Helm Templating: Why Dashes (-) Are a No-Go in Names
Helm, the popular package manager for Kubernetes, relies heavily on templates to create and deploy applications. While the flexibility of templating is a major benefit, one common issue arises when using dashes (-) in the names of resources.
This can be frustrating, especially if you're accustomed to using dashes in naming conventions. Let's dive into the reasons behind this limitation and explore alternative approaches.
Understanding the Problem
Helm templates use the Go templating language, which has specific rules for identifiers. These rules prevent the use of dashes in names, as they are interpreted as operators. For instance, my-service would be parsed as my - service leading to an error.
Example Scenario
Let's consider a simple Helm chart with a deployment resource named my-deployment:
apiVersion: apps/v1
kind: Deployment
metadata:
name: {{ .Values.name }}-deployment
spec:
replicas: 1
selector:
matchLabels:
app: {{ .Values.name }}
template:
metadata:
labels:
app: {{ .Values.name }}
spec:
containers:
- name: my-app
image: nginx:latest
If we try to render this template with name: my-service in our values.yaml file, we'll encounter an error:
Error: template: ...:10:18: executing "...:10:18" at <.Values.name>-deployment: error calling .Values.name: "my-service" is not a valid identifier
Why This Happens
The core reason is that Helm templates leverage Go's templating engine, which requires identifiers to conform to specific syntax. Dashes are reserved for operators, causing the parser to misinterpret the name.
Alternative Approaches
There are two main ways to work around this limitation:
-
Use Underscores: Replace dashes with underscores (
_) in your names. While less visually appealing, this approach is the simplest and ensures the template parses correctly. -
Use Variables: Define separate variables within your values.yaml file to represent the parts of the name. For example, define
appName: "my-service"and use${appName}-deploymentin your template. This offers better readability and maintainability for larger charts.
Example with Underscores:
apiVersion: apps/v1
kind: Deployment
metadata:
name: {{ .Values.name }}_deployment
spec:
replicas: 1
selector:
matchLabels:
app: {{ .Values.name }}
template:
metadata:
labels:
app: {{ .Values.name }}
spec:
containers:
- name: my-app
image: nginx:latest
Example with Variables:
apiVersion: apps/v1
kind: Deployment
metadata:
name: {{ .Values.appName }}-deployment
spec:
replicas: 1
selector:
matchLabels:
app: {{ .Values.appName }}
template:
metadata:
labels:
app: {{ .Values.appName }}
spec:
containers:
- name: my-app
image: nginx:latest
Conclusion
While frustrating at first glance, understanding the reason behind this behavior and adopting the suggested approaches allows for smooth templating within Helm. By using underscores or variables, you can maintain clear and consistent naming practices without encountering errors.
Further Resources:
