Using templates as values

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Using templates as values

You can use any of the current request headers, query parameters, captured URI groups, and shared variables as templates to populate supported config fields.

To escape a template, wrap it inside quotes and pass inside another template. For example:

$('$(something_that_needs_to_escaped)')

Note: The plugin creates a non-mutable table of request headers, query strings, and captured URIs before transformation. So any update or removal of parameters used in the template does not affect the rendered value of template.

Advanced templates

The content of the placeholder $(...) is evaluated as a Lua expression, so logical operators may be used. For example:

Header-Name:$(uri_captures["user-id"] or query_params["user"] or "unknown")

This will first look for the path parameter named user-id (in uri_captures). If not found, it will return the query parameter named user. If that also doesn’t exist, it returns the default value ‘“unknown”’.

Constant parts can be specified as part of the template outside the dynamic placeholders. For example, creating a basic-auth header from a query parameter called auth that only contains the base64-encoded part:

Authorization:Basic $(query_params["auth"])

Lambdas are also supported if wrapped as an expression like this:

$((function() ... implementation here ... end)())

A complete lambda example for prefixing a header value with “Basic “ if not already there:

Authorization:$((function()
    local value = headers.Authorization
    if not value then
      return
    end
    if value:sub(1, 6) == "Basic " then
      return value            -- was already properly formed
    end
    return "Basic " .. value  -- added proper prefix
  end)())

Note: Especially in multi-line templates like the example above, make sure not to add any trailing white-space or new-lines. Since these would be outside the placeholders, they would be considered part of the template, and hence would be appended to the generated value.

The environment is sandboxed, meaning that Lambda’s will not have access to any library functions, except for the string methods (like sub() in the example above).

Examples using template as value

1. Add a service named test which routes requests to the httpbin.konghq.com upstream service:

    curl -X POST http://localhost:8001/services \
        --data 'name=test' \
        --data 'url=https://httpbin.konghq.com/anything'
   

2. Create a route for the test service, capturing a user_id field from the third segment of the request path using a regex:

   Kubernetes users: Version v1beta1 of the Ingress specification does not allow the use of named regex capture groups in paths. If you use the ingress controller, you should use unnamed groups, e.g. (\w+)/instead of (?<user_id>\w+). You can access these based on their order in the URL path. For example $(uri_captures[1]) obtains the value of the first capture group.

    curl -X POST http://localhost:8001/services/test/routes --data "name=test_user" \
        --data-urlencode 'paths=~/requests/user/(?<user_id>\w+)'
   

   You can learn more about using regexes in paths in the Kong Gateway traffic routing reference.

3. Enable the request-transformer-advanced plugin to add a new header, x-user-id, whose value is being set from the captured group in the route path specified above:

    curl -XPOST http://localhost:8001/routes/test_user/plugins --data "name=request-transformer-advanced" --data "config.add.headers=x-user-id:\$(uri_captures['user_id'])"
   

4. Now send a request with a user id in the route path:

    curl -i -X GET localhost:8000/requests/user/foo
   

   You should notice in the response that the x-user-id header has been added with a value of foo.