Ingress vs Load Balancer

I found this very interesting article which explains the differences between NodePort, LoadBalancer and Ingress.

From the content present in the article:

LoadBalancer:

A LoadBalancer service is the standard way to expose a service to the internet. On GKE, this will spin up a Network Load Balancer that will give you a single IP address that will forward all traffic to your service.

If you want to directly expose a service, this is the default method. All traffic on the port you specify will be forwarded to the service. There is no filtering, no routing, etc. This means you can send almost any kind of traffic to it, like HTTP, TCP, UDP, Websockets, gRPC, or whatever.

The big downside is that each service you expose with a LoadBalancer will get its own IP address, and you have to pay for a LoadBalancer per exposed service, which can get expensive!

Ingress:

Ingress is actually NOT a type of service. Instead, it sits in front of multiple services and act as a “smart router” or entrypoint into your cluster.

You can do a lot of different things with an Ingress, and there are many types of Ingress controllers that have different capabilities.

The default GKE ingress controller will spin up a HTTP(S) Load Balancer for you. This will let you do both path based and subdomain based routing to backend services. For example, you can send everything on foo.yourdomain.example to the foo service, and everything under the yourdomain.example/bar/ path to the bar service.

Ingress is probably the most powerful way to expose your services, but can also be the most complicated. There are many types of Ingress controllers, from the Google Cloud Load Balancer, Nginx, Contour, Istio, and more. There are also plugins for Ingress controllers, like the cert-manager, that can automatically provision SSL certificates for your services.

Ingress is the most useful if you want to expose multiple services under the same IP address, and these services all use the same L7 protocol (typically HTTP). You only pay for one load balancer if you are using the native GCP integration, and because Ingress is “smart” you can get a lot of features out of the box (like SSL, Auth, Routing, etc)

There are 4 ways to allow pods in your cluster to receive external traffic:
1.) Pod using HostNetworking: true and (Allows 1 pod per node to listen directly to ports on the host node. Minikube, bare metal, and rasberry pi's sometimes go this route which can allow the host node to listen on port 80/443 allow not using a load balancer or advanced cloud load balancer configurations, it also bypasses Kubernetes Services which can be useful for avoiding SNAT/achieving similar effect of externalTrafficPolicy: Local in scenarios where it's not supported like on AWS.)
2.) NodePort Service
3.) LoadBalancer Service (Which builds on NodePort Service)
4.) Ingress Controller + Ingress Objects (Which builds upon the above)

Lets say you have 10 websites hosted in your cluster and you want to expose them all to external traffic.
*If you use type LoadBalancer Service you'll spawn 10 HA Cloud load balancers (each costs money)
*If you use type Ingress Controller you'll spawn 1 HA Cloud load balancer(saves money), and it'll point to an Ingress Controller running in your cluster.

An Ingress Controller is:

• A service of type Load Balancer backed by a deployment of pods running in your cluster.
• Each pod does 2 things:
  1. Acts as a Layer 7 Load Balancer running inside your cluster. (Comes in many flavors Nginx is popular)
  2. Dynamically configures itself based on Ingress Objects in your cluster
     (Ingress Objects can be thought of as declarative configuration snippits of a Layer 7 Load Balancer.)

The L7 LB/Ingress Controller inside your cluster Load Balances / reverse proxies traffic to Cluster IP Services inside your Cluster, it can also terminate HTTPS if you have a Kubernetes Secret of type TLS cert, and Ingress object that references it.)

TL:DR

1. Ingress sits between the public network (Internet) and the Kubernetes services that publicly expose our Api's implementation.
2. Ingress is capable to provide Load Balancing, SSL termination, and name-based virtual hosting.
3. Ingress capabilities allows to securely expose multiple API's or Applications from a single domain name.

Let's start with practical use case: you have multiple Apis backed by service implementation packages (ASIP for clariy and brevity) to deploy under one single domain name. As you are a cutting edge developer, you implemented a micro-services architecture that requires separate deployments for each ASIP so they can be upgraded or scaled individually. Of course,these ASIPs are encapsulated in individual docker container and available to Kubernetes (K8s) from container repository.

Let's say now that you want to deploy this on Google's GKE K8s. To implement sustained availability, each ASIP instance (replica) is deployed on different nodes (VM) where each VM has it's own cloud internal IP address. Each ASIP deployment is configured in an aptly name "deployment.yaml" file where you declaratively specify, among other things, the number of replicas of the given ASIP K8s should deploy.

The next step is to expose the API to the ouside world and funnel requests to one of the deployed ASIP instance. Since we have many replicas of the same ASIP running on different nodes, we need something that will distribute the request among those replicas. To resolve this, we can create and apply a "service.yaml" file that will configure a K8s service (KServ) that will be externally exposed and accessible through an IP address. This KServ will take charge of the API's request distribution among it's configured ASIPs. Note that a KServ will be automatically reconfigured by the K8s master when an ASIP's node fail and is restarted. Internal IP address are never reused in such case and the KServ must be advised of the new ASIP's deployment location.

But we have other Api service packages that shall be exposed on the same domain name. Spinning a new KServ will create a new external IP address and we won't be able to expose it on the same domain name. Well, this is where Ingress comes in.

