Chapter 2. Infrastructure Components

Starting in OpenShift Container Platform 3.10, the deployment model for installing and operating the core control plane components changed. Prior to 3.10, the API server and the controller manager components ran as stand-alone host processes operated by systemd. In 3.10, these two components are moved to static pods operated by the kubelet.

For masters that have etcd co-located on the same host, etcd is also moved to static pods. RPM-based etcd is still supported on etcd hosts that are not also masters.

In addition, the node components openshift-sdn and openvswitch are now run using a DaemonSet instead of a systemd service.

Figure 2.1. Control plane host architecture changes

Even with control plane components running as static pods, master hosts still source their configuration from the /etc/origin/master/master-config.yaml file, as described in the Master and Node Configuration topic.

Mirror Pods

The kubelet on master nodes automatically creates mirror pods on the API server for each of the control plane static pods so that they are visible in the cluster in the kube-system project. Manifests for these static pods are installed by default by the openshift-ansible installer, located in the /etc/origin/node/pods directory on the master host.

These pods have the following hostPath volumes defined:

The set of operations you can do on the static pods is limited. For example:

$ oc logs master-api-<hostname> -n kube-system

returns the standard output from the API server. However:

$ oc delete pod master-api-<hostname> -n kube-system

will not actually delete the pod.

As another example, a cluster administrator may want to perform a common operation, such as increasing the loglevel of the API server to provide more verbose data if a problem occurs. In OpenShift Container Platform 3.10, you must edit the /etc/origin/master/master.env file, where the --loglevel parameter in the OPTIONS variable can be modified, as this is passed to the process running inside the container. Changes require a restart of the process running inside the container.

Restarting Master Services

To restart control plane services running in control plane static pods, use the master-restart command on the master host.

To restart the master API:

# master-restart api

To restart the controllers:

# master-restart controllers

To restart etcd:

# master-restart etcd

Viewing Master Service Logs

To view logs for control plane services running in control plane static pods, use the master-logs command for the respective component:

# master-logs api api
# master-logs controllers controllers
# master-logs etcd etcd

You can optionally configure your masters for high availability (HA) to ensure that the cluster has no single point of failure.

To mitigate concerns about availability of the master, two activities are recommended:

When using the native HA method with HAProxy, master components have the following availability:

While clustered etcd requires an odd number of hosts for quorum, the master services have no quorum or requirement that they have an odd number of hosts. However, since you need at least two master services for HA, it is common to maintain a uniform odd number of hosts when collocating master services and etcd.

Each node has a kubelet that updates the node as specified by a container manifest, which is a YAML file that describes a pod. The kubelet uses a set of manifests to ensure that its containers are started and that they continue to run.

A container manifest can be provided to a kubelet by:

Each node also runs a simple network proxy that reflects the services defined in the API on that node. This allows the node to do simple TCP and UDP stream forwarding across a set of back ends.

The following is an example node object definition in Kubernetes:

apiVersion: v1 1
kind: Node 2
metadata:
  creationTimestamp: null
  labels: 3
    kubernetes.io/hostname: node1.example.com
  name: node1.example.com 4
spec:
  externalID: node1.example.com 5
status:
  nodeInfo:
    bootID: ""
    containerRuntimeVersion: ""
    kernelVersion: ""
    kubeProxyVersion: ""
    kubeletVersion: ""
    machineID: ""
    osImage: ""
    systemUUID: ""

Starting in OpenShift Container Platform 3.10, a node’s configuration is bootstrapped from the master, which means nodes pull their pre-defined configuration and client and server certificates from the master. This allows faster node start-up by reducing the differences between nodes, as well as centralizing more configuration and letting the cluster converge on the desired state. Certificate rotation and centralized certificate management are enabled by default.

Figure 2.2. Node bootstrapping workflow overview

When node services are started, the node checks if the /etc/origin/node/node.kubeconfig file and other node configuration files exist before joining the cluster. If they do not, the node pulls the configuration from the master, then joins the cluster.

ConfigMaps are used to store the node configuration in the cluster, which populates the configuration file on the node host at /etc/origin/node/node-config.yaml. For definitions of the set of default node groups and their ConfigMaps, see Defining Node Groups and Host Mappings in Installing Clusters.

Node Bootstrap Workflow

The process for automatic node bootstrapping uses the following workflow:

Node Configuration Workflow

Sourcing a node’s configuration uses the following workflow:

Modifying Node Configurations

A node’s configuration is modified by editing the appropriate ConfigMap in the openshift-node project. The /etc/origin/node/node-config.yaml must not be modified directly.

For example, for a node that is in the node-config-compute group, edit the ConfigMap using:

$ oc edit cm node-config-compute -n openshift-node

OpenShift Container Platform can communicate with registries to access private image repositories using credentials supplied by the user. This allows OpenShift to push and pull images to and from private repositories. The Authentication topic has more information.