Cached content expiring too frequently on fastly CDN - heroku

Question

I'm using Fastly as a CDN in front of my Heroku application, and am seeing many requests that I expect to be cached make it through.
An example of this behavior is two requests to the URL:
https://nuu-acceptance-herokuapp-com.global.ssl.fastly.net/attachments/f092ff0398b3bace19fae21b17a22320c3da5428/store/fit/240/160/28515a2fa2e47b59f13b2044ea5b9a7c8c9587ceca7d7dfadb28f08730f7/file.jpg. Here are two responses from the requests, which occurred fifteen minutes apart:
RESPONSE 1:
----------
Strict-Transport-Security: max-age=31536000
Content-Encoding: gzip
X-Content-Type-Options: nosniff
Age: 0
Transfer-Encoding: chunked
X-Cache: MISS
X-Cache-Hits: 0
Content-Disposition: inline; filename="file.jpg"
Connection: keep-alive
Via: 1.1 vegur
Via: 1.1 varnish
X-Request-Id: bc766069-c2ca-4a66-ba88-a8d76da72e2d
X-Served-By: cache-sjc3124-SJC
X-Runtime: 3.711698
Last-Modified: Tue, 23 Jun 2015 18:44:27 GMT
Server: Cowboy
X-Timer: S1435085062.909546,VS0,VE4437
Date: Tue, 23 Jun 2015 18:44:27 GMT
Vary: Accept-Encoding
Content-Type: image/jpeg
Access-Control-Allow-Origin: *
Cache-Control: public, must-revalidate, max-age=31536000
Set-Cookie: __profilin=; path=/; max-age=0; expires=Thu, 01 Jan 1970 00:00:00 -0000; secure
Accept-Ranges: bytes
Expires: Wed, 22 Jun 2016 18:44:27 GMT
----------
RESPONSE 2:
----------
Strict-Transport-Security: max-age=31536000
Content-Encoding: gzip
X-Content-Type-Options: nosniff
Age: 0
Transfer-Encoding: chunked
X-Cache: MISS
X-Cache-Hits: 0
Content-Disposition: inline; filename="file.jpg"
Connection: keep-alive
Via: 1.1 vegur
Via: 1.1 varnish
X-Request-Id: 60ee54b0-9509-42c5-9b03-c0f5854c5524
X-Served-By: cache-sjc3135-SJC
X-Runtime: 0.251021
Last-Modified: Tue, 23 Jun 2015 18:57:44 GMT
Server: Cowboy
X-Timer: S1435085863.749442,VS0,VE560
Date: Tue, 23 Jun 2015 18:57:44 GMT
Vary: Accept-Encoding
Content-Type: image/jpeg
Access-Control-Allow-Origin: *
Cache-Control: public, must-revalidate, max-age=31536000
Accept-Ranges: bytes
Expires: Wed, 22 Jun 2016 18:57:44 GMT
Both are cache misses, even though I expect this content to be cached for a year. It also appears that the same Fastly cluster handled the request. Can anyone point me to what I might be doing wrong? I'm seeing this behavior across many files served by Fastly - fastly seems to serve the files intermittently, but there are cache misses much more often than I expect.
I'd appreciate any help that anyone could give me with this - thanks!

Answer 1

If you look at the HTTP headers from your responses, you will see that they both contain a Set-Cookie header. Responses with cookies will not be cached by Fastly. You can remove them, however, in your app or within your Fastly configuration.