Introduction to Named Templates

Note: An interactive sandbox is located at the end of this lesson.

Named templates#

We’ve got a very simple Helm chart with only two Kubernetes resources, Deployment and Service. If we take a closer look at them we would see that even though they’re two different resources, some of their parts are the same.

For instance, they have the same labels in a metadata property, as shown below:

For now, it’s rather a simple one, but that’ll change once we start to add more resources and extend the existing parts. To reduce the number of duplicated parts of a code we can extract them and define them as named templates ( a template inside a template). Such templates, or as we prefer to call them, snippets, can be defined in an already existing Helm template file or a separate one and then be reused in multiple places.

The `_helpers.tpl` file#

Let’s go back to our example with labels and create a separate named template file— _helpers.tpl—which is located in a /templates folder. What is important here is the name of the file. It can be whatever we would like it to be, but it needs to start with _,otherwise, Helm will try to create a Kubernetes resource file out of it. Since it’s just a utility file that contains reusable parts we don’t want to create a Kubernetes resource out of it.

Here’s the content of a _helpers.tpl file:

Let’s break the above snippet into smaller pieces:

• The part between {{/* and */}} is a comment section that informs the user or other Helm chart developer what the purpose of a specifically named template is (it’s not mandatory, but it’s considered good practice to provide such information).

• {{- define "app.labels" -}} begins the named template, containing a define keyword followed by a "app.labels" which is a name of a template (it will be used later on in template files to reference a definition from here). By design, there is no limitation on how named templates can be named, but usually, their name starts with a chart name, like in our example.

• The actual body of the named template, which remains as it was.

• {{- end }} ends the named template, with only the parts between define and end included in it.

Okay, so now we can add the app.labels named template into the Service definition file (service.yaml):

Instead of a list of labels, we have added a single line with an include action that points out  app.labels named template.

Now, if we try to use this chart and upgrade the kanban-backend release we would get the following result: