Where is it best to store the state in a system…

The state in the system is best stored using specialized state management tools.

Different approaches are suitable for various system components:

Application: Using state management libraries (Redux, Vuex, React Context, etc.) for the client side. For the server side - databases (relational, NoSQL) or distributed caches (Redis, Memcached).
Infrastructure as Code (IaC):
- Terraform: State files .tfstate, stored in a remote backend (S3, Google Cloud Storage, Azure Blob Storage) and locked during changes.
- Ansible: Fact caching on control nodes or in centralized storage.
CI/CD pipelines: Build artifacts and test results can be stored in artifact repositories (Nexus, Artifactory, AWS S3). The pipeline state itself (success/failure, execution stage) is stored in the CI/CD tool (Jenkins, GitLab CI, GitHub Actions).
Containers and orchestration:
- Docker: Data that needs to be preserved is stored in volumes.
- Kubernetes: Cluster state, including information about deployed applications, their configurations, and status, is stored in the distributed key-value store etcd. PersistentVolumeClaims (PVC) and PersistentVolumes (PV) are used for managing long-term data storage for pods.

When choosing a storage location for state, consider:

Durability: Will the state persist after a component or system restart?
Availability: Can the system access the state from anywhere or only locally?
Consistency: Is the data's relevance and integrity ensured?
Security: Who has the right to read and write the state?
Scalability: Can the solution handle increasing amounts of state data?
Atomicity of operations: Are atomic operations guaranteed when changing the state?

Example of storing Terraform state using S3:

// Initializing Terraform backend with S3
terraform {
  backend "s3" {
    bucket         = "my-terraform-state-bucket"  // S3 bucket name
    key            = "path/to/my/state.tfstate"    // Path to the state file
    region         = "us-east-1"                  // S3 region
    encrypt        = true                         // Data encryption in S3
    dynamodb_table = "terraform_lock_table"       // DynamoDB table for locks
  }
}

Example of defining a PersistentVolumeClaim in Kubernetes:

# Defining a PersistentVolumeClaim for data storage
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-data-pvc
spec:
  accessModes:
    - ReadWriteOnce  # Access mode (read/write by a single pod)
  resources:
    requests:
      storage: 10Gi  # Requesting 10 GiB of storage
  storageClassName: standard  # Storage class (defined by provider)

Where is it best to store the state in a system?