Pipeline Stalls: Conflicting CI Agent and Registry Signals

Simulation Label

This is a fictional simulation designed for diagnostic practice. No live systems, credentials or real incident data are involved.

Scenario

You are on call for the platform team. The nightly build pipeline for the core payment service has failed for the third consecutive night. The failure occurs during the image push stage. The CI system reports a generic "upload timeout", but the container registry dashboard shows no incoming traffic for the relevant repository during the failure window. Meanwhile, the CI agents report healthy network connectivity to other internal services.

Evidence

• CI Log Snippet: ERROR: Push to registry.example.com/core/payment:v2.4.1 failed: context deadline exceeded (30s)
• Registry Metrics: Zero write operations recorded for core/payment between 02:00 and 02:15 UTC.
• Agent Health Check: Agents report successful DNS resolution for registry.example.com and successful TCP handshakes to port 443.
• Network Team Alert: A firewall rule update was deployed at 01:55 UTC affecting outbound traffic from the CI subnet to the DMZ.

Hypotheses and Actions

Select the most appropriate immediate action based on the evidence provided.

1. Restart the CI agents to clear potential stale network connections.
2. Increase the push timeout in the pipeline configuration from 30s to 120s.
3. Verify the firewall rules allowing outbound HTTPS from the CI subnet to the registry IP range.
4. Purge the local Docker cache on the CI agents to force a fresh layer upload.

Reveal

The correct action is to verify the firewall rules. The combination of a recent firewall change, successful TCP handshakes (which may complete before deep packet inspection or stateful filtering blocks the sustained data transfer), and zero registry ingress strongly suggests a network-level block rather than an application or agent issue. Increasing timeouts or restarting agents would not resolve a firewall drop.

Learning Outcome

When CI failures present conflicting signals between agent logs and downstream service metrics, correlate recent infrastructure changes. Network security updates often manifest as timeouts or silent drops rather than explicit connection refusals, requiring verification of allow-lists alongside application logs.