> For the complete documentation index, see [llms.txt](https://v2.dataos.info/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://v2.dataos.info/references/dataos-resources/service/core-concepts.md).

# Core concepts

### Ports

`servicePorts` declares the ports exposed by the Service. Each port requires a `name` and a numeric `port`.

```yaml
spec:
  servicePorts:
    - name: postgres
      port: 5432
```

For TCP services, consumers can resolve the Service endpoint from the Service runtime metadata. For HTTP services, configure the container to listen on the declared port and add any ingress configuration supported by the runtime.

### Replicas

`replicas` controls how many identical Service instances are maintained. Use more than one replica for stateless services that can handle traffic concurrently. Stateful services, such as a single Postgres instance, commonly run with `replicas: 1`.

```yaml
spec:
  replicas: 1
```

### Stateful Services

Set `stateful: true` when the Service needs stable identity or attached storage.

```yaml
spec:
  stateful: true
```

Stateful Services can use either inline disk storage or an existing Volume Resource:

* Use `disk` when storage belongs to the Service lifecycle and can be deleted with the Service.
* Use `use.volumes` when the data must persist independently of the Service.

See also: [Provision inline storage](/references/dataos-resources/service.md) and [Persist storage across workloads](/references/dataos-resources/service.md).

### Stack and stackSpec

`stack` selects the runtime stack. `stackSpec` contains stack-specific configuration. For the `container` stack, provide the container image, command, and arguments.

```yaml
spec:
  stack: container
  stackSpec:
    image: docker.io/library/postgres:18-alpine
    command:
      - docker-entrypoint.sh
    arguments:
      - postgres
```

### Projections

Use `use.projection` to project Secrets, environment variables, and files into the Service runtime. The following example reads `POSTGRES_PASSWORD` from a Secret and exposes it as an environment variable.

For projection syntax, template engines, data sources, and troubleshooting, see [Projections](/references/dataos-resources/secret/projections.md).

```yaml
spec:
  use:
    projection:
      templateType: liquid
      secrets:
        - id: ${{tenant-id}}:${{secret-name}}
          contextAlias: pgSecret
      projections:
        envVars:
          - key: POSTGRES_PASSWORD
            template: "{{ secrets['pgSecret'].POSTGRES_PASSWORD }}"
```


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