If a client sends a request with `Accept-Encoding: gzip' header, OpenShift will reply with a compressed response. My tests showed that this happens even if the server running on the gear doesn't provide compressed responses, which means that the compression is performed by OpenShift proxy.

For example, in one of my tests, the origin server sent the following response:

    HTTP/1.1 200 OK
    Content-Type: text/plain
    Cache-Control: no-transform
    Content-Length: 11

    Hello world

The client, however, received the following response:

    HTTP/1.1 200 OK
    Date: Fri, 26 Jun 2015 17:25:11 GMT
    Content-Type: text/plain
    Cache-Control: no-transform
    Vary: Accept-Encoding
    Content-Encoding: gzip
    Content-Length: 31
    Keep-Alive: timeout=15, max=100
    Connection: Keep-Alive

    [gzip-compressed data]

As you can see, the proxy compressed the response, even though it contained the `Cache-Control: no-transform' header. Quoting the relevant RFC:

    A proxy MUST NOT transform the payload (Section 3.3 of [RFC7231]) of
    a message that contains a no-transform cache-control directive
    (Section 5.2 of [RFC7234]).

    A proxy MAY transform the payload of a message that does not contain
    a no-transform cache-control directive.  A proxy that transforms a
    payload MUST add a Warning header field with the warn-code of 214
    ("Transformation Applied") if one is not already in the message (see
    Section 5.5 of [RFC7234]).  A proxy that transforms the payload of a
    200 (OK) response can further inform downstream recipients that a
    transformation has been applied by changing the response status code
    to 203 (Non-Authoritative Information) (Section 6.3.4 of [RFC7231]).

Also, it would be nice if such transparent compression could be disabled by an administrator for his own server, as it is sometimes undesirable [1].

[1]: https://stackoverflow.com/questions/30397457/disable-gzip-compression-on-openshift-php-cartridge