Learn Kubernetes Basics

Concepts

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Nodes

A node is a worker machine in Kubernetes, previously known as a minion. A node may be a VM or physical machine, depending on the cluster. Each node contains the services necessary to run pods and is managed by the master components. The services on a node include the container runtime, kubelet and kube-proxy. See The Kubernetes Node section in the architecture design doc for more details.

Node Status

A node’s status contains the following information:

Node status and other details about a node can be displayed using below command:

kubectl describe node <insert-node-name-here>

Each section is described in detail below.

Addresses

The usage of these fields varies depending on your cloud provider or bare metal configuration.

Conditions

The conditions field describes the status of all Running nodes. Examples of conditions include:

Node Condition Description
OutOfDisk True if there is insufficient free space on the node for adding new pods, otherwise False
Ready True if the node is healthy and ready to accept pods, False if the node is not healthy and is not accepting pods, and Unknown if the node controller has not heard from the node in the last node-monitor-grace-period (default is 40 seconds)
MemoryPressure True if pressure exists on the node memory – that is, if the node memory is low; otherwise False
PIDPressure True if pressure exists on the processes – that is, if there are too many processes on the node; otherwise False
DiskPressure True if pressure exists on the disk size – that is, if the disk capacity is low; otherwise False
NetworkUnavailable True if the network for the node is not correctly configured, otherwise False

The node condition is represented as a JSON object. For example, the following response describes a healthy node.

"conditions": [
  {
    "type": "Ready",
    "status": "True",
    "reason": "KubeletReady",
    "message": "kubelet is posting ready status",
    "lastHeartbeatTime": "2019-06-05T18:38:35Z",
    "lastTransitionTime": "2019-06-05T11:41:27Z"
  }
]

If the Status of the Ready condition remains Unknown or False for longer than the pod-eviction-timeout, an argument is passed to the kube-controller-manager and all the Pods on the node are scheduled for deletion by the Node Controller. The default eviction timeout duration is five minutes. In some cases when the node is unreachable, the apiserver is unable to communicate with the kubelet on the node. The decision to delete the pods cannot be communicated to the kubelet until communication with the apiserver is re-established. In the meantime, the pods that are scheduled for deletion may continue to run on the partitioned node.

In versions of Kubernetes prior to 1.5, the node controller would force delete these unreachable pods from the apiserver. However, in 1.5 and higher, the node controller does not force delete pods until it is confirmed that they have stopped running in the cluster. You can see the pods that might be running on an unreachable node as being in the Terminating or Unknown state. In cases where Kubernetes cannot deduce from the underlying infrastructure if a node has permanently left a cluster, the cluster administrator may need to delete the node object by hand. Deleting the node object from Kubernetes causes all the Pod objects running on the node to be deleted from the apiserver, and frees up their names.

In version 1.12, TaintNodesByCondition feature is promoted to beta, so node lifecycle controller automatically creates taints that represent conditions. Similarly the scheduler ignores conditions when considering a Node; instead it looks at the Node’s taints and a Pod’s tolerations.

Now users can choose between the old scheduling model and a new, more flexible scheduling model. A Pod that does not have any tolerations gets scheduled according to the old model. But a Pod that tolerates the taints of a particular Node can be scheduled on that Node.

Caution: Enabling this feature creates a small delay between the time when a condition is observed and when a taint is created. This delay is usually less than one second, but it can increase the number of Pods that are successfully scheduled but rejected by the kubelet.

Capacity and Allocatable

Describes the resources available on the node: CPU, memory and the maximum number of pods that can be scheduled onto the node.

The fields in the capacity block indicate the total amount of resources that a Node has. The allocatable block indicates the amount of resources that on a Node that are available to be consumed by normal Pods.

You may read more about capacity and allocatable resources while learning how to reserve compute resources on a Node.

Info

Describes general information about the node, such as kernel version, Kubernetes version (kubelet and kube-proxy version), Docker version (if used), and OS name. This information is gathered by Kubelet from the node.

Management

Unlike pods and services, a node is not inherently created by Kubernetes: it is created externally by cloud providers like Google Compute Engine, or it exists in your pool of physical or virtual machines. So when Kubernetes creates a node, it creates an object that represents the node. After creation, Kubernetes checks whether the node is valid or not. For example, if you try to create a node from the following content:

{
  "kind": "Node",
  "apiVersion": "v1",
  "metadata": {
    "name": "10.240.79.157",
    "labels": {
      "name": "my-first-k8s-node"
    }
  }
}

Kubernetes creates a node object internally (the representation), and validates the node by health checking based on the metadata.name field. If the node is valid – that is, if all necessary services are running – it is eligible to run a pod. Otherwise, it is ignored for any cluster activity until it becomes valid.

Note: Kubernetes keeps the object for the invalid node and keeps checking to see whether it becomes valid. You must explicitly delete the Node object to stop this process.

Currently, there are three components that interact with the Kubernetes node interface: node controller, kubelet, and kubectl.

Node Controller

The node controller is a Kubernetes master component which manages various aspects of nodes.

The node controller has multiple roles in a node’s life. The first is assigning a CIDR block to the node when it is registered (if CIDR assignment is turned on).

