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Showing content from https://www.fastly.com/blog/best-practices-using-vary-header below:

Best practices for using the Vary header

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GET **/somepage.php** HTTP/1.1
Host: **example.com**
Connection: keep-alive
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/36.0.1985.125 Safari/537.36
Accept-Language: en-US,en;q=0.8
Accept-Encoding: gzip,deflate,sdch

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HTTP/1.1 200 OK
Content-Length: 3458
Cache-Control: max-age=86400
Content-Encoding: gzip
**Vary: Accept-Encoding**

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HTTP/1.1 200 OK
Content-Length: 8365
Cache-Control: max-age=86400
**Vary: Accept-Encoding**

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# do this only once per request
if (req.restarts == 0) {
  # normalize Accept-Encoding to reduce vary
  if (req.http.Accept-Encoding ~ "gzip") {
    set req.http.Accept-Encoding = "gzip";
  } else {
    unset req.http.Accept-Encoding;
  }
}

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# do this only once per request
if (req.restarts == 0) {
  # normalize Accept-Encoding to reduce vary
  if (req.http.Accept-Encoding) {
    if (req.http.User-Agent ~ "MSIE 6") {
      unset req.http.Accept-Encoding;
    } elsif (req.http.Accept-Encoding ~ "gzip") {
      set req.http.Accept-Encoding = "gzip";
    } elsif (req.http.Accept-Encoding ~ "deflate") {
      set req.http.Accept-Encoding = "deflate";
    } else {
      unset req.http.Accept-Encoding;
    }
  }
}

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if (req.http.User-Agent ~ "(Mobile|Android|iPhone|iPad)") {
  set req.http.User-Agent = "mobile";
} else {
  set req.http.User-Agent = "desktop";
}

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if (req.http.Referer ~ "^https?://www.example.com/") {
  set req.http.Referer = "http://www.example.com/";
} else {
  unset req.http.Referer;
}

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sub vcl_recv {
  # set the custom header
  if (req.http.Cookie ~ "ABtesting=B") {
    set req.http.X-ABtesting = "B";
  } else {
    set req.http.X-ABtesting = "A";
  }
...
}

...

sub vcl_fetch {
  # vary on the custom header
  if (beresp.http.Vary) {
    set beresp.http.Vary = beresp.http.Vary ", X-ABtesting";
  } else {
    set beresp.http.Vary = "X-ABtesting";
  }
  ...
}

...

sub vcl_deliver {
  # Hide the existence of the header from downstream
  if (resp.http.Vary) {
    set resp.http.Vary = regsub(resp.http.Vary, "X-ABtesting", "Cookie");
  }
  # Set the ABtesting cookie if not present in the request
  if (req.http.Cookie !~ "ABtesting=") {
    # 75% A, 25% B
    if (randombool(3, 4)) {
      add resp.http.Set-Cookie = "ABtesting=A; expires=" now + 180d "; path=/;";
    } else {
      add resp.http.Set-Cookie = "ABtesting=B; expires=" now + 180d "; path=/;";
    }
  }

  ...
}

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