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Detailed explanations of Kubernetes system concepts and abstractions.

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Services

Kubernetes Pods are mortal. They are born and when they die, they are not resurrected. ReplicationControllers in particular create and destroy Pods dynamically (e.g. when scaling up or down or when doing rolling updates). While each Pod gets its own IP address, even those IP addresses cannot be relied upon to be stable over time. This leads to a problem: if some set of Pods (let’s call them backends) provides functionality to other Pods (let’s call them frontends) inside the Kubernetes cluster, how do those frontends find out and keep track of which backends are in that set?

Enter Services.

A Kubernetes Service is an abstraction which defines a logical set of Pods and a policy by which to access them - sometimes called a micro-service. The set of Pods targeted by a Service is (usually) determined by a Label Selector (see below for why you might want a Service without a selector).

As an example, consider an image-processing backend which is running with 3 replicas. Those replicas are fungible - frontends do not care which backend they use. While the actual Pods that compose the backend set may change, the frontend clients should not need to be aware of that or keep track of the list of backends themselves. The Service abstraction enables this decoupling.

For Kubernetes-native applications, Kubernetes offers a simple Endpoints API that is updated whenever the set of Pods in a Service changes. For non-native applications, Kubernetes offers a virtual-IP-based bridge to Services which redirects to the backend Pods.

Defining a service

A Service in Kubernetes is a REST object, similar to a Pod. Like all of the REST objects, a Service definition can be POSTed to the apiserver to create a new instance. For example, suppose you have a set of Pods that each expose port 9376 and carry a label "app=MyApp".

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376

This specification will create a new Service object named “my-service” which targets TCP port 9376 on any Pod with the "app=MyApp" label. This Service will also be assigned an IP address (sometimes called the “cluster IP”), which is used by the service proxies (see below). The Service’s selector will be evaluated continuously and the results will be POSTed to an Endpoints object also named “my-service”.

Note that a Service can map an incoming port to any targetPort. By default the targetPort will be set to the same value as the port field. Perhaps more interesting is that targetPort can be a string, referring to the name of a port in the backend Pods. The actual port number assigned to that name can be different in each backend Pod. This offers a lot of flexibility for deploying and evolving your Services. For example, you can change the port number that pods expose in the next version of your backend software, without breaking clients.

Kubernetes Services support TCP and UDP for protocols. The default is TCP.

Services without selectors

Services generally abstract access to Kubernetes Pods, but they can also abstract other kinds of backends. For example:

In any of these scenarios you can define a service without a selector:

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376

Because this service has no selector, the corresponding Endpoints object will not be created. You can manually map the service to your own specific endpoints:

kind: Endpoints
apiVersion: v1
metadata:
  name: my-service
subsets:
  - addresses:
      - ip: 1.2.3.4
    ports:
      - port: 9376

NOTE: Endpoint IPs may not be loopback (127.0.0.0/8), link-local (169.254.0.0/16), or link-local multicast (224.0.0.0/24).

Accessing a Service without a selector works the same as if it had selector. The traffic will be routed to endpoints defined by the user (1.2.3.4:9376 in this example).

An ExternalName service is a special case of service that does not have selectors. It does not define any ports or endpoints. Rather, it serves as a way to return an alias to an external service residing outside the cluster.

kind: Service
apiVersion: v1
metadata:
  name: my-service
  namespace: prod
spec:
  type: ExternalName
  externalName: my.database.example.com

When looking up the host my-service.prod.svc.CLUSTER, the cluster DNS service will return a CNAME record with the value my.database.example.com. Accessing such a service works in the same way as others, with the only difference that the redirection happens at the DNS level and no proxying or forwarding occurs. Should you later decide to move your database into your cluster, you can start its pods, add appropriate selectors or endpoints and change the service type.

Virtual IPs and service proxies

Every node in a Kubernetes cluster runs a kube-proxy. kube-proxy is responsible for implementing a form of virtual IP for Services of type other than ExternalName. In Kubernetes v1.0 the proxy was purely in userspace. In Kubernetes v1.1 an iptables proxy was added, but was not the default operating mode. Since Kubernetes v1.2, the iptables proxy is the default.

As of Kubernetes v1.0, Services are a “layer 4” (TCP/UDP over IP) construct. In Kubernetes v1.1 the Ingress API was added (beta) to represent “layer 7” (HTTP) services.

Proxy-mode: userspace

In this mode, kube-proxy watches the Kubernetes master for the addition and removal of Service and Endpoints objects. For each Service it opens a port (randomly chosen) on the local node. Any connections to this “proxy port” will be proxied to one of the Service’s backend Pods (as reported in Endpoints). Which backend Pod to use is decided based on the SessionAffinity of the Service. Lastly, it installs iptables rules which capture traffic to the Service’s clusterIP (which is virtual) and Port and redirects that traffic to the proxy port which proxies the backend Pod.

