K3s Config Deep Dive: Metrics, Order & Best Practices

Hey guys! Ever found yourself scratching your head over K3s configurations, especially when things don't seem to be behaving as expected? You're not alone! In this article, we're diving deep into a tricky K3s issue involving supervisor metrics, the order in which nodes are enabled, and some quirky behavior with embedded registries. We'll break down the problem, explore the steps to reproduce it, and, most importantly, understand how to avoid these pitfalls. So, let's get started and unravel this K3s puzzle together!

The K3s Configuration Conundrum

At the heart of our discussion is a K3s configuration issue that can lead to unexpected behavior in your cluster. The main problem? K3s configurations, specifically settings like supervisor-metrics and embedded-registry, aren't always applied consistently across all nodes. This inconsistency stems from how K3s handles configurations during bootstrap and how etcd-only nodes interact with the control plane. The core issue revolves around the fact that certain configuration checks are skipped after the initial bootstrap process, leading to discrepancies if nodes are configured differently post-initialization. This can be particularly problematic in setups with mixed roles, such as etcd-only and control-plane-only nodes, where configurations intended for specific node types might bleed over to others. The situation becomes even more complex when considering that etcd-only nodes might inadvertently adopt configurations from the initial control plane node, rather than adhering to their local settings. This behavior defies the expectation that each server should operate based on its local configuration, potentially resulting in a cluster that doesn't function as intended. Understanding this issue is crucial for anyone managing a K3s cluster, as it highlights the importance of consistent configuration practices and the potential pitfalls of diverging setups. Let's delve into the specifics and see how this issue manifests in practice.

Environmental Factors and Cluster Setup

Before we dive into the specifics, let's set the stage. Our test environment involves a K3s cluster with a specific configuration. We have three etcd-only nodes and two control-plane-only nodes. This mixed setup is where things can get interesting, as it highlights the potential for configuration discrepancies. The K3s version isn't explicitly mentioned (n/a), but the issue is related to a tracking item in RKE2 (https://github.com/rancher/rke2/issues/8465), which gives us a clue about the context. The crux of the matter is that certain configurations, such as supervisor-metrics and embedded-registry, should ideally be set consistently across the cluster or at least be applied based on the node's role. However, the initial checks for these configurations happen only during the bootstrap process. This means that if a node already has etcd data on disk, these checks are skipped. Consequently, if you try to enable or disable features like the embedded registry on some nodes but not others after the initial setup, you might run into trouble. The expectation is that each node should use its local configuration, but the actual behavior can deviate from this, especially for etcd-only nodes that might end up using the configuration from the initial control plane node. This inconsistency can lead to a cluster that doesn't behave as expected, making it crucial to understand and address this issue. So, with our environment defined, let's explore the steps to reproduce the bug and see this behavior in action.

Steps to Reproduce the Bug: A Hands-On Approach

Alright, let's get our hands dirty and walk through the steps to reproduce this K3s configuration bug. This is where things get real, and you'll see exactly how the issue manifests. Follow these steps closely, and you'll be able to observe the inconsistent behavior firsthand. This exercise is invaluable for understanding the problem and ensuring you can avoid it in your own K3s deployments.

1. Create a Cluster: Start by setting up a K3s cluster with our specified configuration: three etcd-only nodes and one control-plane-only node. This mixed setup is crucial for demonstrating the issue.
2. Initial Configuration: After the cluster is up and running, we'll introduce the configuration discrepancy. Add embedded-registry: true to the configuration of etcd nodes 2 and 3. This step is key to triggering the bug.
3. Restart Nodes 2 and 3: Go ahead and restart etcd nodes 2 and 3 to apply the new configuration. Now, here's where it gets interesting. Check the logs of these nodes. You'll notice that the embedded registry is not enabled, despite being set in the config. This is the first sign of the problem.
4. Configure Node 1: Next, add embedded-registry: true to etcd node 1 and restart it. This node is different because it's the first etcd node we're configuring with the embedded registry.
5. Verify Node 1: After restarting node 1, check its logs. You'll see that the embedded registry is enabled, as expected. This highlights the inconsistency – why did it work on node 1 but not on nodes 2 and 3 initially?
6. Final Restart: Now, restart etcd nodes 2 and 3 again. This is the crucial step that reveals the underlying issue. After the restart, check their logs once more.
7. Observe the Change: You'll find that the embedded registry is now enabled on nodes 2 and 3! This is because they've picked up the configuration from the initial control plane node after node 1 was configured. This behavior is not what we'd expect; each node should ideally use its local configuration.

By following these steps, you've witnessed the K3s configuration bug in action. The key takeaway is that the order in which nodes are configured and restarted can significantly impact the effective configuration, especially for etcd-only nodes. This hands-on demonstration underscores the importance of understanding how K3s handles configurations and the potential pitfalls of inconsistent settings.

Expected vs. Actual Behavior: A Tale of Two Outcomes

Now that we've reproduced the bug, let's clearly contrast the expected behavior with what actually happens. Understanding this discrepancy is crucial for grasping the severity of the issue and why it needs addressing. So, what did we expect, and what did we actually observe?

Expected Behavior

The expected behavior in this scenario is straightforward: each server in the K3s cluster should use its local configuration. If we set embedded-registry: true in the configuration file of a specific node, that node should enable the embedded registry upon restart. This is the fundamental principle of configuration management – local settings should dictate local behavior. In our case, etcd nodes 2 and 3 were configured with embedded-registry: true, so we expected them to start with the embedded registry enabled after their initial restart. This expectation aligns with the principle of least surprise and ensures that administrators can confidently manage individual nodes without unintended side effects.

