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Compress Response

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.

Supported Phases​

on_http_request, on_http_response

Type​

compress-response

Configuration Fields​

  • algorithmsarray of strings

    A list of allowed compression algorithms to be considered during encoding type negotiation. Each element must be unique.

    Defaults to gzip, deflate, br, compress.

    • Supported algorithms
    • br
    • compress
    • deflate
    • gzip

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.

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.

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.

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:

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The algorithm gzip would be selected because it has the highest q-value of 1.0.

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.

Example 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.

Traffic Policy Document​

You can use one of the following snippets:

Policy​

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Agent Config​

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For this example, we are assuming that an ngrok agent is pointing at the upstream service https://jsonplaceholder.typicode.com.

Request​

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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.

Action Result Variables​

  • actions.ngrok.compress.already_compressedboolean

    Indicates whether the body was already compressed before the action was applied. Returns true if no further compression was performed.

  • actions.ngrok.compress.negotiated_algorithmstring

    The compression algorithm selected and applied by the action, based on the client's request and action configuration.