Transformations

One of the core features of any API Gateway is the ability to transform the traffic that it manages. To really enable the decoupling of your services, the API Gateway should be able to mutate requests before forwarding them to your upstream services and do the same with the resulting responses before they reach the downstream clients. Gloo Edge delivers on this promise by providing you with a powerful transformation API.

To review where transformations happen as Gloo Edge filters traffic, see Traffic processing.

Defining a transformation

Transformations are defined by adding the transformations attribute to your Virtual Services. You can define this attribute on three different Virtual Service sub-resources:

• VirtualHosts
• Routes
• WeightedDestinations

The configuration format is the same in all three cases and must be specified under the relevant options attribute. For example, to configure transformations for all traffic matching a Virtual Host, you need to add the following attribute to your Virtual Host definition:

# This snippet has been abridged for brevity
virtualHost:
  options:
    stagedTransformations:
      regular:
        requestTransforms:
        - requestTransformation:
            transformationTemplate:
              headers:
                foo:
                  text: 'bar'

Inheritance rules

By default, a transformation defined on a Virtual Service attribute is inherited by all the child attributes:

• transformations defined on VirtualHosts are inherited by Routes and WeightedDestinations.
• transformations defined on Routes are inherited by WeightedDestinations.

If a child attribute defines its own transformation, it overrides the configuration on its parent. However, if inheritTransformation is set to true on the stagedTransformations for a Route, it can inherit transformations from its parent as illustrated below.

Let’s define the virtualHost and its child route is defined as follows:

# This snippet has been abridged for brevity, and only includes transformation-relevant config
virtualHost:
  options:
    stagedTransformations:
      regular:
        requestTransforms:
        - matcher:
            - prefix: '/parent'
          requestTransformation:
            transformationTemplate:
              headers:
                foo:
                  text: 'bar'
  routes:
    - options:
        stagedTransformations:
          inheritTransformation: true
          regular:
            requestTransforms:
            - matchers:
              - prefix: '/child'
              requestTransformation:
                transformationTemplate:
                  headers:
                    foo:
                      text: 'baz'

Because inheritTransformation is set to true on the child route, the parent virtualHost transformation config is merged into the child. The child route’s transformations look like the following.

# This snippet has been abridged for brevity, and only includes transformation-relevant config
routes:
- options:
  stagedTransformations:
    inheritTransformation: true
    regular:
      requestTransforms:
      - matchers:
        - prefix: '/child'
        requestTransformation:
          transformationTemplate:
            headers:
              foo:
                text: 'baz'
      - matchers:
        - prefix: '/parent'
        requestTransformation:
          transformationTemplate:
            headers:
              foo:
                text: 'bar'

As stated above, the route’s configuration overrides its parent’s, but now it also inherits the parent’s transformations. So in this case, routes matching /parent are also transformed. If inheritTransformation were set to false, the matching /parent routes would not be transformed. Note that only the first matched transformation runs, so if both the child and the parent had the same matchers, the child’s transformation would run.

Configuration format

Learn more about the properties that you can set in the stagedTransformations object section of your YAML file.

The following YAML file shows a sample structure for how to configure request and response transformations in the stagedTransformations section:

stagedTransformations:
  early:
    # early transformations
  regular:
    requestTransforms:
      - matcher:
          prefix : '/'
        clearRouteCache: bool
        requestTransformation: {}
        responseTransformation: {}
    responseTransforms: {}
  inheritTransformations: bool

The early and regular attributes are used to specify when in the envoy filter chain the transformations run. For request transformations, early transformations are applied before regular transformations as shown in the following diagram. For response transformations, this order is reversed, and regular transformations are applied before early transformations. To learn more about the order in which envoy filters are applied, see HTTP filter chain processing in the envoy documentation.

The requestTransforms attribute specifies a list of transformations that are applied when a matching request is found. The first transformation that specifies a matcher that matches the request attributes is applied.

