Deploying Postgresql as a Statefulset in Kubernetes

There are different types of applications, from single large applications to microservices-based applications that cater to different needs. When it comes to the states of those applications, there are two states:

• Stateless applications can be run independently in isolation without any knowledge of past transactions.
• Stateful applications have full knowledge of the past information (state).

Most applications we use are stateful applications, and their state data may consist of user preferences, recent activity, database transactions, credentials, settings, etc.

Kubernetes provides StatefulSets when creating a stateful application in a Kubernetes cluster. Managing states within a containerized environment has become even more significant with the popularity of deploying database clusters in Kubernetes.

In this article, we will focus on how to deploy a PostgreSQL database on a Kubernetes cluster using StatefulSets.

(This article is part of our Kubernetes Guide. Use the right-hand menu to navigate.)

What is Kubernetes StatefulSets?

StatefulSet is a Kubernetes workload API object that can be used to manage stateful applications.

In a normal deployment, the user does not care how the pods are scheduled as long as it doesn’t have a negative impact on the deployed application. However, there is the need to properly identify pods to preserve the state in stateful applications with persistent storage.

StatefulSet provides this functionality by creating pods with a persistent identifier that will pertain to its value across rescheduling. This way, a pod will get correctly mapped to the storage volumes even if it is recreated, and the application state will be preserved.

StatefulSets use cases

There are several use cases for StatefulSets.

Ordered deploying & scaling

When an application relies on multiple containers, the ordered approach to scaling ensures that dependent containers are created in an orderly manner at deployments and scaling scenarios.

Ordered automated rolling updates

Updating applications or microservices that are dependent should also be updated in an orderly manner. Additionally, an update should not affect the functionality.

Therefore, users can decide the order in which the applications or microservices should be updated by using a StatefulSet.

Mapping persistent storage

When considering databases, persistent storage is the most critical part as applications need to store data. With a StatefulSet, users can:

1. Define which pods correspond to each persistent storage
2. Create resilient application deployments

Using unique network identifiers to create persistent network connectivity

With unique identifiers, network users can manage and route traffic for specific pods without worrying about IP changes at rescheduling. This provides greater control over the network communications between pods by providing the ability to configure persistent routing, policies, and security configs for desired pods.

Even with these benefits, StatefulSets do not provide a solution for all requirements. For instance, StatefulSets are not interchangeable with deployments or ReplicaSets—these are instead geared to stateless configurations.