Ingress sit's between the Internet and all the KServices we expose to to the outside world. Ingress is capable to provide load balancing, SSL termination and name-based virtual hosting. The latter capacity is able to route an incoming request to the right service by analysing it's URL. Of Course, Ingress must be configured and applied with an... "ingress.yaml" file that will specify the rewrites and the routes required to send a request to the right KServ.

Internet -> Ingress -> K8s Services -> Replicas

So, with the right ingress, KServices and ASIPs configuration, we can securely expose many API's using the same domain name.

In simple words, load balancer distributes the requests among multiple backend services (of same type) whereas ingress is more like an API gateway (reverse proxy) which routes the request to a specific backend service based on, for instance, the URL.

Short Version:

In Kubernetes, Object definitions define desired state, while controllers watch Object definitions to achieve that state.

Ingress:

• "Ingress" Object that does little on its own, but defines L7 Load Balancing rules
• "Ingress Controller" that watches state of Ingress Objects to create L7 LB configuration based on rules defined in the Ingress Objects

LoadBalancer:

• "Service" Object of type "LoadBalancer" that allows a service to be attached to a LoadBalancer
• "Load Balancer Controller" that creates Load Balancers based on rules defined in the Service Object

Ingress

Ingress Object:

A kubernetes Object that does not do anything on its own because an Ingress Controller is not included by default. An Ingress Object just describes a way to route Layer 7 traffic into your cluster by specifying things like the request path, request domain, and target kubernetes service, while adding a service object may actually creates services because a service controller is included in Kubernetes by default.

Ingress Controller:

A Kubernetes Deployment/DaemonSet + Service which:

  1. listens on specific ports (usually 80 and 443) for web traffic
  2. Watches for the creation, modification, or deletion of Ingress Resources
  3. Creates internal L7 routing rules based on desired state indicated by Ingress Objects

For example, the Nginx Ingress Controller may:

1. Use a service to listen on port 80 and 443 for incoming traffic
2. Watch for creation of Ingress Objects and convert the desired state into new server{} sections which are dynamically placed into nginx.conf

LoadBalancer

Load Balancer Controller:

Load Balancer Controllers may be configured in platforms such as AWS and GKE and provide a way to assign external IPs through the creation of external load balancers. This functionality can be used by:

1. Deploying the Load Balancer Controller if it wasn't deployed yet
2. Setting a service to type "LoadBalancer"
3. Setting appropriate annotations in the Service to configure the Load Balancer

Service Type:

When the service type is set to LoadBalancer and there is a Cloud provided Load Balancer Controller present, the service is exposed externally using a cloud provider's load balancer. NodePort and ClusterIP Services, to which the external load balancer routes, are automatically created thereby assigning a service external IPs and or DNS.

Relationships

Ingress Controller Services are often provisioned as LoadBalancer type, so that http and https requests can be proxied / routed to specific internal services through an external ip.

However, a LoadBalancer is not strictly needed for this. Since, through the use of hostNetwork or hostPort you can technically bind a port on the host to a service (allowing you to visit it via the hosts external ip:port). Though officially this is not recommended as it uses up ports on the actual node.

References

https://kubernetes.io/docs/concepts/configuration/overview/#services

https://kubernetes.io/docs/tasks/access-application-cluster/create-external-load-balancer/

https://kubernetes.io/docs/tasks/access-application-cluster/create-external-load-balancer/#external-load-balancer-providers

https://kubernetes.io/docs/concepts/services-networking/ingress/ https://kubernetes.io/docs/concepts/architecture/cloud-controller/ https://kubernetes-sigs.github.io/aws-load-balancer-controller/v2.3/

I hight recommend reading NodePort vs LoadBalancer vs Ingress? for Knowledge ++

Loadbalancer service: is a layer 4 proxy (TCP, UDP,..). It runs outside Kubernetes cluster-network. It doesn't have features of layer 7: CircuitBreaker, measure number of request, request's latency, failure, routing,...

ingress: is a layer 7 proxy (http, https, gRPC,..). It is an application in a pod inside Kubernetes cluster-network. If the ingress pod dies, Kubernetes will restart it or move it to other node in the cluster.

Pods have their own IP:PORT but it is dynamic in nature and changes if deleted or redeployed.

Services are assigned ClusterIPor NodePort(a port in a VM where service resource is created) which can be mapped to set of pods or other backend [see: headless services]

• To access correct Pod use ClusterIP(from within clutser)

• NodePort can be used to access pods from outside of the cluster

LoadBalancer[External/Internal]: provided by cloud providers points to either ClusterIP or NodePort. You can access the service by using LB's IP

LB ~> SERVICE(ClusterIP or NodePort) ~> POD

Ingress resource is an entrypoint to cluster.A LB can listen to ingress rules and can route to specific service.[see this example]

LB(Ingress-managed) ~> SERVICE(ClusterIP or NodePort) ~> POD

In order to understand this, I think it is good to ask "why do we need ingress why load balancer is not sufficient"

The best answer I found in the book Kubernetes in action.

Understanding why Ingresses are needed

One important reason is that each LoadBalancer service requires its own load balancer with its own public IP address, whereas an Ingress only requires one, even when providing access to dozens of services. When a client sends an HTTP request to the Ingress, the host and path in the request determine which service the request is forwarded to