Template Directory Rendering Tutorial#

This tutorial walks you through the recommended pattern for rendering a directory tree of Go templates into output files while preserving the directory structure. You’ll learn how to combine three providers — directory, go-template, and file — in a single solution to scaffold entire projects from templates.

Prerequisites#

scafctl installed and available in your PATH
Basic familiarity with solution files and Go template syntax

Table of Contents#

Overview
Setting Up Templates
Reading Templates with the Directory Provider
Batch Rendering with render-tree
Writing Files with write-tree
Path Transformation with outputPath
Putting It All Together
Advanced Patterns

Overview#

A common scaffolding task is:

You have a directory of template files (.tpl, .tmpl, .gotmpl, etc.)
You want to render each template with shared variables
You want to write the rendered output preserving the directory structure

scafctl solves this with three operations:

Step	Provider	Operation	Purpose
1	`directory`	`list`	Read template files with their content
2	`go-template`	`render-tree`	Batch-render all templates at once
3	`file`	`write-tree`	Write rendered files preserving structure

Setting Up Templates#

Create a templates/ directory with your Go template files:

templates/
├── README.md.tpl
├── config/
│   └── app.yaml.tpl
└── k8s/
    ├── deployment.yaml.tpl
    └── service.yaml.tpl

Each .tpl file is a standard Go template. For example, k8s/deployment.yaml.tpl:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: {{ .appName }}
  namespace: {{ .namespace }}
spec:
  replicas: {{ .replicas }}
  selector:
    matchLabels:
      app: {{ .appName }}
  template:
    spec:
      containers:
        - name: {{ .appName }}
          image: {{ .registry }}/{{ .appName }}:{{ .appVersion }}
          ports:
            - containerPort: {{ .containerPort }}

Reading Templates with the Directory Provider#

Use the directory provider with includeContent: true to read all templates recursively:

resolvers:
  templateFiles:
    type: any
    resolve:
      with:
        - provider: directory
          inputs:
            operation: list
            path: ./templates
            recursive: true
            filterGlob: "*.tpl"
            includeContent: true

This returns an object with an entries array. Each entry has:

path — the relative path (e.g. k8s/deployment.yaml.tpl)
content — the file’s raw content (the unrendered template text)
Plus metadata like name, size, extension, etc.

Batch Rendering with render-tree#

The go-template provider’s render-tree operation takes an array of {path, content} entries and renders each content as a Go template:

resolvers:
  rendered:
    type: any
    resolve:
      with:
        - provider: go-template
          inputs:
            operation: render-tree
            entries:
              expr: '_.templateFiles.entries'
            data:
              appName: myapp
              namespace: production
              replicas: 3
              registry: ghcr.io/myorg
              appVersion: "1.0.0"
              containerPort: 8080

The output is an array of {path, content} where each content is now the rendered result. The path is passed through unchanged from the input.

Per-entry data (fan-out)#

Each entry may carry an optional data map that is shallow-merged over the shared top-level data for that entry only. On key conflicts, per-entry values win over shared data, iteration variables, and resolver context. This turns the common “render one template per item, each with its own variables and its own output path” pattern into a single resolver – no separate forEach render resolver and no manual index-zip:

resolvers:
  backendConfigs:
    type: any
    resolve:
      with:
        - provider: go-template
          inputs:
            operation: render-tree
            data:
              platformAppName: myapp      # shared default (merge base)
            entries:
              expr: |
                _.environments.map(env, {
                  "path":    "envs/" + env.name + "/backend.tf",
                  "content": _.backendTemplate.entries[0].content,
                  "data":    {"environment": env}   # per-entry, wins over shared
                })

Entries without a data field render against the shared data alone, so existing render-tree usage is unaffected. A non-map data value fails with a clear error naming the entry index and path.

Using a Separate Vars Resolver#

For cleaner solutions, define variables in their own resolver and reference them:

resolvers:
  vars:
    type: any
    resolve:
      with:
        - provider: static
          inputs:
            value:
              appName: myapp
              namespace: production
              # ... more vars

  rendered:
    type: any
    resolve:
      with:
        - provider: go-template
          inputs:
            operation: render-tree
            entries:
              expr: '_.templateFiles.entries'
            data:
              rslvr: vars

Writing Files with write-tree#

The file provider’s write-tree operation takes an array of {path, content} entries and writes them under a basePath:

workflow:
  actions:
    write-output:
      provider: file
      inputs:
        operation: write-tree
        basePath: ./output
        entries:
          rslvr: rendered

This writes each entry as a file relative to basePath, automatically creating subdirectories as needed. Without outputPath, files keep their original paths:

./output/k8s/deployment.yaml.tpl    # Still has .tpl extension!
./output/k8s/service.yaml.tpl
./output/config/app.yaml.tpl
./output/README.md.tpl

To strip the .tpl extension, use outputPath.

Path Transformation with outputPath#

The outputPath field is a Go template that transforms each entry’s path before writing. It receives these variables:

Variable	Description	Example for `k8s/deployment.yaml.tpl`
`__filePath`	Original relative path	`k8s/deployment.yaml.tpl`
`__fileName`	File name only	`deployment.yaml.tpl`
`__fileStem`	File name without last extension	`deployment.yaml`
`__fileExtension`	Last extension including dot	`.tpl`
`__fileDir`	Directory part (empty for root files)	`k8s`

Resolver values are also available in the template (for example {{ .environment }}), so you can route files into dynamic, resolver-derived directories. The __file* variables take precedence if a resolver has the same name.