The responseTransforms attribute specifies a list of transformations that are applied when a matching response is found. A transformation is applied only if no transformation in requestTransforms matched the request.

The clearRouteCache attribute is a boolean value that determines whether the route cache should be cleared if the request transformation was applied. If the transformation modifies the headers in a way that affects routing, this attribute must be set to true. The default value is false.

The requestTransformation and responseTransformation attributes have the same format and specify transformations that are applied to requests and responses respectively. The format can be one of the following:

• headerBodyTransform: This type of transformation makes all the headers available in the body and returns a JSON body that consists of two attributes: headers, containing the headers, and body, containing the original body.
  • If addRequestMetadata is true, queryString, queryStringParameters, multiValueQueryStringParameters, httpMethod, path, and multiValueHeaders will additionally be present in the body.
• transformationTemplate: This type of transformation allows you to define transformation templates. This is the more powerful and flexible type of transformation. For more information, see Transformation templates.
• xsltTransformation: This type of transformation allows you to use the XSLT transformation language to describe your transformation. For more information, see XSLT Transformation.

The inheritTransformation attribute allows child routes to inherit transformations from their parent RouteTables and/or VirtualHosts. For more information, see Inheritance rules.

Transformation templates

Templates are the core of Gloo Edge’s transformation API. They allow you to mutate the headers and bodies of requests and responses based on the properties of the headers and bodies themselves. The following snippet illustrates the structure of the transformationTemplate object:

transformationTemplate:
  parseBodyBehavior: {}
  ignoreErrorOnParse: bool
  extractors:  {}
  headers: {}
  # Only one of body, passthrough, and mergeExtractorsToBody can be specified
  body: {}
  passthrough: {}
  mergeExtractorsToBody: {}
  dynamicMetadataValues: []
  advancedTemplates: bool
  escapeCharacters: bool

Let’s go ahead and describe each one of these attributes in detail.

Body Parsing Behavior

parseBodyBehavior

This attribute determines how the request/response body will be parsed and can have one of two values:

• ParseAsJson: Gloo Edge will attempt to parse the body as a JSON structure. This is the default behavior.
• DontParse: the body will be treated as plain text.

The important part to know about the DontParse setting is that the body will be buffered and available, but will not be parsed. If you’re looking to skip any body buffering completely, see the section on passthrough: {}

As we will see later, some of the templating features won’t be available when treating the body as plain text.

ignoreErrorOnParse

By default, Gloo Edge will attempt to parse the body as JSON, unless you have DontParse set as the parseBodyBehavior. If ignoreErrorOnParse is set to true, Envoy will not throw an exception in case the body parsing fails. Defaults to false.

Implicit in this setting is that the body will be buffered and available. If you’re looking to skip any body buffering completely, see the section on passthrough: {}

extractors

Use this attribute to extract information from a request or response. It consists of a set of mappings from a string to an extraction:

• the extraction defines which information will be extracted
• the string key will provide the extractor with a name it can be referred by.

An extraction must have one of two sources:

• header: extract information from a header with the given name.

  extractors:
    myExtractor:
      header: 'foo'
      regex: '.*'
  

• body: extract information from the body. This attribute takes an empty value (as there is always only one body).

  extractors:
    myExtractor:
      body: {}
      regex: '.*'
  

Extracting the body is generally not useful when Gloo Edge has already parsed it as JSON, the default behavior. The parsed body data can be directly referenced using standard JSON syntax. The body extractor treats the body as plaintext, and is interpreted using a regular expression as noted below. This can be useful for body data that cannot be parsed as JSON.

An extraction must also define which information is to be extracted from the source. This can be done by providing a regular expression via the regex attribute. The regular expression will be applied to the body or to the value of relevant header and must match the entire source. If your regular expression uses capturing groups, you can select the group match you want to use via the subgroup attribute.

