Replacing control nodes with Kolla Ansible

Maintaining an OpenStack cloud in production requires constant care, and one of the tasks that may arise is replacing or adding new control nodes. Whether due to expansion, hardware failure, or infrastructure upgrades, the process needs to be secure and well-planned to avoid downtime.

We went through this experience in our environment, using kolla-ansible as the deployment tool. We followed the official documentation, but as with any real-world environment, we encountered some challenges. In this post, I share the step-by-step process we executed, the checks we performed, and an unexpected issue with RabbitMQ – and how we resolved it. Additionally, I present the complete process for removing control nodes, based on the documentation and lessons learned.

Adding a new control node

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Prerequisites

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The new control node needs to be properly prepared. In our case, all servers use bonding for high availability of network interfaces. Therefore, it was necessary to:

• Install Ubuntu 22.04 on the machine.
• Configure bonding (with two interfaces) and, on top of it, create two VLANs:
  • One for internal communication of cloud services.
  • Another for external communication (external services, API).
• Assign an IP to each VLAN and ensure the new node was reachable via both addresses.

This step is important because kolla-ansible uses these IPs to configure services and communication between nodes.

Adding the node to the cluster with Kolla-Ansible

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We used a multinode inventory file listing all nodes (including the new one). The addition was done in stages, always limiting execution to the control group (or to the new host specifically).

# Prepares the node: installs dependencies, configures repositories, etc.
kolla-ansible bootstrap-servers -i multinode --limit control

# Pulls container images for the new node
kolla-ansible pull -i multinode --limit <new-host>

# Runs the deployment only on control nodes (updates configuration)
kolla-ansible deploy -i multinode --limit control

After the deployment, the new node should already be participating in the cluster. However, verification is necessary.

Post-deployment checks

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We validated the main services running on the control nodes: MariaDB (Galera), RabbitMQ, Neutron, and Nova.

MariaDB (Galera)

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The MariaDB (Galera) cluster needs to be synchronized and have the correct size.

# Retrieves the MariaDB root password from the configuration file
ROOT_PASS=$(cat /etc/kolla/mariadb/galera.cnf | grep wsrep_sst_auth | cut -f 2 -d:)

# Checks the number of nodes in the cluster
docker exec -t mariadb mysql --user root --password=$ROOT_PASS -e "SHOW STATUS LIKE 'wsrep_cluster_size';"

# Confirms the cluster is primary (accepts read/write)
docker exec -t mariadb mysql --user root --password=$ROOT_PASS -e "SHOW STATUS LIKE 'wsrep_cluster_status';"

# Verifies if the current node is synchronized (synced)
docker exec -t mariadb mysql --user root --password=$ROOT_PASS -e "SHOW STATUS LIKE 'wsrep_local_state_comment';"

# Lists the addresses of cluster members
docker exec -t mariadb mysql --user root --password=$ROOT_PASS -e "SHOW STATUS LIKE 'wsrep_incoming_addresses';"

RabbitMQ

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RabbitMQ is OpenStack’s message bus. We checked the cluster status and connectivity:

# RabbitMQ cluster status
docker exec -it rabbitmq rabbitmqctl cluster_status

# Tests connectivity between listener ports
docker exec -it rabbitmq rabbitmq-diagnostics check_port_connectivity

The new node appeared in the cluster, and connectivity was ok.

Neutron and Nova

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Finally, we checked if agents and compute services were present:

openstack network agent list --host <new-host>
openstack compute service list --host <new-host>

Up to this point, everything indicated that the addition had been a success. However, when attempting to create an instance, the error appeared.

Error: “missing queue” in RabbitMQ

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When creating a VM, the operation failed with the following message in the nova-conductor log:

The reply XXXX failed to send after 60 seconds due to a missing queue
oslo_messaging.exceptions.MessageUndeliverable

Analyzing further, we realized that RabbitMQ was in HA (high availability), but the queues had not been created on the new node. The queues existing on the other control nodes were not automatically replicated to the new cluster member. When nova-conductor tried to send a message to a queue that only existed on the old nodes, the new node couldn’t deliver it, resulting in the error.

Solution: restart RabbitMQ on all nodes

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After some investigation, the solution found was to restart the RabbitMQ service on all control nodes. This forced queue reconciliation and complete cluster synchronization. For this, it is necessary to perform a rolling restart to ensure that a subsequent container on another node can only be restarted when the previous one is functional.

