body-generator.md

title	Body Generator
sidebar_position	8

The body generator creates a request body from a Go text/template, populated with request context (path params, query params, headers, original body, client IP). This is useful for GET-to-POST translation, constructing backend API payloads from URL parameters, or normalizing request formats.

How It Works

A Go template is compiled at startup. On each request, the template is executed with request context data and the result replaces r.Body. The Content-Type and Content-Length headers are updated accordingly.

Configuration

Body generator is configured per route on RouteConfig.

routes:
  - id: search
    path: /search
    backends:
      - url: http://search-service:8080
    body_generator:
      enabled: true
      template: '{"query": "{{.Query.Get "q"}}", "client": "{{.ClientIP}}"}'
      content_type: application/json

Field	Type	Default	Description
enabled	bool	false	Enable body generation
template	string	-	Go text/template string (required)
content_type	string	application/json	Content-Type header for the generated body

Template Data

The following fields are available in the template:

Field	Type	Description
.Method	string	HTTP method (GET, POST, etc.)
.URL	string	Full request URL
.Host	string	Request host
.Path	string	URL path
.PathParams	map[string]string	Route path parameters
.Query	url.Values	Query string parameters (use .Query.Get "key")
.Headers	http.Header	Request headers (use .Headers.Get "key")
.Body	string	Original request body (if present)
.ClientIP	string	Client IP address
.RouteID	string	Route identifier

Template Functions

All Sprig v3 functions (string, math, date, encoding, collections, etc.) are available in addition to the Runway-specific helpers:

Function	Description	Example
json	JSON-encodes a value	{{json .PathParams}}
first	Returns first element of a string slice	{{first (.Query "tags")}}

Middleware Position

Step 16.05 in the per-route middleware chain -- after request transforms (step 16) and before backend auth (step 16.25).

Admin API

GET /body-generator

Returns per-route stats:

{
  "search": {
    "generated": 5000,
    "content_type": "application/json"
  }
}

Example: GET-to-POST Translation

Convert a GET search request into a POST JSON body for the backend:

routes:
  - id: search-proxy
    path: /api/search
    backends:
      - url: http://elasticsearch:9200/index/_search
    body_generator:
      enabled: true
      content_type: application/json
      template: |
        {
          "query": {
            "match": {
              "content": "{{.Query.Get "q"}}"
            }
          },
          "size": 10
        }

Example: Path-to-Body Mapping

Use path parameters to construct a backend request:

routes:
  - id: user-lookup
    path: /users/:id/profile
    backends:
      - url: http://user-service:8080/graphql
    body_generator:
      enabled: true
      template: '{"query": "{ user(id: \"{{index .PathParams "id"}}\") { name email } }"}'