The Compress Response Traffic Policy action enables you to improve the performance of your web applications by compressing HTTP response bodies returned by your upstream service.

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Configuration Reference

The Traffic Policy configuration reference for this action.

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Supported Phases

on_http_response

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Type

compress-response

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Configuration Fields

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algorithms
array of strings
default:['gzip', 'deflate', 'br', 'compress']
A list of allowed compression algorithms to be considered during encoding type negotiation. Each element must be unique.
Supported algorithms
br
compress
deflate
gzip

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Behavior

When an HTTP request includes an Accept-Encoding header, HTTP responses are automatically compressed, and a Content-Encoding header is added to the response. If the response is already compressed by your upstream service, no additional compression will be applied.

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Streaming Compression

When compression is applied, the response body is not buffered; it is compressed in real-time as it streams through the ngrok endpoint.

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Algorithm Choice

If a request specifies Accept-Encoding but none of the requested algorithms are supported, no compression is applied, and the upstream response is returned unmodified.

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Quality Values

Compression is negotiated based on the Accept-Encoding header as defined by the RFC, respecting q-values and selecting the supported algorithm with the highest q-value. For example: 
Accept-Encoding: gzip;q=1.0, br; q=0.5, *;q=0
The algorithm gzip would be selected because it has the highest q-value of 1.0.

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Response Headers

When this action performs compression, the following changes are made to the HTTP response headers:
  • The Content-Length header is removed.
  • A Content-Encoding header is added, specifying the negotiated algorithm.
  • A Vary: Accept-Encoding header is added to indicate that the response varies based on the Accept-Encoding request header.

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Examples

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Compressing API Responses Based on URL Path

The following Traffic Policy configuration demonstrates how to set this up using the compress-response action with an expression to compress traffic on specific URL paths.

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Example Traffic Policy Document

on_http_response:
  - expressions:
      - "req.url.path.startsWith('/api/')"
    actions:
      - type: "compress-response"
        config:
          algorithms:
            - "gzip"
            - "br"
            - "deflate"
            - "compress"
For this example, we are assuming that an ngrok agent is pointing at the upstream service https://jsonplaceholder.typicode.com.

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Example Request

$ curl -i https://jsonplaceholder.ngrok.app/api/posts

HTTP/2 200 OK
content-encoding: gzip
content-type: application/json; charset=utf-8

<compressed-body-here>
In this example, when a request is made to /api/posts, ngrok compresses the response using one of the specified algorithms (e.g., gzip). The response includes the content-encoding: gzip header, and the body is compressed accordingly. This setup ensures that only API responses for paths starting with /api/ are compressed, enhancing performance and responsiveness for only those endpoints.

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Action Result Variables

The following variables are made available for use in subsequent expressions and CEL interpolations after the action has run. Variable values will only apply to the last action execution, results are not concatenated.
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actions.ngrok.compress.already_compressed
boolean
Indicates whether the body was already compressed before the action was applied. Returns true if no further compression was performed.
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actions.ngrok.compress.negotiated_algorithm
string
The compression algorithm selected and applied by the action, based on the client’s request and action configuration.