Ingresses & Ingress-Controller
A Kubernetes service is by default exclusive to the cluster.
Only applications running on the cluster can access them because of this.
An ingress in Kubernetes allows us to direct traffic from outside the cluster to one or more services there.
For all incoming traffic, the ingress typically serves as a single point of entry.
An ingress is assigned a public IP address (provisioned by your cloud provider), making it reachable from outside the cluster.
It then directs all of its traffic to the proper service using a set of rules, however, most Ingress-Controllers directly serve traffic to the pods and not through the service (by constantly checking the endpoint object).
When creating an ingress, there are a few things to consider.
They are initially made to manage web traffic (HTTP or HTTPS).
Although it is possible, using an ingress with other kinds of protocols usually requires additional configuration. Most importantly, the ingress object doesn't actually accomplish anything on its own. Therefore, we must have an ingress controller on hand for an ingress to actually function. Most cloud platforms provide their own ingress controllers, but there are also plenty of open-source options to choose from.
LoadBalancer
Ingresses and LoadBalancers in Kubernetes overlap quite a bit.
This is due to the fact that they are primarily employed to expose services to the internet.
LoadBalancers, however, differ from ingresses in several ways, a load balancer is merely an addition to a service, not a separate entity like an ingress.
For this to work, the cluster must be running on a provider that supports external load balancers. All of the major cloud providers support external load balancers using their own resource types:
- AWS uses a Network Load Balancer
- GKE also uses a Network Load Balancer
- Azure uses a Public Load Balancer
Load balancers can only route to a single service because they are defined per service. As opposed to an ingress, which can route to numerous services within the cluster, this is different. As you've noted, a LoadBalancer operatores on Layer 4 whereas most Ingress Controllers operate on Layer 7; however the Ingress-Controller is usually "exposed" by an external Layer 4 LB to make it accessible in the first place.
That being said, your Cloud-provider doesn't decide wether to create an Ingress or LoadBalancer, it obviously depends on what resource you're creating and your CNI (Container Network Interface), which most Cloud providers also have their own implementation will notify the necessary service to create the resource you want. Also, keep in mind that regardless of the provider, using an external LoadBalancer will typically come with additional costs.
