Operator and Custom Resource (CR) Relationship

Understand how an Operator and a Custom Resource (CR) work together to manage applications in Kubernetes.

1. Core Concepts and Analogy

The easiest way to understand the relationship between an Operator and a CR is to think of a ‘factory’ and an ‘order form’.

Item	Concept	Analogy	Main Components
Operator	Automation software that embeds operational knowledge	A factory run by management robots	Controller, CRD, domain knowledge
Custom Resource	A user-defined resource instance	A customer’s order form (blueprint)	Defines the desired state (spec)

    graph TD
    User[User] -- "1. Submit order form (CR)" --> API[K8s API Server]
    API -- "2. Detect create event" --> OP[Operator / Controller]
    
    subgraph Operator_Logic
    OP -- "3. Interpret spec" --> Reconcile{Reconcile Loop}
    Reconcile -- "4. Create actual resources" --> Real[Pods, Services, PVCs]
    Real -- "5. Observe current state" --> Reconcile
    end
    
    Reconcile -- "6. Update status" --> API

2. Why Must the Operator Be Installed First?

Here are four reasons why the Operator must already be running before you deploy a CR to the cluster.

CRD registration: Kubernetes must first be given the CustomResourceDefinition (CRD) so that it can recognize a new resource type (e.g., kind: MySQL).
Controller activation: When an order form (CR) arrives, the ‘brain’ (controller) that reads it and creates the actual Pods must already be running.
RBAC permissions: The permissions (Role and RoleBinding) the Operator needs to create Pods and Services on your behalf must be set up first.
Dependency preparation: The Operator first sets up the basic infrastructure needed to run the app, such as a Secret for storing passwords and the storage configuration.

3. What a CR Contains (spec vs status)

Through spec, the user states the “desired state”, and through status, the Operator reports the “current state”.

Field	Role	Typical Contents
spec	Defined by the user (desired state)	Version, replicas, resources (CPU/memory), backup schedule, ports, etc.
status	Recorded by the Operator (current state)	Number of running Pods, access endpoint, last backup time, error messages, etc.

4. Summary: Recap with Analogies

Operator = Apartment complex management office: The office must be established (installed) first so that staff (controllers) and equipment (permissions) are in place.
CR = Move-in application: Only after the management office opens can you submit an application, be assigned a unit number (name), and receive a key (Service).

If you submit an application (CR) without a management office (Operator), that application is just a piece of paper no one reads.

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