Creating ReplicaSet in Kubernetes Cluster using Terraform
Kubernetes is a portable, extensible, open-source platform for managing containerized workloads and services, that facilitates both declarative configuration and automation. It has a large, rapidly growing ecosystem. Kubernetes services, support, and tools are widely available.
As a container management tool, Kubernetes was designed to orchestrate multiple containers and replication, and in fact there are currently several ways to do it.
What is Kubernetes replication for?
Before we go into how you would do replication, let’s talk about why. Typically you would want to replicate your containers (and thereby your applications) for several reasons, including:
- Reliability: By having multiple versions of an application, you prevent problems if one or more fails. This is particularly true if the system replaces any containers that fail.
- Load balancing: Having multiple versions of a container enables you to easily send traffic to different instances to prevent overloading of a single instance or node. This is something that Kubernetes does out of the box, making it extremely convenient.
- Scaling: When load does become too much for the number of existing instances, Kubernetes enables you to easily scale up your application, adding additional instances as needed.
Replication is appropriate for numerous use cases, including:
- Microservices-based applications: In these cases, multiple small applications provide very specific functionality.
- Cloud native applications: Because cloud-native applications are based on the theory that any component can fail at any time, replication is a perfect environment for implementing them, as multiple instances are baked into the architecture.
- Mobile applications: Mobile applications can often be architected so that the mobile client interacts with an isolated version of the server application.
ReplicaSet
A ReplicaSet’s purpose is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods.
How a ReplicaSet works
A ReplicaSet is defined with fields, including a selector that specifies how to identify Pods it can acquire, a number of replicas indicating how many Pods it should be maintaining, and a pod template specifying the data of new Pods it should create to meet the number of replicas criteria. A ReplicaSet then fulfills its purpose by creating and deleting Pods as needed to reach the desired number. When a ReplicaSet needs to create new Pods, it uses its Pod template.
A ReplicaSet is linked to its Pods via the Pods’ metadata.ownerReferences field, which specifies what resource the current object is owned by. All Pods acquired by a ReplicaSet have their owning ReplicaSet’s identifying information within their ownerReferences field. It’s through this link that the ReplicaSet knows of the state of the Pods it is maintaining and plans accordingly.
A ReplicaSet identifies new Pods to acquire by using its selector. If there is a Pod that has no OwnerReference or the OwnerReference is not a Controller and it matches a ReplicaSet’s selector, it will be immediately acquired by said ReplicaSet.
When to use a ReplicaSet
A ReplicaSet ensures that a specified number of pod replicas are running at any given time. However, a Deployment is a higher-level concept that manages ReplicaSets and provides declarative updates to Pods along with a lot of other useful features. Therefore, we recommend using Deployments instead of directly using ReplicaSets, unless you require custom update orchestration or don’t require updates at all.
This actually means that you may never need to manipulate ReplicaSet objects: use a Deployment instead, and define your application in the spec section.
Terraform
Terraform is an open-source infrastructure as code software tool created by HashiCorp. It enables users to define and provision a datacenter infrastructure using a high-level configuration language known as HashiCorp Configuration Language, or optionally JSON.
Practical
In this practical, we are going to create a ReplicaSet in Kubernetes cluster(minikube) using Terraform code.
Note
Terraform by-default uses Deployment for ReplicaSet. A Deployment ensures that a specified number of pod “replicas” are running at any one time. In other words, a Deployment makes sure that a pod or homogeneous set of pods are always up and available. If there are too many pods, it will kill some. If there are too few, the Deployment will start more.
Prerequisites
- Minikube.
- Kubectl(configured to minikube).
- Terraform.
- Visual Studio Code(with Terraform extension).
If all the prerequisites are downloaded and configured in the right way, then we start with our idea.
Here we are going to create ReplicaSet of WordPress(Web Application) and MySQL(Database) pods within minikube.
After writing the code, we have to initialize the working directory. The terraform init command is used to initialize a working directory containing Terraform configuration files. This is the first command that should be run after writing a new Terraform configuration or cloning an existing one from version control. It is safe to run this command multiple times.
Then we have to validate our code to check any possible errors before applying the changes. The terraform validate command validates the configuration files in a directory, referring only to the configuration and not accessing any remote services such as remote state, provider APIs, etc.
Validate runs checks that verify whether a configuration is syntactically valid and internally consistent, regardless of any provided variables or existing state. It is thus primarily useful for general verification of reusable modules, including correctness of attribute names and value types.
If the configuration is valid , then apply the changes. The terraform apply command is used to apply the changes required to reach the desired state of the configuration, or the pre-determined set of actions generated by a terraform plan execution plan.
So, we have successfully create our ReplicaSet using Terraform code.
In order to delete all pods and ReplicaSets we can use the destroy command in terraform.
terraform destroy 