Routing into a Resolver-Derived Directory#

The outputPath template renders against the solution’s top-level resolver values (keyed by resolver name), so to use {{ .environment }} you need a top-level environment resolver:

resolvers:
  environment:
    type: string
    resolve:
      with:
        - provider: static
          inputs:
            value: prod

Then reference it in outputPath:

outputPath: "{{ .environment }}/{{ .__fileDir }}/{{ .__fileStem }}"

Result (with environment: prod): k8s/deployment.yaml.tpl -> prod/k8s/deployment.yaml

Stripping the `.tpl` Extension#

The most common pattern – reconstruct the path using the stem (which drops .tpl):

outputPath: >-
  {{ if .__fileDir }}{{ .__fileDir }}/{{ end }}{{ .__fileStem }}

Result: k8s/deployment.yaml.tpl → k8s/deployment.yaml

Flattening All Files into One Directory#

Ignore the directory structure, keeping only file names:

outputPath: "{{ .__fileName }}"

Result: k8s/deployment.yaml.tpl → deployment.yaml.tpl

Using Sprig Functions#

All Sprig template functions are available:

outputPath: "{{ lower .__filePath }}"

Result: SRC/MyFile.TXT → src/myfile.txt

Replacing Extensions#

Change .tpl to .generated.yaml:

outputPath: >-
  {{ if .__fileDir }}{{ .__fileDir }}/{{ end }}{{ trimSuffix ".tpl" .__fileName }}.generated.yaml

Putting It All Together#

Here is a complete solution that reads templates, renders them, and writes the output:

apiVersion: scafctl.io/v1
kind: Solution
metadata:
  name: template-directory-rendering
  version: 1.0.0
  description: Render a directory of Go templates into output files

spec:
  resolvers:
    # Variables shared across all templates
    vars:
      type: any
      resolve:
        with:
          - provider: static
            inputs:
              value:
                appName: myapp
                appVersion: "1.2.0"
                namespace: production
                environment: prod
                replicas: 3
                registry: ghcr.io/myorg
                containerPort: 8080
                servicePort: 80
                serviceType: ClusterIP
                logLevel: info

    # Read all template files
    templateFiles:
      type: any
      resolve:
        with:
          - provider: directory
            inputs:
              operation: list
              path: ./templates
              recursive: true
              filterGlob: "*.tpl"
              includeContent: true

    # Render all templates with vars
    rendered:
      type: any
      resolve:
        with:
          - provider: go-template
            inputs:
              operation: render-tree
              entries:
                expr: '_.templateFiles.entries'
              data:
                rslvr: vars

  workflow:
    actions:
      # Write rendered results, stripping .tpl extension
      write-output:
        provider: file
        inputs:
          operation: write-tree
          basePath: ./output
          entries:
            rslvr: rendered
          outputPath: >-
            {{ if .__fileDir }}{{ .__fileDir }}/{{ end }}{{ .__fileStem }}

Run it:

Bash

scafctl run solution -f solution.yaml

PowerShell

scafctl run solution -f solution.yaml

Inspect the output:

find ./output -type f
# output/README.md
# output/config/app.yaml
# output/k8s/deployment.yaml
# output/k8s/service.yaml

cat ./output/k8s/deployment.yaml
# apiVersion: apps/v1
# kind: Deployment
# metadata:
#   name: myapp
#   namespace: production
# ...

Advanced Patterns#

Combining with CEL Filtering#

Use a cel transform to filter entries before rendering — for instance, skip files larger than 10KB:

resolvers:
  small-templates:
    type: any
    resolve:
      with:
        - provider: directory
          inputs:
            operation: list
            path: ./templates
            recursive: true
            filterGlob: "*.tpl"
            includeContent: true
      transforms:
        - provider: cel
          inputs:
            expression: >-
              {"entries": data.entries.filter(e, e.size < 10000)}

Multiple Template Sets#

Render different template directories with different variables by defining separate resolver chains:

resolvers:
  frontendTemplates:
    type: any
    resolve:
      with:
        - provider: directory
          inputs:
            operation: list
            path: ./templates/frontend
            recursive: true
            filterGlob: "*.tpl"
            includeContent: true

  backendTemplates:
    type: any
    resolve:
      with:
        - provider: directory
          inputs:
            operation: list
            path: ./templates/backend
            recursive: true
            filterGlob: "*.tpl"
            includeContent: true

  renderedFrontend:
    type: any
    resolve:
      with:
        - provider: go-template
          inputs:
            operation: render-tree
            entries:
              expr: '_.frontendTemplates.entries'
            data:
              framework: react
              # ...

  renderedBackend:
    type: any
    resolve:
      with:
        - provider: go-template
          inputs:
            operation: render-tree
            entries:
              expr: '_.backendTemplates.entries'
            data:
              framework: gin
              # ...

Then write each set to a different output directory:

workflow:
  actions:
    write-frontend:
      provider: file
      inputs:
        operation: write-tree
        basePath: ./output/frontend
        entries:
          rslvr: renderedFrontend
        outputPath: >-
          {{ if .__fileDir }}{{ .__fileDir }}/{{ end }}{{ .__fileStem }}

    write-backend:
      provider: file
      inputs:
        operation: write-tree
        basePath: ./output/backend
        entries:
          rslvr: renderedBackend
        outputPath: >-
          {{ if .__fileDir }}{{ .__fileDir }}/{{ end }}{{ .__fileStem }}

Dry Run#

Use --dry-run to preview what would be written without touching the filesystem: