pods are the smallest deployable units of computing that can be created and managed in Kubernetes.
A pod (as in a pod of whales or pea pod) is a group of one or more containers (such as Docker containers), the shared storage for those containers, and options about how to run the containers. Pods are always co-located and co-scheduled, and run in a shared context. A pod models an application-specific “logical host” - it contains one or more application containers which are relatively tightly coupled — in a pre-container world, they would have executed on the same physical or virtual machine.
While Kubernetes supports more container runtimes than just Docker, Docker is the most commonly known runtime, and it helps to describe pods in Docker terms.
The shared context of a pod is a set of Linux namespaces, cgroups, and potentially other facets of isolation - the same things that isolate a Docker container. Within a pod’s context, the individual applications may have further sub-isolations applied.
Containers within a pod share an IP address and port space, and
can find each other via
localhost. They can also communicate with each
other using standard inter-process communications like SystemV semaphores or
POSIX shared memory. Containers in different pods have distinct IP addresses
and can not communicate by IPC.
Applications within a pod also have access to shared volumes, which are defined as part of a pod and are made available to be mounted into each application’s filesystem.
Like individual application containers, pods are considered to be relatively ephemeral (rather than durable) entities. As discussed in life of a pod, pods are created, assigned a unique ID (UID), and scheduled to nodes where they remain until termination (according to restart policy) or deletion. If a node dies, the pods scheduled to that node are deleted, after a timeout period. A given pod (as defined by a UID) is not “rescheduled” to a new node; instead, it can be replaced by an identical pod, with even the same name if desired, but with a new UID (see replication controller for more details). (In the future, a higher-level API may support pod migration.)
When something is said to have the same lifetime as a pod, such as a volume, that means that it exists as long as that pod (with that UID) exists. If that pod is deleted for any reason, even if an identical replacement is created, the related thing (e.g. volume) is also destroyed and created anew.
A multi-container pod that contains a file puller and a web server that uses a persistent volume for shared storage between the containers.
Pods are a model of the pattern of multiple cooperating processes which form a cohesive unit of service. They simplify application deployment and management by providing a higher-level abstraction than the set of their constituent applications. Pods serve as unit of deployment, horizontal scaling, and replication. Colocation (co-scheduling), shared fate (e.g. termination), coordinated replication, resource sharing, and dependency management are handled automatically for containers in a pod.
Pods enable data sharing and communication among their constituents.
The applications in a pod all use the same network namespace (same IP and port
space), and can thus “find” each other and communicate using
Because of this, applications in a pod must coordinate their usage of ports.
Each pod has an IP address in a flat shared networking space that has full
communication with other physical computers and pods across the network.
The hostname is set to the pod’s Name for the application containers within the pod. More details on networking.
In addition to defining the application containers that run in the pod, the pod specifies a set of shared storage volumes. Volumes enable data to survive container restarts and to be shared among the applications within the pod.
Pods can be used to host vertically integrated application stacks (e.g. LAMP), but their primary motivation is to support co-located, co-managed helper programs, such as:
Individual pods are not intended to run multiple instances of the same application, in general.
For a longer explanation, see The Distributed System ToolKit: Patterns for Composite Containers.
Why not just run multiple programs in a single (Docker) container?
Why not support affinity-based co-scheduling of containers?
That approach would provide co-location, but would not provide most of the benefits of pods, such as resource sharing, IPC, guaranteed fate sharing, and simplified management.
Pods aren’t intended to be treated as durable pets. They won’t survive scheduling failures, node failures, or other evictions, such as due to lack of resources, or in the case of node maintenance.
In general, users shouldn’t need to create pods directly. They should almost always use controllers (e.g., replication controller), even for singletons. Controllers provide self-healing with a cluster scope, as well as replication and rollout management.
Pod is exposed as a primitive in order to facilitate:
The current best practice for pets is to create a replication controller with
replicas equal to
1 and a corresponding service. If you find this cumbersome, please comment on issue #260.
Because pods represent running processes on nodes in the cluster, it is important to allow those processes to gracefully terminate when they are no longer needed (vs being violently killed with a KILL signal and having no chance to clean up). Users should be able to request deletion and know when processes terminate, but also be able to ensure that deletes eventually complete. When a user requests deletion of a pod the system records the intended grace period before the pod is allowed to be forcefully killed, and a TERM signal is sent to the main process in each container. Once the grace period has expired the KILL signal is sent to those processes and the pod is then deleted from the API server. If the Kubelet or the container manager is restarted while waiting for processes to terminate, the termination will be retried with the full grace period.
An example flow:
preStophook is still running after the grace period expires, step 2 is then invoked with a small (2 second) extended grace period.
By default, all deletes are graceful within 30 seconds. The
kubectl delete command supports the
--grace-period=<seconds> option which allows a user to override the default and specify their own value. The value
0 indicates that delete should be immediate, and removes the pod in the API immediately so a new pod can be created with the same name. On the node pods that are set to terminate immediately will still be given a small grace period before being force killed.
From kubernetes v1.1, any container in a pod can enable privileged mode, using the
privileged flag on the
SecurityContext of the container spec. This is useful for containers that want to use linux capabilities like manipulating the network stack and accessing devices. Processes within the container get almost the same privileges that are available to processes outside a container. With privileged mode, it should be easier to write network and volume plugins as separate pods that don’t need to be compiled into the kubelet.
If the master is running kubernetes v1.1 or higher, and the nodes are running a version lower than v1.1, then new privileged pods will be accepted by api-server, but will not be launched. They will be pending state.
If user calls
kubectl describe pod FooPodName, user can see the reason why the pod is in pending state. The events table in the describe command output will say:
Error validating pod "FooPodName"."FooPodNamespace" from api, ignoring: spec.containers.securityContext.privileged: forbidden '<*>(0xc2089d3248)true'
If the master is running a version lower than v1.1, then privileged pods cannot be created. If user attempts to create a pod, that has a privileged container, the user will get the following error:
The Pod "FooPodName" is invalid.
spec.containers.securityContext.privileged: forbidden '<*>(0xc20b222db0)true'
Pod is a top-level resource in the kubernetes REST API. More details about the API object can be found at: Pod API object.