K3s: Supervisor Metrics Bug & Configuration Order

Introduction

Hey guys! Let's dive into a fascinating discussion surrounding a tricky issue in K3s, specifically concerning the behavior of supervisor metrics and other critical configurations across different nodes in a cluster. We're going to break down a bug report that highlights how K3s nodes sometimes pick up configurations from the initial node, rather than adhering to their local settings. This can lead to some unexpected behavior, especially when you're trying to manage features like the embedded registry. So, buckle up, and let's get started!

Background on the Issue

The heart of the matter lies in K3s's configuration management, particularly how it handles settings like supervisor-metrics and embedded-registry. Imagine you're setting up a cluster with different roles for your nodes – some are etcd-only, dedicated to handling your cluster's data, while others are control-plane-only, managing the cluster's operations. Now, you might want to enable certain features, like the embedded registry, on some nodes but not others. This is where things can get a bit dicey.

The bug we're discussing is rooted in how K3s applies configurations. Ideally, each server should use its local config file. However, there's a catch: these configuration checks primarily happen during the bootstrap process. If a node already has etcd data on its disk, these checks are skipped. This can lead to a situation where etcd-only nodes inadvertently adopt the configuration from the initial control-plane node, rather than using their own local settings. This can lead to discrepancies and unexpected behavior, especially when you're trying to manage features like the embedded registry.

Environmental Context

Before we dive deeper, let's set the stage with some environmental details:

• K3s Version: This issue isn't tied to a specific version, but it's crucial to be aware of your K3s version when troubleshooting.
• Node Architecture: The CPU architecture, OS, and version of your nodes are relevant, but this issue is more about configuration management than the underlying infrastructure.
• Cluster Configuration: The specific setup that triggers this issue involves a cluster with 3 etcd-only nodes and 2 control-plane-only nodes. This separation of roles is common in production environments for better scalability and resilience.

The Bug Unveiled

The core issue stems from a K3s tracking issue originally reported for RKE2 (https://github.com/rancher/rke2/issues/8465). The configuration described in this issue, where supervisor-metrics, embedded-registry, and other critical control arguments are set inconsistently across servers, should ideally be prevented altogether. However, the checks designed to enforce this only occur during the bootstrap phase. This means that if a node already possesses etcd data on its disk, these checks are bypassed.

This behavior uncovers a critical flaw: etcd-only nodes might end up using the configuration of the remote server (the initial control-plane node) instead of their own local configuration. This deviation from the expected behavior can lead to significant operational challenges, especially in managing features that should be locally scoped.

Steps to Reproduce the Issue

To really understand what's going on, let's walk through the steps to reproduce this bug. This will give you a hands-on understanding of how the configuration mix-up occurs.

1. Initial Cluster Setup: Start by creating a K3s cluster with the specific configuration that triggers the issue: 3 etcd-only nodes and 1 control-plane-only node. This separation is crucial for demonstrating the problem. Think of it like setting up a mini-lab where the conditions are just right to observe the bug.
2. Selective Configuration: This is where the divergence begins. Add the line embedded-registry: true to the configuration files of etcd nodes 2 and 3. Crucially, do not add this line to the init node (the first control-plane node). After making these changes, restart etcd nodes 2 and 3. Now, check the logs on these nodes. You'll notice something peculiar: the embedded registry is not enabled, despite being explicitly set in their configuration files. This is our first clue that something's amiss.
3. Enabling on the Init Node: Next, add embedded-registry: true to the configuration of etcd node 1 (the init node) and restart it. Observe the logs on node 1, and you'll see that the embedded registry is enabled, as expected. This confirms that local configuration works when it's the first node to be configured.
4. The Twist: Now, here's where things get interesting. Restart etcd nodes 2 and 3 again. Check their logs, and you'll find that the embedded registry is now enabled on these nodes as well. This is despite the fact that their local configurations should have taken effect initially. This demonstrates that the configuration from the init node is being propagated and overriding the local settings.

These steps clearly illustrate the bug: the etcd-only nodes are initially ignoring their local configurations and, after the init node is configured, adopting its settings. This is not the intended behavior and can lead to significant problems in a production environment.

Expected vs. Actual Behavior

Let's clearly define what should happen and what's actually happening.

• Expected Behavior: Each server in the K3s cluster should adhere to its local configuration settings. If embedded-registry: true is set in the configuration file of a specific node, that node should enable the embedded registry, regardless of the settings on other nodes. This ensures that each component of the cluster can be managed independently and predictably.
• Actual Behavior: Servers are not consistently using their local configurations. Specifically, etcd-only nodes initially ignore their local settings. They appear to be influenced by the configuration of the init node (the first control-plane node). This means that a change on the init node can inadvertently affect other nodes in the cluster, leading to configuration drift and unexpected behavior.

This discrepancy between expected and actual behavior is the crux of the bug. It highlights a potential vulnerability in K3s's configuration management that can lead to significant operational challenges.

Implications and Additional Context

This bug has significant implications for K3s cluster management, especially in environments with specific configuration requirements for different node roles. Imagine a scenario where you want to isolate certain services to specific nodes or enforce security policies at the node level. If configurations are being propagated from the init node, it becomes much harder to maintain this isolation and control.

The fact that this issue was initially reported for RKE2, another Kubernetes distribution by Rancher, suggests that the underlying problem might stem from shared codebase or architectural patterns. This highlights the importance of cross-project collaboration and knowledge sharing in the open-source community.

The provided steps to reproduce offer a clear and concise way to demonstrate the bug. This is invaluable for developers working on a fix, as it allows them to quickly verify whether their changes have addressed the issue. It also helps users understand the conditions under which the bug occurs, so they can avoid it in their own deployments.

Impact on Cluster Operations

Understanding the impact on cluster operations is critical. If nodes are not using their local configurations, it can lead to a range of problems:

• Inconsistent Behavior: Nodes might behave differently than expected, leading to unpredictable application deployments and service disruptions.
• Security Risks: Misconfigured nodes can create security vulnerabilities, exposing the cluster to potential attacks.
• Maintenance Challenges: Troubleshooting and maintenance become significantly more difficult when configurations are not consistent across nodes.
• Scalability Issues: If nodes are not configured correctly, it can hinder the cluster's ability to scale and handle increased workloads.

Mitigating the Issue

While a permanent fix is being developed, there are some steps you can take to mitigate the issue:

• Careful Planning: Plan your cluster configuration carefully, ensuring that all nodes have the correct settings before joining the cluster.
• Consistent Configuration: Strive for consistent configurations across all nodes, especially for critical settings like embedded-registry.
• Avoid Dynamic Changes: Minimize dynamic configuration changes after the cluster is bootstrapped, as these can trigger the bug.
• Monitoring: Implement robust monitoring to detect any configuration drift or unexpected behavior.

Conclusion

Alright, guys, we've journeyed through a fascinating, albeit tricky, bug in K3s! We've seen how the order in which you enable supervisor metrics and other settings can lead to unexpected configuration behavior, particularly when dealing with etcd-only nodes. The key takeaway here is the importance of understanding how K3s handles configurations and the potential pitfalls of relying on the init node's settings. By following the reproduction steps and keeping the implications in mind, you can better navigate this issue and ensure a smoother K3s experience.

This discussion underscores the complexities of distributed systems and the importance of meticulous configuration management. As K3s continues to evolve, addressing these kinds of bugs will be crucial for maintaining its reliability and usability. Keep experimenting, keep learning, and let's all contribute to making K3s even better! Thanks for diving deep with me on this one!