As an example, to define an extraction named foo which will contain the value of the foo query parameter you can apply the following configuration:

extractors:
  # This is the name of the extraction
  foo:
    # The :path pseudo-header contains the URI
    header: ':path'
    # Use a nested capturing group to extract the query param
    regex: '(.*foo=([^&]*).*)'
    # Select the second group match
    subgroup: 2

Another example shows how to extract a value from the body using a regex. If the body is:

Here is a body containing the identification number of the object.

id: 4423

and we want to extract the id number, we can define an extractor:

extractors:
  # This is the name of the extraction
  bodyIdExtractor:
    # This empty struct must be included to extract from the body
    body: {}
    # The regex must match the ENTIRE body, else the extraction will not work
    # The [\s\S]* allows us to match any character, making the regex match the entire source.
    regex: '[\s\S]*id: ([0-9]{4})[\s\S]*'
    # Select the first subgroup match ([0-9]{4}) since we only want the id number
    subgroup: 1

The result will be the bodyIdExtractor having value 4423.

Extracted values can be used in two ways:

• You can reference extractors by their name in template strings, e.g. {{ my-extractor }} (or {{ extraction(my-extractor) }}, if you are setting advancedTemplates to true) will render to the value of the my-extractor extractor.
• You can use them in conjunction with the mergeExtractorsToBody body transformation type to merge them into the body.

headers

Use this attribute to apply templates to request/response headers. It consists of a map where each key determines the name of the resulting header, while the corresponding value is a template which will determine the value.

For example, to set the header foo to the value of the header bar, you could use the following configuration:

transformationTemplate:
  headers:
    foo:
      text: '{{ header("bar") }}'

You could also use the parsed data from the body and add information to the header. For instance, given a request body:

{
  "Name": "Gloo",
  "Favorites": {
    "Color": "Blue"
  }
}

You could reference the value stored in Color and place it into the headers like so:

transformationTemplate:
  headers:
    color:
      text: '{{ Favorites.Color }}'

See the template language section for more details about template strings.

body

Use this attribute to apply templates to the request/response body. It consists of a template string that will determine the content of the resulting body.

As an example, the following configuration snippet could be used to transform a response body only if the HTTP response code is 404 and preserve the original response body in other cases.

transformationTemplate:
  # [...]
  body:
    text: '{% if header(":status") == "404" %}{ "error": "Not found!" }{% else %}{{ body() }}{% endif %}'
  # [...]

See the template language section for more details about template strings.

passthrough

In some cases your do not need to transform the request/response body nor extract information from it. In these cases, particularly if the payload is large, you should use the passthrough attribute to instruct Gloo Edge to ignore the body (i.e. to not buffer it). The attribute always takes just the empty value:

transformationTemplate:
  # [...]
  passthrough: {}
  # [...]

If you’re looking to parse the body, and either ignore errors on parsing, or just disable JSON parsing, see those sections in this document, respectively.

mergeExtractorsToBody

Use this type of body transformation to merge all the extractions defined in the transformationTemplate to the body. The values of the extractions will be merged to a location in the body JSON determined by their names. You can use separators in the extractor names to nest elements inside the body.

For an example, see the following configuration:

transformationTemplate:
  mergeExtractorsToBody: {}
  extractors:
  path:
    header: ':path'
    regex: '.*'
  # The name of this attribute determines where the value will be nested in the body
  host.name:
    header: 'host'
    regex: '.*'

This will cause the resulting body to include the following extra attributes (in additional to the original ones):

{
  "path": "/the/request/path",
  "host": {
    "name": "value of the 'host' header"
  }
}

dynamicMetadataValues

This attribute can be used to define an Envoy Dynamic Metadata entry. This metadata can be used by other filters in the filter chain to implement custom behavior.

As an example, the following configuration creates a dynamic metadata entry in the com.example namespace with key foo and value equal to that of the foo header.

dynamicMetadataValues:
- metadataNamespace: "com.example"
  key: 'foo'
  value:
    text: '{{ header("foo") }}'

The metadataNamespace is optional. It defaults to the namespace of the Gloo Edge transformation filter name, i.e. io.solo.transformation.

