instruction: Explain EndpointSlices
output: EndpointSlices
  The EndpointSlice API is the mechanism that Kubernetes uses to let your Service scale to handle large numbers of backends, and allows the cluster to update its list of healthy backends efficiently.
  FEATURE STATE:
  Kubernetes v1.21 [stable]
  Kubernetes'
  EndpointSlice
  API provides a way to track network endpoints
  within a Kubernetes cluster.
  EndpointSlice API
  In Kubernetes, an EndpointSlice contains references to a set of network
  endpoints. The control plane automatically creates EndpointSlices
  for any Kubernetes Service that has a
  selector
  specified. These EndpointSlices include
  references to all the Pods that match the Service selector. EndpointSlices group
  network endpoints together by unique combinations of IP family, protocol,
  port number, and Service name.
  The name of a EndpointSlice object must be a valid
  DNS subdomain name
  As an example, here's a sample EndpointSlice object, that's owned by the
  example
  Kubernetes Service.
  apiVersion
  discovery.k8s.io/v1
  kind
  EndpointSlice
  metadata
  name
  example-abc
  labels
  kubernetes.io/service-name
  example
  addressType
  IPv4
  ports
  name
  http
  protocol
  port
  endpoints
  addresses
  "10.1.2.3"
  conditions
  ready
  true
  hostname
  pod-1
  nodeName
  node-1
  zone
  us-west2-a
  By default, the control plane creates and manages EndpointSlices to have no
  more than 100 endpoints each. You can configure this with the
  --max-endpoints-per-slice
  kube-controller-manager
  flag, up to a maximum of 1000.
  EndpointSlices act as the source of truth for
  kube-proxy
  when it comes to
  how to route internal traffic.
  Address types
  EndpointSlices support two address types:
  IPv4
  IPv6
  Each
  EndpointSlice
  object represents a specific IP address type. If you have
  a Service that is available via IPv4 and IPv6, there will be at least two
  EndpointSlice
  objects (one for IPv4, and one for IPv6).
  Conditions
  The EndpointSlice API stores conditions about endpoints that may be useful for consumers.
  The three conditions are
  serving
  terminating
  , and
  ready
  Serving
  FEATURE STATE:
  Kubernetes v1.26 [stable]
  serving
  condition indicates that the endpoint is currently serving responses, and
  so it should be used as a target for Service traffic. For endpoints backed by a Pod, this
  maps to the Pod's
  Ready
  condition.
  Terminating
  FEATURE STATE:
  Kubernetes v1.26 [stable]
  terminating
  condition indicates that the endpoint is
  terminating. For endpoints backed by a Pod, this condition is set when
  the Pod is first deleted (that is, when it receives a deletion
  timestamp, but most likely before the Pod's containers exit).
  Service proxies will normally ignore endpoints that are
  terminating
  but they may route traffic to endpoints that are both
  serving
  terminating
  if all available endpoints are
  terminating
  . (This
  helps to ensure that no Service traffic is lost during rolling updates
  of the underlying Pods.)
  Ready
  ready
  condition is essentially a shortcut for checking
  serving
  and not
  terminating
  " (though it will also always be
  true
  for Services with
  spec.publishNotReadyAddresses
  set to
  true
  Topology information
  Each endpoint within an EndpointSlice can contain relevant topology information.
  The topology information includes the location of the endpoint and information
  about the corresponding Node and zone. These are available in the following
  per endpoint fields on EndpointSlices:
  nodeName
  - The name of the Node this endpoint is on.
  zone
  - The zone this endpoint is in.
  Management
  Most often, the control plane (specifically, the endpoint slice
  controller
  ) creates and
  manages EndpointSlice objects. There are a variety of other use cases for
  EndpointSlices, such as service mesh implementations, that could result in other
  entities or controllers managing additional sets of EndpointSlices.
  To ensure that multiple entities can manage EndpointSlices without interfering
  with each other, Kubernetes defines the
  label
  endpointslice.kubernetes.io/managed-by
  , which indicates the entity managing
  an EndpointSlice.
  The endpoint slice controller sets
  endpointslice-controller.k8s.io
  as the value
  for this label on all EndpointSlices it manages. Other entities managing
  EndpointSlices should also set a unique value for this label.