The net result is that any traffic bound for the Service’s IP:Port is proxied to an appropriate backend without the clients knowing anything about Kubernetes or Services or Pods.

By default, the choice of backend is round robin. Client-IP based session affinity can be selected by setting service.spec.sessionAffinity to "ClientIP" (the default is "None").

Services overview diagram for userspace proxy

Proxy-mode: iptables

In this mode, kube-proxy watches the Kubernetes master for the addition and removal of Service and Endpoints objects. For each Service it installs iptables rules which capture traffic to the Service’s clusterIP (which is virtual) and Port and redirects that traffic to one of the Service’s backend sets. For each Endpoints object it installs iptables rules which select a backend Pod.

By default, the choice of backend is random. Client-IP based session affinity can be selected by setting service.spec.sessionAffinity to "ClientIP" (the default is "None").

As with the userspace proxy, the net result is that any traffic bound for the Service’s IP:Port is proxied to an appropriate backend without the clients knowing anything about Kubernetes or Services or Pods. This should be faster and more reliable than the userspace proxy. However, unlike the userspace proxier, the iptables proxier cannot automatically retry another Pod if the one it initially selects does not respond, so it depends on having working readiness probes.

Services overview diagram for iptables proxy

Multi-Port Services

Many Services need to expose more than one port. For this case, Kubernetes supports multiple port definitions on a Service object. When using multiple ports you must give all of your ports names, so that endpoints can be disambiguated. For example:

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
    selector:
      app: MyApp
    ports:
      - name: http
        protocol: TCP
        port: 80
        targetPort: 9376
      - name: https
        protocol: TCP
        port: 443
        targetPort: 9377

Choosing your own IP address

You can specify your own cluster IP address as part of a Service creation request. To do this, set the spec.clusterIP field. For example, if you already have an existing DNS entry that you wish to replace, or legacy systems that are configured for a specific IP address and difficult to re-configure. The IP address that a user chooses must be a valid IP address and within the service-cluster-ip-range CIDR range that is specified by flag to the API server. If the IP address value is invalid, the apiserver returns a 422 HTTP status code to indicate that the value is invalid.

Why not use round-robin DNS?

A question that pops up every now and then is why we do all this stuff with virtual IPs rather than just use standard round-robin DNS. There are a few reasons:

We try to discourage users from doing things that hurt themselves. That said, if enough people ask for this, we may implement it as an alternative.

Discovering services

Kubernetes supports 2 primary modes of finding a Service - environment variables and DNS.

Environment variables

When a Pod is run on a Node, the kubelet adds a set of environment variables for each active Service. It supports both Docker links compatible variables (see makeLinkVariables) and simpler {SVCNAME}_SERVICE_HOST and {SVCNAME}_SERVICE_PORT variables, where the Service name is upper-cased and dashes are converted to underscores.

For example, the Service "redis-master" which exposes TCP port 6379 and has been allocated cluster IP address 10.0.0.11 produces the following environment variables:

REDIS_MASTER_SERVICE_HOST=10.0.0.11
REDIS_MASTER_SERVICE_PORT=6379
REDIS_MASTER_PORT=tcp://10.0.0.11:6379
REDIS_MASTER_PORT_6379_TCP=tcp://10.0.0.11:6379
REDIS_MASTER_PORT_6379_TCP_PROTO=tcp
REDIS_MASTER_PORT_6379_TCP_PORT=6379
REDIS_MASTER_PORT_6379_TCP_ADDR=10.0.0.11

This does imply an ordering requirement - any Service that a Pod wants to access must be created before the Pod itself, or else the environment variables will not be populated. DNS does not have this restriction.

DNS

An optional (though strongly recommended) cluster add-on is a DNS server. The DNS server watches the Kubernetes API for new Services and creates a set of DNS records for each. If DNS has been enabled throughout the cluster then all Pods should be able to do name resolution of Services automatically.

For example, if you have a Service called "my-service" in Kubernetes Namespace "my-ns" a DNS record for "my-service.my-ns" is created. Pods which exist in the "my-ns" Namespace should be able to find it by simply doing a name lookup for "my-service". Pods which exist in other Namespaces must qualify the name as "my-service.my-ns". The result of these name lookups is the cluster IP.

Kubernetes also supports DNS SRV (service) records for named ports. If the "my-service.my-ns" Service has a port named "http" with protocol TCP, you can do a DNS SRV query for "_http._tcp.my-service.my-ns" to discover the port number for "http".

The Kubernetes DNS server is the only way to access services of type ExternalName. More information is available in the DNS Pods and Services.