Actual Behavior

However, the actual behavior deviates significantly from this expectation. Initially, when we restarted etcd nodes 2 and 3 after adding embedded-registry: true, the embedded registry did not get enabled. This was the first red flag, indicating that something was amiss. The nodes were not behaving according to their local configuration. Instead, they seemed to be ignoring the setting. The real kicker is what happened after we configured etcd node 1 and restarted it. Suddenly, when we restarted nodes 2 and 3 again, the embedded registry did get enabled. This reveals the underlying issue: the nodes were picking up the configuration from the initial control plane node (node 1) rather than adhering to their local settings. This behavior is problematic because it introduces inconsistency and makes it difficult to reason about the state of the cluster. It also violates the principle of local configuration, making management and troubleshooting much harder.

The stark contrast between the expected and actual behavior highlights the severity of this K3s configuration bug. The fact that nodes can inadvertently adopt configurations from other nodes undermines the predictability and manageability of the cluster. This discrepancy underscores the importance of addressing the issue to ensure that K3s behaves as expected and that administrators can rely on local configurations to dictate local behavior.

Diving Deeper: Additional Context and Logs

To truly understand the root cause of this K3s configuration bug, we need to delve into some additional context and logs. While the provided information doesn't include specific log snippets, we can infer some key details based on the observed behavior and the issue description. This section is all about piecing together the puzzle and figuring out why K3s is behaving in this unexpected way.

The Bootstrap Check Bypass

The core issue, as mentioned earlier, is that K3s performs certain configuration checks only during the bootstrap process. This means that if a node already has etcd data on disk, these checks are skipped. This is a critical detail because it explains why the initial configuration of etcd nodes 2 and 3 failed to enable the embedded registry. Since these nodes likely had existing etcd data from the initial cluster setup, the configuration check for embedded-registry: true was bypassed. This design choice, while potentially intended to optimize the bootstrap process, introduces a loophole that can lead to configuration inconsistencies.

The Init Node Influence

The second key piece of the puzzle is the influence of the initial control plane node (etcd node 1 in our example). The behavior we observed suggests that etcd-only nodes can inadvertently pick up configurations from this node, especially if their local configurations are not properly applied during the initial bootstrap. This is likely due to the way K3s handles configuration distribution and synchronization within the cluster. While the exact mechanism is not explicitly detailed, it's clear that the initial control plane node plays a significant role in shaping the configuration landscape of the cluster.

Implications for Mixed Clusters

This behavior has significant implications for mixed clusters, where you have different types of nodes (e.g., etcd-only, control-plane-only, worker nodes) with potentially different configuration requirements. If configurations intended for specific node types bleed over to others, it can lead to unexpected behavior and even cluster instability. For example, enabling the embedded registry on etcd-only nodes might not be desirable in all scenarios, as it can consume resources and potentially impact performance. The fact that this can happen unintentionally due to the configuration bug is a serious concern.

The Need for Consistent Configuration

The underlying message here is clear: consistent configuration is paramount in K3s clusters. Diverging configurations across nodes can lead to a tangled web of issues, making it difficult to manage and troubleshoot the cluster. While the K3s documentation and best practices emphasize the importance of consistent configurations, this bug highlights the potential consequences of deviating from this principle. It also underscores the need for K3s to provide better mechanisms for enforcing configuration consistency and preventing unintended behavior.

Repair Input Keyword

• What happens if supervisor metrics, embedded-registry, and other critical control args are set on some K3s servers but not others? Why does K3s skip these checks if the node already has etcd data on disk? Why do etcd-only nodes use the remote server's config instead of the local config? What should be the expected behavior when servers use their local config in K3s? What is the actual behavior when servers use config from init node in K3s?

Conclusion: Navigating the K3s Configuration Maze

So, guys, we've journeyed through a fascinating yet complex K3s configuration issue. We've seen how seemingly straightforward settings like supervisor-metrics and embedded-registry can lead to unexpected behavior if not handled carefully. The key takeaway is that K3s configuration can be tricky, especially in mixed-node setups. The issue stems from how K3s handles configurations during bootstrap and the potential for etcd-only nodes to inherit settings from the initial control plane node. This can lead to a situation where nodes don't behave according to their local configurations, making the cluster unpredictable and harder to manage.

We've walked through the steps to reproduce the bug, clearly contrasting the expected behavior (local configuration dictating local behavior) with the actual behavior (nodes picking up configurations from the init node). This hands-on demonstration is crucial for understanding the problem and recognizing the potential pitfalls. We've also explored the additional context, highlighting the importance of the bootstrap check bypass and the influence of the initial control plane node. The implications for mixed clusters are significant, as inconsistent configurations can lead to a variety of issues.

The ultimate lesson here is that consistent configuration is king in K3s. While K3s is a powerful and flexible platform, it's essential to adhere to best practices and ensure that your configurations are aligned across the cluster. This bug underscores the need for careful planning and execution when setting up and managing K3s clusters. It also highlights the importance of staying informed about potential issues and workarounds. By understanding the nuances of K3s configuration, you can navigate the maze with confidence and build robust, reliable clusters. So, keep experimenting, keep learning, and keep those K3s clusters humming!