The second is keeping the node controller’s internal list of nodes up to date with the cloud provider’s list of available machines. When running in a cloud environment, whenever a node is unhealthy, the node controller asks the cloud provider if the VM for that node is still available. If not, the node controller deletes the node from its list of nodes.

The third is monitoring the nodes’ health. The node controller is responsible for updating the NodeReady condition of NodeStatus to ConditionUnknown when a node becomes unreachable (i.e. the node controller stops receiving heartbeats for some reason, e.g. due to the node being down), and then later evicting all the pods from the node (using graceful termination) if the node continues to be unreachable. (The default timeouts are 40s to start reporting ConditionUnknown and 5m after that to start evicting pods.) The node controller checks the state of each node every --node-monitor-period seconds.

In versions of Kubernetes prior to 1.13, NodeStatus is the heartbeat from the node. Starting from Kubernetes 1.13, node lease feature is introduced as an alpha feature (feature gate NodeLease, KEP-0009). When node lease feature is enabled, each node has an associated Lease object in kube-node-lease namespace that is renewed by the node periodically, and both NodeStatus and node lease are treated as heartbeats from the node. Node leases are renewed frequently while NodeStatus is reported from node to master only when there is some change or enough time has passed (default is 1 minute, which is longer than the default timeout of 40 seconds for unreachable nodes). Since node lease is much more lightweight than NodeStatus, this feature makes node heartbeat significantly cheaper from both scalability and performance perspectives.

In Kubernetes 1.4, we updated the logic of the node controller to better handle cases when a large number of nodes have problems with reaching the master (e.g. because the master has networking problem). Starting with 1.4, the node controller looks at the state of all nodes in the cluster when making a decision about pod eviction.

In most cases, node controller limits the eviction rate to --node-eviction-rate (default 0.1) per second, meaning it won’t evict pods from more than 1 node per 10 seconds.

The node eviction behavior changes when a node in a given availability zone becomes unhealthy. The node controller checks what percentage of nodes in the zone are unhealthy (NodeReady condition is ConditionUnknown or ConditionFalse) at the same time. If the fraction of unhealthy nodes is at least --unhealthy-zone-threshold (default 0.55) then the eviction rate is reduced: if the cluster is small (i.e. has less than or equal to --large-cluster-size-threshold nodes - default 50) then evictions are stopped, otherwise the eviction rate is reduced to --secondary-node-eviction-rate (default 0.01) per second. The reason these policies are implemented per availability zone is because one availability zone might become partitioned from the master while the others remain connected. If your cluster does not span multiple cloud provider availability zones, then there is only one availability zone (the whole cluster).

A key reason for spreading your nodes across availability zones is so that the workload can be shifted to healthy zones when one entire zone goes down. Therefore, if all nodes in a zone are unhealthy then node controller evicts at the normal rate --node-eviction-rate. The corner case is when all zones are completely unhealthy (i.e. there are no healthy nodes in the cluster). In such case, the node controller assumes that there’s some problem with master connectivity and stops all evictions until some connectivity is restored.

Starting in Kubernetes 1.6, the NodeController is also responsible for evicting pods that are running on nodes with NoExecute taints, when the pods do not tolerate the taints. Additionally, as an alpha feature that is disabled by default, the NodeController is responsible for adding taints corresponding to node problems like node unreachable or not ready. See this documentation for details about NoExecute taints and the alpha feature.

Starting in version 1.8, the node controller can be made responsible for creating taints that represent Node conditions. This is an alpha feature of version 1.8.

Self-Registration of Nodes

When the kubelet flag --register-node is true (the default), the kubelet will attempt to register itself with the API server. This is the preferred pattern, used by most distros.

For self-registration, the kubelet is started with the following options:

When the Node authorization mode and NodeRestriction admission plugin are enabled, kubelets are only authorized to create/modify their own Node resource.

Manual Node Administration

A cluster administrator can create and modify node objects.

If the administrator wishes to create node objects manually, set the kubelet flag --register-node=false.

The administrator can modify node resources (regardless of the setting of --register-node). Modifications include setting labels on the node and marking it unschedulable.

Labels on nodes can be used in conjunction with node selectors on pods to control scheduling, e.g. to constrain a pod to only be eligible to run on a subset of the nodes.

Marking a node as unschedulable prevents new pods from being scheduled to that node, but does not affect any existing pods on the node. This is useful as a preparatory step before a node reboot, etc. For example, to mark a node unschedulable, run this command:

kubectl cordon $NODENAME
Note: Pods created by a DaemonSet controller bypass the Kubernetes scheduler and do not respect the unschedulable attribute on a node. This assumes that daemons belong on the machine even if it is being drained of applications while it prepares for a reboot.

Node capacity

The capacity of the node (number of cpus and amount of memory) is part of the node object. Normally, nodes register themselves and report their capacity when creating the node object. If you are doing manual node administration, then you need to set node capacity when adding a node.

The Kubernetes scheduler ensures that there are enough resources for all the pods on a node. It checks that the sum of the requests of containers on the node is no greater than the node capacity. It includes all containers started by the kubelet, but not containers started directly by the container runtime nor any process running outside of the containers.

If you want to explicitly reserve resources for non-Pod processes, follow this tutorial to reserve resources for system daemons.

API Object

Node is a top-level resource in the Kubernetes REST API. More details about the API object can be found at: Node API object.

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