> ## 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 namespace `FireboltEnginePreset` value, when one object exists
3. The referenced EngineClass value
4. 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 Preset, and Preset overrides the class, for conflicting scalar values. Collection behavior depends on the field:

| Field                                                          | Resolution                                                                                                                      |
| -------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------- |
| `serviceAccountName`                                           | Engine, then Preset, then class                                                                                                 |
| `nodeSelector`                                                 | Maps merge; Engine keys win over Preset keys, which win over class keys                                                         |
| `tolerations`, `topologySpreadConstraints`, `imagePullSecrets` | Class entries, then Preset entries, then Engine entries                                                                         |
| `affinity`, pod `securityContext`                              | Engine value replaces Preset, which replaces class, when set                                                                    |
| labels and annotations                                         | Maps merge; Engine keys win; `firebolt.io/` keys are reserved                                                                   |
| volumes                                                        | Class entries, then Preset entries, then Engine entries; Firebolt Operator volume names are reserved                            |
| init containers and sidecars                                   | Class entries, then Preset entries, then Engine entries                                                                         |
| Engine container image and pull policy                         | Engine, then Preset, then class, then Firebolt Operator default                                                                 |
| Engine container resources                                     | The Engine block replaces the Preset or class block when the Engine sets requests, limits, or claims                            |
| Engine container environment and mounts                        | Firebolt Operator entries, then class entries, then Preset 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 Preset; otherwise the class; default is `emptyDir`.                                                                                            |
| `customEngineConfig` | Deep merge: Firebolt Operator configuration, class configuration, Preset configuration, then Engine configuration. Engine keys win. Protected identity, routing, and topology paths cannot be overridden. |
| `rollout`            | Engine, class, then `graceful`. Preset does not carry this field.                                                                                                                                         |
| `drainCheckEnabled`  | Engine, class, then `true`. Preset does not carry this field.                                                                                                                                             |
| `drainCheckInterval` | Engine, class, then Firebolt Operator default. Preset does not carry this field.                                                                                                                          |
| `autoStop`           | Whole setting. The Engine policy replaces the class policy when set. Preset does not carry this field.                                                                                                    |
| `uiSidecar`          | Engine, class, then `false`. Preset does not carry this field.                                                                                                                                            |

## FireboltEnginePreset

`FireboltEnginePreset` is an optional ambient overlay in the same namespace. Engines do not reference it by name. The object must be named `firebolt` — a validation rule baked into the CRD pins the name, so name uniqueness caps a namespace at one object.

Set `spec.requirePreset: true` on an Engine to keep it from becoming Ready until that object exists and is Ready. A present but unready Preset object fails closed for every Engine in the namespace even when `requirePreset` is unset.

See [FireboltEnginePreset overview](../enginepreset/overview).

## 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. A `FireboltEnginePreset` edit to the pod template, storage, or custom Engine configuration rolls every Engine in the namespace.

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.