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Jérôme Petazzoni e6eb157cc6 🪓 Split "kubectl expose" and "service types"

2023-01-13 17:50:22 +01:00

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Service Types

There are different types of services:

ClusterIP, NodePort, LoadBalancer, ExternalName
There are also headless services
Services can also have optional external IPs
There is also another resource type called Ingress

(specifically for HTTP services)
Wow, that's a lot! Let's start with the basics ...

`ClusterIP`

It's the default service type
A virtual IP address is allocated for the service

(in an internal, private range; e.g. 10.96.0.0/12)
This IP address is reachable only from within the cluster (nodes and pods)
Our code can connect to the service using the original port number
Perfect for internal communication, within the cluster

`LoadBalancer`

An external load balancer is allocated for the service

(typically a cloud load balancer, e.g. ELB on AWS, GLB on GCE ...)
This is available only when the underlying infrastructure provides some kind of "load balancer as a service"
Each service of that type will typically cost a little bit of money

(e.g. a few cents per hour on AWS or GCE)
Ideally, traffic would flow directly from the load balancer to the pods
In practice, it will often flow through a NodePort first

`NodePort`

A port number is allocated for the service

(by default, in the 30000-32767 range)
That port is made available on all our nodes and anybody can connect to it

(we can connect to any node on that port to reach the service)
Our code needs to be changed to connect to that new port number
Under the hood: kube-proxy sets up a bunch of iptables rules on our nodes
Sometimes, it's the only available option for external traffic

(e.g. most clusters deployed with kubeadm or on-premises)

`ExternalName`

Services of type ExternalName are quite different
No load balancer (internal or external) is created
Only a DNS entry gets added to the DNS managed by Kubernetes
That DNS entry will just be a CNAME to a provided record

Example:

kubectl create service externalname k8s --external-name kubernetes.io

Creates a CNAME k8s pointing to kubernetes.io

External IPs

We can add an External IP to a service, e.g.:

kubectl expose deploy my-little-deploy --port=80 --external-ip=1.2.3.4

1.2.3.4 should be the address of one of our nodes

(it could also be a virtual address, service address, or VIP, shared by multiple nodes)
Connections to 1.2.3.4:80 will be sent to our service
External IPs will also show up on services of type LoadBalancer

(they will be added automatically by the process provisioning the load balancer)

Headless services

Sometimes, we want to access our scaled services directly:
- if we want to save a tiny little bit of latency (typically less than 1ms)
- if we need to connect over arbitrary ports (instead of a few fixed ones)
- if we need to communicate over another protocol than UDP or TCP
- if we want to decide how to balance the requests client-side
- ...
In that case, we can use a "headless service"

Creating a headless services

A headless service is obtained by setting the clusterIP field to None

(Either with --cluster-ip=None, or by providing a custom YAML)
As a result, the service doesn't have a virtual IP address
Since there is no virtual IP address, there is no load balancer either
CoreDNS will return the pods' IP addresses as multiple A records
This gives us an easy way to discover all the replicas for a deployment

Services and endpoints

A service has a number of "endpoints"
Each endpoint is a host + port where the service is available
The endpoints are maintained and updated automatically by Kubernetes

Check the endpoints that Kubernetes has associated with our blue service:
```
kubectl describe service blue
```

]

In the output, there will be a line starting with Endpoints:.

That line will list a bunch of addresses in host:port format.

Viewing endpoint details

When we have many endpoints, our display commands truncate the list
```
kubectl get endpoints
```
If we want to see the full list, we can use one of the following commands:
```
kubectl describe endpoints blue
kubectl get endpoints blue -o yaml
```
These commands will show us a list of IP addresses
These IP addresses should match the addresses of the corresponding pods:
```
kubectl get pods -l app=blue -o wide
```

`endpoints` not `endpoint`

endpoints is the only resource that cannot be singular

$ kubectl get endpoint
error: the server doesn't have a resource type "endpoint"

This is because the type itself is plural (unlike every other resource)
There is no endpoint object: type Endpoints struct
The type doesn't represent a single endpoint, but a list of endpoints

`Ingress`

Ingresses are another type (kind) of resource
They are specifically for HTTP services

(not TCP or UDP)
They can also handle TLS certificates, URL rewriting ...
They require an Ingress Controller to function

???

:EN:- Service types: ClusterIP, NodePort, LoadBalancer

:FR:- Différents types de services : ClusterIP, NodePort, LoadBalancer

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Service Types

ClusterIP

LoadBalancer

NodePort

ExternalName