Modify the Keeper configuration

Oct 13, 2025 by wbrum.org 102 views

Can Become Leader Cannot be Switched in Keeper's Config: A Critical Issue in ClickHouse

Unexpected Behavior: Switching Node Roles Proves Impossible

In the world of distributed databases, ClickHouse is a popular choice for its high-performance capabilities and scalability. However, like any complex system, it's not immune to unexpected behaviors. One such issue has been reported in the ClickHouse community, where switching a node's role from member to observer and back is impossible. This behavior is particularly puzzling, as it seems to defy the fundamental principles of distributed systems.

Reproducing the Issue: A Step-by-Step Guide

To understand this issue better, let's dive into the steps required to reproduce it. We'll start by creating two ClickHouse Keeper instances with different roles: leader and observer. The configuration for these instances will be as follows:

### Raft Configuration

<raft_configuration>
  <server>
    <id>1</id>
    <hostname>k1</hostname>
    <port>9444</port>
    <can_become_leader>true</can_become_leader>
  </server>
  <server>
    <id>2</id>
    <hostname>k2</hostname>
    <port>9444</port>
    <can_become_leader>false</can_become_leader>
  </server>
</raft_configuration>

Steps to Reproduce the Issue

Stop Both Nodes: The first step is to stop both Keeper instances. This will ensure that we're starting from a clean slate.
Exchange Roles: Next, we'll exchange the roles of the two nodes by disabling can_become_leader for node 1 and enabling it for node 2.
Start Again: After exchanging roles, we'll start both Keeper instances again.
Check Roles: Finally, we'll check the roles of the nodes by using the echo stat command and connecting to the ZooKeeper instance at 127.0.0.1:2181.

Expected Behavior: Switching Roles Should be Possible

In an ideal world, switching a node's role from member to observer and back should be possible, even for Keeper instances with a snapshot on disk. This is because the can_become_leader parameter is meant to be a dynamic configuration that can be changed as needed.

However, as we've seen, this is not the case. The issue seems to be related to the fact that the can_become_leader parameter cannot be switched once a Keeper instance has been created. This means that if we want to change the role of a node, we're stuck with the initial configuration.

Implications and Workarounds

The implications of this issue are significant. In a distributed system like ClickHouse, being able to switch node roles is crucial for maintaining high availability and scalability. If we're unable to do so, it can lead to a range of problems, including:

Reduced availability: If a node's role cannot be switched, it may become unavailable for certain tasks, leading to reduced availability and performance.
Increased complexity: Workarounds for this issue may involve creating new Keeper instances or using complex configuration scripts, which can increase the complexity of the system.
Security risks: If a node's role cannot be switched, it may become a single point of failure, which can lead to security risks if the node is compromised.

To work around this issue, we may need to create new Keeper instances or use complex configuration scripts. However, these workarounds are not ideal and may not be feasible in all scenarios.

Conclusion

In conclusion, the issue of can_become_leader not being switchable in Keeper's config is a critical problem that needs to be addressed. It's essential to understand the implications of this issue and explore workarounds that can mitigate its effects. By doing so, we can ensure that ClickHouse remains a high-performance and scalable distributed database solution.

Future Work

To resolve this issue, we may need to explore the following options:

Modify the Keeper configuration: We may need to modify the Keeper configuration to allow for dynamic switching of the can_become_leader parameter.
Create new Keeper instances: We may need to create new Keeper instances with the desired configuration, which can be more complex and time-consuming.
Use complex configuration scripts: We may need to use complex configuration scripts to work around this issue, which can increase the complexity of the system.

By exploring these options, we can ensure that ClickHouse remains a reliable and scalable distributed database solution.
Can Become Leader Cannot be Switched in Keeper's Config: A Q&A Article

Introduction

In our previous article, we discussed the issue of can_become_leader not being switchable in Keeper's config. This issue has significant implications for ClickHouse users, particularly those who rely on high availability and scalability. In this article, we'll answer some of the most frequently asked questions about this issue.

Q: What is the `can_become_leader` parameter?

A: The can_become_leader parameter is a configuration option in ClickHouse that determines whether a node can become the leader of a cluster. This parameter is crucial for maintaining high availability and scalability in a distributed system.

Q: Why can't I switch the `can_become_leader` parameter?

A: The can_become_leader parameter cannot be switched once a Keeper instance has been created. This is because the parameter is tied to the node's identity and cannot be changed dynamically.

Q: What are the implications of this issue?

A: The implications of this issue are significant. If a node's role cannot be switched, it may become unavailable for certain tasks, leading to reduced availability and performance. Additionally, workarounds for this issue may involve creating new Keeper instances or using complex configuration scripts, which can increase the complexity of the system.

Q: How can I work around this issue?

A: There are several workarounds for this issue, including:

Creating new Keeper instances with the desired configuration
Using complex configuration scripts to modify the can_become_leader parameter
Modifying the Keeper configuration to allow for dynamic switching of the can_become_leader parameter

Q: What are the security risks associated with this issue?

A: If a node's role cannot be switched, it may become a single point of failure, which can lead to security risks if the node is compromised. Additionally, the inability to switch roles may make it more difficult to detect and respond to security threats.

Q: Can I switch the `can_become_leader` parameter if I'm using a snapshot on disk?

A: Unfortunately, the can_become_leader parameter cannot be switched even if you're using a snapshot on disk. This is because the parameter is tied to the node's identity and cannot be changed dynamically.

Q: How can I prevent this issue from occurring in the future?

A: To prevent this issue from occurring in the future, it's essential to carefully plan and configure your ClickHouse cluster. This includes:

Ensuring that all nodes have the same configuration
Using a consistent naming convention for nodes
Regularly backing up your data and configuration

Q: Is this issue specific to ClickHouse or can it occur in other distributed systems?

A: While this issue is specific to ClickHouse, it can occur in other distributed systems that use a similar configuration model. However, the specific implementation and configuration of each system may affect the likelihood and severity of this issue.

Q: What is the current status of this issue?

A: The current status of this issue is that it is being actively discussed and investigated by the ClickHouse community. While there is no official fix or workaround available at this time, the community is working to resolve this issue and provide a solution.

Conclusion

In conclusion, the issue of can_become_leader not being switchable in Keeper's config is a critical problem that needs to be addressed. By understanding the implications and workarounds for this issue, ClickHouse users can take steps to mitigate its effects and ensure the high availability and scalability of their distributed system.