Headless services

Sometimes you don’t need or want load-balancing and a single service IP. In this case, you can create “headless” services by specifying "None" for the cluster IP (spec.clusterIP).

This option allows developers to reduce coupling to the Kubernetes system by allowing them freedom to do discovery their own way. Applications can still use a self-registration pattern and adapters for other discovery systems could easily be built upon this API.

For such Services, a cluster IP is not allocated, kube-proxy does not handle these services, and there is no load balancing or proxying done by the platform for them. How DNS is automatically configured depends on whether the service has selectors defined.

With selectors

For headless services that define selectors, the endpoints controller creates Endpoints records in the API, and modifies the DNS configuration to return A records (addresses) that point directly to the Pods backing the Service.

Without selectors

For headless services that do not define selectors, the endpoints controller does not create Endpoints records. However, the DNS system looks for and configures either:

Publishing services - service types

For some parts of your application (e.g. frontends) you may want to expose a Service onto an external (outside of your cluster) IP address.

Kubernetes ServiceTypes allow you to specify what kind of service you want. The default is ClusterIP.

Type values and their behaviors are:

Type NodePort

If you set the type field to "NodePort", the Kubernetes master will allocate a port from a flag-configured range (default: 30000-32767), and each Node will proxy that port (the same port number on every Node) into your Service. That port will be reported in your Service’s spec.ports[*].nodePort field.

If you want a specific port number, you can specify a value in the nodePort field, and the system will allocate you that port or else the API transaction will fail (i.e. you need to take care about possible port collisions yourself). The value you specify must be in the configured range for node ports.

This gives developers the freedom to set up their own load balancers, to configure environments that are not fully supported by Kubernetes, or even to just expose one or more nodes’ IPs directly.

Note that this Service will be visible as both <NodeIP>:spec.ports[*].nodePort and spec.clusterIp:spec.ports[*].port.

Type LoadBalancer

On cloud providers which support external load balancers, setting the type field to "LoadBalancer" will provision a load balancer for your Service. The actual creation of the load balancer happens asynchronously, and information about the provisioned balancer will be published in the Service’s status.loadBalancer field. For example:

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376
      nodePort: 30061
  clusterIP: 10.0.171.239
  loadBalancerIP: 78.11.24.19
  type: LoadBalancer
status:
  loadBalancer:
    ingress:
      - ip: 146.148.47.155

Traffic from the external load balancer will be directed at the backend Pods, though exactly how that works depends on the cloud provider. Some cloud providers allow the loadBalancerIP to be specified. In those cases, the load-balancer will be created with the user-specified loadBalancerIP. If the loadBalancerIP field is not specified, an ephemeral IP will be assigned to the loadBalancer. If the loadBalancerIP is specified, but the cloud provider does not support the feature, the field will be ignored.

Special notes for Azure: To use user-specified public type loadBalancerIP, a static type public IP address resource needs to be created first, and it should be in the same resource group of the cluster. Then you could specify the assigned IP address as loadBalancerIP.

Internal load balancer

In a mixed environment it is sometimes necessary to route traffic from services inside the same VPC.

In a split-horizon DNS environment you would need two services to be able to route both external and internal traffic to your endpoints.

This can be achieved by adding the following annotations to the service based on cloud provider.

For AWS:

[...]
metadata:
    name: my-service
    annotations:
        service.beta.kubernetes.io/aws-load-balancer-internal: 0.0.0.0/0
[...]

For Azure:

[...]
metadata: 
    name: my-service
    annotations: 
        service.beta.kubernetes.io/azure-load-balancer-internal: "true"
[...]

SSL support on AWS

For partial SSL support on clusters running on AWS, starting with 1.3 two annotations can be added to a LoadBalancer service:

    metadata:
      name: my-service
      annotations:
        service.beta.kubernetes.io/aws-load-balancer-ssl-cert: arn:aws:acm:us-east-1:123456789012:certificate/12345678-1234-1234-1234-123456789012

The first specifies which certificate to use. It can be either a certificate from a third party issuer that was uploaded to IAM or one created within AWS Certificate Manager.

    metadata:
      name: my-service
      annotations:
         service.beta.kubernetes.io/aws-load-balancer-backend-protocol: (https|http|ssl|tcp)

The second annotation specifies which protocol a pod speaks. For HTTPS and SSL, the ELB will expect the pod to authenticate itself over the encrypted connection.

HTTP and HTTPS will select layer 7 proxying: the ELB will terminate the connection with the user, parse headers and inject the X-Forwarded-For header with the user’s IP address (pods will only see the IP address of the ELB at the other end of its connection) when forwarding requests.