A common use case for this attribute is to define custom data to be included in your access logs. See the dedicated tutorial for an example of how this can be achieved.

advancedTemplates

This attribute determines which notation to use when accessing elements in JSON structures. If set to true, Gloo Edge will expect JSON pointer notation (e.g. “time/start”) instead of dot notation (e.g. “time.start”). Defaults to false.

Please note that, if set to true, you will need to use the extraction function to access extractors in template strings (e.g. {{ extraction("myExtractor") }}); if the default value of false is used, extractors will simply be available by their name (e.g. {{ myExtractor }}).

escapeCharacters

This attribute is used to set for the entire transformation whether Inja should be configured to preserve escaped characters in strings. This is particularly useful when using context from the request or response body to construct new JSON bodies.

Templating language

Gloo Edge templates are powered by v3.4 of the Inja template engine, which is inspired by the popular Jinja templating language in Python. When writing your templates, you can take advantage of all the core Inja features, i.a. loops, conditional logic, and functions.

In addition to the standard functions available in the core Inja library, you can use additional custom functions that we have added:

• header(header_name): returns the value of the header with the given name.
• extraction(extractor_name): returns the value of the extractor with the given name.
• env(env_var_name): returns the value of the environment variable with the given name.
• body(): returns the request/response body.
• context(): returns the base JSON context (allowing for example to range on a JSON body that is an array).
• request_header(header_name): return the value of the request header with the given name. Useful for including request header values in response transformations.
• base64_encode(string): encodes the input string to base64.
• base64_decode(string): decodes the input string from base64.
• substring(string, start_pos, substring_len): returns a substring of the input string, starting at start_pos and extending for substring_len characters. If no substring_len is provided or substring_len is <= 0, the substring extends to the end of the input string.
• replace_with_random(string, pattern): returns the input string with instances matching the pattern replaced with random characters.
• raw_string(string): returns the input string with escaped characters intact. Useful for constructing JSON request or response bodies.

You can use templates to mutate headers, the body, and dynamic metadata.

XSLT Transformation

XSLT transformations allow transformations on HTTP requests using the XSLT transformation language. The following snippet illustrates the structure of the xsltTransformation object.

xsltTransformation:
  xslt: string
  setContentType: string
  nonXmlTransform: bool

xslt

The XSLT transformation is specified in this field as a string. Like other transformations, an invalid XSLT transformation will not be accepted and envoy validation will reject the transformation configuration.

setContentType

XSLT transformations can be used to transform HTTP body between content type. For example, from XML to JSON, or JSON to XML. In the case of these transformations, the content-type HTTP header is set to the value of setContentType. If left empty, the content-type header is unchanged.

nonXmlTransform

XSLT transformations typically accept only XML as input. If the input to the transformation is not XML, this should be set to true. For example, if the XSLT transformation is transforming a JSON input to XML, this would be set to true. By default, this is false and the XSLT transformation will only accept XML input.

Common use cases

On this page we have seen all the properties of the Gloo Edge Transformation API as well as some simple example snippets. If are looking for complete examples, please check out the following tutorials, which will guide you through some of the most common transformation use cases.

• Change response status: Conditionally update the status of an HTTP response

• Decode and modify base64 request headers: Decode and modify base64 encoded request headers before forwarding the requests upstream.

• Inject response header: Inject request headers into response

• SOAP/XSLT transformation (Enterprise): Transforming the request body from SOAP/XML to a JSON.

• Extract query parameters: Extract query parameters from a request

• Update request path: Conditionally update the request path

• Add headers to the body: Adding request/response headers to the body

• Debug logging for transformations: Debug complex sequences of transformations.

• Enriching access logs: Use transformations to craft custom attributes in access logs.