This document introduces how to use Go Template to process request and response data in MCP Gateway. Go Template provides powerful template functionality that helps us flexibly handle data transformation and formatting.

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Basic Syntax

Go Template uses {{}} as delimiters, and various functions and variables can be used within the delimiters. In MCP Gateway, we mainly use the following variables:

Configuration Variables

  • .Config: Service-level configuration
  • .Args: Request parameters

Runtime Variables

  • .Request: Original request information
  • .Response: Upstream service response information

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Common Use Cases

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1. Getting Configuration from Environment Variables

config:
  Authorization: 'Bearer {{ env "AUTH_TOKEN" }}'  # Get configuration from environment variables
Using the env function allows you to safely retrieve sensitive information from environment variables, avoiding hardcoding in configuration files.

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2. Extracting Values from Request Headers

headers:
  Authorization: "{{.Request.Headers.Authorization}}"   # Pass through client's Authorization header
  Cookie: "{{.Config.Cookie}}"                          # Use value from service configuration

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3. Building Request Body

requestBody: |-
  {
    "username": "{{.Args.username}}",
    "email": "{{.Args.email}}"
  }

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4. Processing Response Data

responseBody: |-
  {
    "id": "{{.Response.Data.id}}",
    "username": "{{.Response.Data.username}}",
    "email": "{{.Response.Data.email}}",
    "createdAt": "{{.Response.Data.createdAt}}"
  }

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5. Handling Nested Response Data

For complex nested data structures, you can access them layer by layer: 
responseBody: |-
  {
    "id": "{{.Response.Data.id}}",
    "username": "{{.Response.Data.username}}",
    "email": "{{.Response.Data.email}}",
    "createdAt": "{{.Response.Data.createdAt}}",
    "preferences": {
      "isPublic": {{.Response.Data.preferences.isPublic}},
      "showEmail": {{.Response.Data.preferences.showEmail}},
      "theme": "{{.Response.Data.preferences.theme}}",
      "tags": {{.Response.Data.preferences.tags}}
    }
  }

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6. Handling Array Data

When you need to process array data in responses, you can use Go Template’s range functionality: 
responseBody: |-
  {
    "total": "{{.Response.Data.total}}",
    "rows": [
      {{- $len := len .Response.Data.rows -}}
      {{- $rows := fromJSON .Response.Data.rows }}
      {{- range $i, $e := $rows }}
      {
        "id": {{ $e.id }},
        "detail": "{{ $e.detail }}",
        "deviceName": "{{ $e.deviceName }}"
      }{{ if lt (add $i 1) $len }},{{ end }}
      {{- end }}
    ]
  }

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7. Using Parameters in URLs

Dynamically building URL paths: 
endpoint: "http://localhost:5236/users/{{.Args.email}}/preferences"

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8. Handling Complex Object Data

When you need to convert complex structures like objects and arrays from requests or responses to JSON, you can use the toJSON function: 
requestBody: |-
  {
    "isPublic": {{.Args.isPublic}},
    "showEmail": {{.Args.showEmail}},
    "theme": "{{.Args.theme}}",
    "tags": {{.Args.tags}},
    "settings": {{ toJSON .Args.settings }}
  }
Here, settings is a complex object. Using the toJSON function will automatically convert settings to a JSON string.

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Built-in Functions

Currently supports the following built-in functions:

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Template Debugging Tips

1

Use Log Output

During development, you can view actual values of template variables through logs
2

Step-by-step Construction

For complex templates, it’s recommended to first build a simple version, then gradually add complex logic
3

Test Edge Cases

Ensure templates work correctly under various data conditions, including empty values, null values, etc.

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Best Practices

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Extension Features

If you need to add new template functions, you can:
  1. Describe specific use cases, create an issue explaining the requirements
  2. Welcome PR after implementation, but currently only accept general-purpose functions

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More Resources

Go Template Official Documentation

View complete Go Template documentation and advanced usage