TCP and SSL will select layer 4 proxying: the ELB will forward traffic without modifying the headers.

External IPs

If there are external IPs that route to one or more cluster nodes, Kubernetes services can be exposed on those externalIPs. Traffic that ingresses into the cluster with the external IP (as destination IP), on the service port, will be routed to one of the service endpoints. externalIPs are not managed by Kubernetes and are the responsibility of the cluster administrator.

In the ServiceSpec, externalIPs can be specified along with any of the ServiceTypes. In the example below, my-service can be accessed by clients on 80.11.12.10:80 (externalIP:port)

kind: Service
apiVersion: v1
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - name: http
      protocol: TCP
      port: 80
      targetPort: 9376
  externalIPs:
    - 80.11.12.10

Shortcomings

Using the userspace proxy for VIPs will work at small to medium scale, but will not scale to very large clusters with thousands of Services. See the original design proposal for portals for more details.

Using the userspace proxy obscures the source-IP of a packet accessing a Service. This makes some kinds of firewalling impossible. The iptables proxier does not obscure in-cluster source IPs, but it does still impact clients coming through a load-balancer or node-port.

The Type field is designed as nested functionality - each level adds to the previous. This is not strictly required on all cloud providers (e.g. Google Compute Engine does not need to allocate a NodePort to make LoadBalancer work, but AWS does) but the current API requires it.

Future work

In the future we envision that the proxy policy can become more nuanced than simple round robin balancing, for example master-elected or sharded. We also envision that some Services will have “real” load balancers, in which case the VIP will simply transport the packets there.

We intend to improve our support for L7 (HTTP) Services.

We intend to have more flexible ingress modes for Services which encompass the current ClusterIP, NodePort, and LoadBalancer modes and more.

The gory details of virtual IPs

The previous information should be sufficient for many people who just want to use Services. However, there is a lot going on behind the scenes that may be worth understanding.

Avoiding collisions

One of the primary philosophies of Kubernetes is that users should not be exposed to situations that could cause their actions to fail through no fault of their own. In this situation, we are looking at network ports - users should not have to choose a port number if that choice might collide with another user. That is an isolation failure.

In order to allow users to choose a port number for their Services, we must ensure that no two Services can collide. We do that by allocating each Service its own IP address.

To ensure each service receives a unique IP, an internal allocator atomically updates a global allocation map in etcd prior to creating each service. The map object must exist in the registry for services to get IPs, otherwise creations will fail with a message indicating an IP could not be allocated. A background controller is responsible for creating that map (to migrate from older versions of Kubernetes that used in memory locking) as well as checking for invalid assignments due to administrator intervention and cleaning up any IPs that were allocated but which no service currently uses.

IPs and VIPs

Unlike Pod IP addresses, which actually route to a fixed destination, Service IPs are not actually answered by a single host. Instead, we use iptables (packet processing logic in Linux) to define virtual IP addresses which are transparently redirected as needed. When clients connect to the VIP, their traffic is automatically transported to an appropriate endpoint. The environment variables and DNS for Services are actually populated in terms of the Service’s VIP and port.

We support two proxy modes - userspace and iptables, which operate slightly differently.

Userspace

As an example, consider the image processing application described above. When the backend Service is created, the Kubernetes master assigns a virtual IP address, for example 10.0.0.1. Assuming the Service port is 1234, the Service is observed by all of the kube-proxy instances in the cluster. When a proxy sees a new Service, it opens a new random port, establishes an iptables redirect from the VIP to this new port, and starts accepting connections on it.

When a client connects to the VIP the iptables rule kicks in, and redirects the packets to the Service proxy’s own port. The Service proxy chooses a backend, and starts proxying traffic from the client to the backend.

This means that Service owners can choose any port they want without risk of collision. Clients can simply connect to an IP and port, without being aware of which Pods they are actually accessing.

Iptables

Again, consider the image processing application described above. When the backend Service is created, the Kubernetes master assigns a virtual IP address, for example 10.0.0.1. Assuming the Service port is 1234, the Service is observed by all of the kube-proxy instances in the cluster. When a proxy sees a new Service, it installs a series of iptables rules which redirect from the VIP to per-Service rules. The per-Service rules link to per-Endpoint rules which redirect (Destination NAT) to the backends.

When a client connects to the VIP the iptables rule kicks in. A backend is chosen (either based on session affinity or randomly) and packets are redirected to the backend. Unlike the userspace proxy, packets are never copied to userspace, the kube-proxy does not have to be running for the VIP to work, and the client IP is not altered.

This same basic flow executes when traffic comes in through a node-port or through a load-balancer, though in those cases the client IP does get altered.

API Object

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

For More Information

Read Connecting a Front End to a Back End Using a Service.

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