  Ownership
  In most use cases, EndpointSlices are owned by the Service that the endpoint
  slice object tracks endpoints for. This ownership is indicated by an owner
  reference on each EndpointSlice as well as a
  kubernetes.io/service-name
  label that enables simple lookups of all EndpointSlices belonging to a Service.
  Distribution of EndpointSlices
  Each EndpointSlice has a set of ports that applies to all endpoints within the
  resource. When named ports are used for a Service, Pods may end up with
  different target port numbers for the same named port, requiring different
  EndpointSlices.
  The control plane tries to fill EndpointSlices as full as possible, but does not
  actively rebalance them. The logic is fairly straightforward:
  Iterate through existing EndpointSlices, remove endpoints that are no longer
  desired and update matching endpoints that have changed.
  Iterate through EndpointSlices that have been modified in the first step and
  fill them up with any new endpoints needed.
  If there's still new endpoints left to add, try to fit them into a previously
  unchanged slice and/or create new ones.
  Importantly, the third step prioritizes limiting EndpointSlice updates over a
  perfectly full distribution of EndpointSlices. As an example, if there are 10
  new endpoints to add and 2 EndpointSlices with room for 5 more endpoints each,
  this approach will create a new EndpointSlice instead of filling up the 2
  existing EndpointSlices. In other words, a single EndpointSlice creation is
  preferable to multiple EndpointSlice updates.
  With kube-proxy running on each Node and watching EndpointSlices, every change
  to an EndpointSlice becomes relatively expensive since it will be transmitted to
  every Node in the cluster. This approach is intended to limit the number of
  changes that need to be sent to every Node, even if it may result with multiple
  EndpointSlices that are not full.
  In practice, this less than ideal distribution should be rare. Most changes
  processed by the EndpointSlice controller will be small enough to fit in an
  existing EndpointSlice, and if not, a new EndpointSlice is likely going to be
  necessary soon anyway. Rolling updates of Deployments also provide a natural
  repacking of EndpointSlices with all Pods and their corresponding endpoints
  getting replaced.
  Duplicate endpoints
  Due to the nature of EndpointSlice changes, endpoints may be represented in more
  than one EndpointSlice at the same time. This naturally occurs as changes to
  different EndpointSlice objects can arrive at the Kubernetes client watch / cache
  at different times.
  Note:
  Clients of the EndpointSlice API must iterate through all the existing EndpointSlices
  associated to a Service and build a complete list of unique network endpoints. It is
  important to mention that endpoints may be duplicated in different EndpointSlices.
  You can find a reference implementation for how to perform this endpoint aggregation
  and deduplication as part of the
  EndpointSliceCache
  code within
  kube-proxy
  EndpointSlice mirroring
  FEATURE STATE:
  Kubernetes v1.33 [deprecated]
  The EndpointSlice API is a replacement for the older Endpoints API. To
  preserve compatibility with older controllers and user workloads that
  expect
  kube-proxy
  to route traffic based on Endpoints resources, the cluster's control
  plane mirrors most user-created Endpoints resources to corresponding
  EndpointSlices.
  (However, this feature, like the rest of the Endpoints API, is
  deprecated. Users who manually specify endpoints for selectorless
  Services should do so by creating EndpointSlice resources directly,
  rather than by creating Endpoints resources and allowing them to be
  mirrored.)
  The control plane mirrors Endpoints resources unless:
  the Endpoints resource has a
  endpointslice.kubernetes.io/skip-mirror
  label
  set to
  true
  the Endpoints resource has a
  control-plane.alpha.kubernetes.io/leader
  annotation.
  the corresponding Service resource does not exist.
  the corresponding Service resource has a non-nil selector.
  Individual Endpoints resources may translate into multiple EndpointSlices. This
  will occur if an Endpoints resource has multiple subsets or includes endpoints
  with multiple IP families (IPv4 and IPv6). A maximum of 1000 addresses per
  subset will be mirrored to EndpointSlices.
  What's next
  Follow the
  Connecting Applications with Services
  tutorial
  Read the
  API reference
  for the EndpointSlice API
  Read the
  API reference
  for the Endpoints API
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  Last modified April 09, 2025 at 5:08 AM PST:
  Update docs for deprecation of Endpoints API (#49831) (649bda2cbd)