Parameter Store Rollout

Postmortem: 502s During Parameter Store Rollout

Ryan Lopopolo | November 16, 2018

Attempting to deploy 0.149.2 brought down hyperbo.la and caused all requests to return 502 status code. Root cause is a misconfigured PrivateLink endpoint and an unsafe ASG cycler script.

Context

Since 2014, hyperbo.la secrets were distributed via flat files with each deployment. .env files were necessary when hyperbo.la was a single Linode VPS, but AWS has better tools for distributing secrets.

GH-100 converted settings.py to fetch secrets from SSM Parameter Store.

App backend instances run in private VPC subnets with no Internet egress. Packer builds run in a single public VPC subnet with a public IP.

I recently refactored terraform config to use a launch template instead of a launch configuration. Deploys are now managed by an untested ASG cycler script.

0.149.0 includes a new task runner which includes a new and untested deploy task, inv deploy, and a rollback task, inv deploy.rollback.

Timeline

0.149.1

0.149.2

  • [2018-11-11 00:04] inv deploy kicks off Packer build.
  • [00:15] inv deploy finishes cycling the ASG.
  • [00:16] Manual smoke test of https://hyperbo.la/ shows nginx 502 page.
  • [00:17] Uptime Robot reports hyperbo.la hard down.
  • [00:17] inv deploy.rollback.
  • [00:19] inv deploy.rollback finishes cycling the ASG.
  • [00:20] Manual smoke test of https://hyperbo.la/ shows we are recovered.
  • [00:23] Uptime Robot reports we are back up.
  • [01:08] Reconfigure SSM endpoint security groups.
  • [01:09] Attempt to deploy 0.149.2 again.
  • [01:11] Manual smoke test of https://hyperbo.la/ shows nginx 502 page.
  • [01:13] inv deploy.rollback.
  • [01:14] inv deploy.rollback finishes cycling the ASG.
  • [01:15] Manual smoke test of https://hyperbo.la/ shows we are recovered.
  • [01:19] Enable private DNS for SSM endpoint.
  • [01:22] Attempt to deploy 0.149.2 again.
  • [01:25] Manual smoke test of https://hyperbo.la/ shows deploy succeeds.
  • [01:26] https://hyperbo.la/healthz confirms 0.149.2 is deployed.

At this point the incident is mitigated but hyperbo.la is undeployable.

0.149.3

  • [08:59] Cut 0.149.3 to package terraform changes into a release and clean up.
  • [09:07] inv deploy kicks off Packer build.
  • [09:11] inv deploy.ami fails when running manage.py collectstatic.

At this point I spent time to deep dive into VPC terraform to properly configure the SSM PrivateLink endpoint.

0.149.4

  • [18:48] inv deploy kicks off Packer build.
  • [18:59] Packer build finishes successfully.
  • [19:05] inv deploy finishes cycling the ASG.
  • [19:06] Manual smoke test of https://hyperbo.la/ shows deploy succeeds.

At this point, the incident is over and we are stable.

Root Cause Analysis

Implemented SSM PrivateLink endpoint without reading documentation

For most AWS features, the terraform documentation is descriptive enough that implementation is straightforward. PrivateLink was non-trivial to set up. Errors I made:

  • Did not allow egress from Packer builder to endpoint.
  • Did not allow ingress to endpoint from Packer.
  • Did not allow egress from app backend to endpoint.
  • Did not allow ingress to endpoint from app backend.
  • Did not enable private DNS for zero-configuration use with boto3.

ASG cycler did not validate instances are healthy in ALB

The ALB already hits /healthz. Automate checking that hosts come up cleanly to not rely on manual smoke testing. Automatically detach unhealthy hosts from the ALB and halt the rollout.

Remediation Items

  • GH-103: add ALB healthchecks to cycle_asg script.
  • GH-101 (Done): create a management domain in VPC for shared infrastructure.
  • Done: add a lab terraform environment for testing VPC changes.
  • GH-104: export instance journald logs for gunicorn and nginx to CloudWatch.