Services in Kubernetes

Table of Contents

Services in Kubernetes

Overview

Kubernetes Pods are mortal. They are born and they die, and 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:

You want to have an external database cluster in production, but in test you use your own databases.
You want to point your service to a service in another Namespace or on another cluster.
You are migrating your workload to Kubernetes and some of your backends run outside of Kubernetes.

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 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": 80 }
            ]
        }
    ]
}

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 (1.2.3.4:80 in this example).

Virtual IPs and service proxies

Every node in a Kubernetes cluster runs a kube-proxy. This application 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 made to that port will be proxied to one of the corresponding backend Pods. Which backend to use is decided based on the SessionAffinity of the Service. Lastly, it installs iptables rules which capture traffic to the Service's cluster IP (which is virtual) and Port and redirects that traffic to the previously described port.

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

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 of Kubernetes 1.0, Services are a "layer 3" (TCP/UDP over IP) construct. We do not yet have a concept of "layer 7" (HTTP) services.

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:

There is a long history of DNS libraries not respecting DNS TTLs and caching the results of name lookups.
Many apps do DNS lookups once and cache the results.
Even if apps and libraries did proper re-resolution, the load of every client re-resolving DNS over and over would be difficult to manage.

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".

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).

For such Services, a cluster IP is not allocated. DNS is configured to return multiple A records (addresses) for the Service name, which point directly to the Pods backing the Service. Additionally, the kube proxy does not handle these services and there is no load balancing or proxying done by the platform for them. The endpoints controller will still create Endpoints records in the API.

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

External services

For some parts of your application (e.g. frontends) you may want to expose a Service onto an external (outside of your cluster, maybe public internet) IP address. Kubernetes supports two ways of doing this: NodePorts and LoadBalancers.

Every Service has a type field which defines how the Service can be accessed. Valid values for this field are:

ClusterIP: use a cluster-internal IP only - this is the default and is discussed above
NodePort: use a cluster IP, but also expose the service on a port on each node of the cluster (the same port on each node)
LoadBalancer: use a ClusterIP and a NodePort, but also ask the cloud provider for a load balancer which forwards to the Service

Note that while NodePorts can be TCP or UDP, LoadBalancers only support TCP as of Kubernetes 1.0.

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. 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 cloud environments that are not fully supported by Kubernetes, or even to just expose one or more nodes' IPs directly.

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",
        "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.

Shortcomings

We expect that using iptables and userspace proxies for VIPs will work at small to medium scale, but may not scale to very large clusters with thousands of Services. See the original design proposal for portals for more details.

Using the kube-proxy obscures the source-IP of a packet accessing a Service. This makes some kinds of firewalling impossible.

LoadBalancers only support TCP, not UDP.

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.

There's a proposal to eliminate userspace proxying in favor of doing it all in iptables. This should perform better and fix the source-IP obfuscation, though is less flexible than arbitrary userspace code.

We intend to have first-class 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 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.

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.

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.

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