# Restart RabbitMQ on the first node
docker restart rabbitmq

# Check if the container is HEALTHY
watch 'docker ps -a --filter name=rabbitmq'

When RabbitMQ is working correctly on the first node, repeat the same procedure for the next ones.

After the restart, the cluster recreated the queues, and VM creation returned to normal.

Removing a control node

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Like addition, removing a control node needs to be done carefully to avoid affecting cluster availability. The official Kolla-Ansible documentation recommends a step-by-step process, which includes reallocating resources, stopping services, and cleaning up registries.

Step 1: Move Neutron resources

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Before removing the node, it is necessary to reallocate routers and DHCP networks managed by the L3 and DHCP agents residing on the host to be removed.

For L3 agents (routers):

# Identifies the L3 agent ID on the host to be removed
l3_id=$(openstack network agent list --host <host> --agent-type l3 -f value -c ID)

# Identifies the ID of an L3 agent on another node (target)
target_l3_id=$(openstack network agent list --host <target-host> --agent-type l3 -f value -c ID)

# Moves each router from the old agent to the new one
openstack router list --agent $l3_id -f value -c ID | while read router; do
  openstack network agent remove router $l3_id $router --l3
  openstack network agent add router $target_l3_id $router --l3
done

# Disables the old L3 agent
openstack network agent set $l3_id --disable

For DHCP agents:

# Identifies the DHCP agent ID on the host to be removed
dhcp_id=$(openstack network agent list --host <host> --agent-type dhcp -f value -c ID)

# Identifies the ID of a DHCP agent on another node (target)
target_dhcp_id=$(openstack network agent list --host <target-host> --agent-type dhcp -f value -c ID)

# Moves each network from the old agent to the new one
openstack network list --agent $dhcp_id -f value -c ID | while read network; do
  openstack network agent remove network $dhcp_id $network --dhcp
  openstack network agent add network $target_dhcp_id $network --dhcp
done

Step 2: Stop services on the removed host

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With resources reallocated, we can stop all containers on the host to be removed:

kolla-ansible stop -i multinode --yes-i-really-really-mean-it --limit <host>

Step 3: Remove the host from the Ansible inventory

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Edit your inventory file (multinode) and remove or comment out the lines referring to the host being retired.

Step 4: Reconfigure the remaining controllers

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Extra care with RabbitMQ

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As with addition, RabbitMQ deserves special attention. There are indications that, even after removing the node from the inventory and reconfiguring, RabbitMQ might still list the removed node in the cluster. This can cause connectivity issues.

Check if the host to be removed is still listed:

docker exec -it rabbitmq rabbitmqctl cluster_status

If it is still listed in any way (in our case, the host still appeared under Disk Nodes), remove the host:

docker exec -it rabbitmq rabbitmqctl forget_cluster_node <node-name>

Verify again that the host has indeed been removed, then proceed.

Deploy

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Now it’s necessary to reconfigure the remaining control nodes so they update the cluster state (MariaDB, RabbitMQ, etc.).

kolla-ansible deploy -i multinode --limit control

Step 5: Clean up services from the removed host

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Finally, remove the registries of OpenStack agents and services that might still be visible:

# Removes network agents
openstack network agent list --host <host> -f value -c ID | while read id; do
  openstack network agent delete $id
done

# Removes compute services
openstack compute service list --os-compute-api-version 2.53 --host <host> -f value -c ID | while read id; do
  openstack compute service delete --os-compute-api-version 2.53 $id
done

Conclusion

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Replacing control nodes in an OpenStack environment with kolla is a well-documented process, but it’s not always free of surprises. The main lesson we learned was: always check the state of RabbitMQ queues after adding a new node and, when removing, ensure the node has been completely forgotten by the messaging cluster.

A simple queue listing (rabbitmqctl list_queues) or cluster check (rabbitmqctl cluster_status) before and after could have indicated the problem earlier.

Fortunately, the solutions were simple (although they involve restarting critical services). In production environments, it’s advisable to plan a maintenance window for this type of operation.

In the end, the entire process was much smoother than we thought, and this really shows how fascinating the kolla-ansible project is.