> ## Documentation Index
> Fetch the complete documentation index at: https://docs.firebolt.io/llms.txt
> Use this file to discover all available pages before exploring further.

# EngineClass configuration inheritance

> Share Engine settings while allowing per-Engine overrides.

A `FireboltEngineClass` provides reusable defaults to Engines in the same namespace. Set `FireboltEngine.spec.engineClassRef` to the class name.

Values resolve in this order:

1. The Engine value
2. The referenced EngineClass value
3. The Firebolt Operator default

An EngineClass is namespaced because its pod template can reference namespaced resources such as ServiceAccounts, Secrets, ConfigMaps, and persistent volume claims.

## Pod template precedence

The Engine template overrides the class template for conflicting scalar values. Collection behavior depends on the field:

| Field                                                          | Resolution                                                                                                 |
| -------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------- |
| `serviceAccountName`                                           | Engine, then class                                                                                         |
| `nodeSelector`                                                 | Maps merge; Engine keys win                                                                                |
| `tolerations`, `topologySpreadConstraints`, `imagePullSecrets` | Class entries followed by Engine entries                                                                   |
| `affinity`, pod `securityContext`                              | Engine value replaces class value when set                                                                 |
| labels and annotations                                         | Maps merge; Engine keys win; `firebolt.io/` keys are reserved                                              |
| volumes                                                        | Class entries followed by Engine entries; Firebolt Operator volume names are reserved                      |
| init containers and sidecars                                   | Class entries followed by Engine entries                                                                   |
| Engine container image and pull policy                         | Engine, then class, then Firebolt Operator default                                                         |
| Engine container resources                                     | The Engine block replaces the class block when the Engine sets requests, limits, or claims                 |
| Engine container environment and mounts                        | Firebolt Operator entries, then class entries, then Engine entries; reserved names are rejected or omitted |

The Firebolt Operator owns fields required for Engine identity, networking, probes, and StatefulSet behavior. See the [FireboltEngineClass CRD reference](../crd-reference/fireboltengineclass-crd-reference) for the complete allowed-field table.

## Override a class image for one Engine

Use the class image as the shared default:

```yaml theme={"theme":{"light":"css-variables","dark":"css-variables"}}
apiVersion: compute.firebolt.io/v1alpha1
kind: FireboltEngineClass
metadata:
  name: production
  namespace: firebolt
spec:
  template:
    spec:
      containers:
        - name: engine
          image: example.com/firebolt/engine:stable
```

Override it on one Engine:

```yaml theme={"theme":{"light":"css-variables","dark":"css-variables"}}
apiVersion: compute.firebolt.io/v1alpha1
kind: FireboltEngine
metadata:
  name: canary
  namespace: firebolt
spec:
  instanceRef: production
  engineClassRef: production
  replicas: 1
  template:
    spec:
      containers:
        - name: engine
          image: example.com/firebolt/engine:candidate
```

## Inherited Engine settings

| Field                | Resolution                                                                                                                                                                          |
| -------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `storage`            | Whole setting. The Engine backend wins when set; otherwise the class backend applies; default is `emptyDir`.                                                                        |
| `customEngineConfig` | Deep merge: Firebolt Operator configuration, class configuration, then Engine configuration. Engine keys win. Protected identity, routing, and topology paths cannot be overridden. |
| `rollout`            | Engine, class, then `graceful`                                                                                                                                                      |
| `drainCheckEnabled`  | Engine, class, then `true`                                                                                                                                                          |
| `drainCheckInterval` | Engine, class, then Firebolt Operator default                                                                                                                                       |
| `autoStop`           | Whole setting. The Engine policy replaces the class policy when set.                                                                                                                |
| `uiSidecar`          | Engine, class, then `false`                                                                                                                                                         |

## Class changes

Changes to class settings that alter Engine pods roll every bound Engine in the namespace. These include the pod template, storage, custom Engine configuration, and UI sidecar setting.

Changes to rollout, drain-check, and auto-stop policy take effect without forcing a pod rollout by themselves.

Inspect the number of referencing Engines with:

```bash theme={"theme":{"light":"css-variables","dark":"css-variables"}}
kubectl get firengc production -n firebolt
```

## Delete a class

Class deletion remains blocked while Engines in the same namespace reference it. Remove or replace `spec.engineClassRef` on every bound Engine, wait for those updates to reconcile, then delete the class.

If deletion is blocked, `status.conditions[type=Ready]` reports reason `DeletionBlocked` and `status.boundEngines` shows the current consumer count.