Sentry is a service for archiving URL's with a clean, current, auditable trail of digital provenance. It's main job is to issue GET requests to a given URL, record all information related to the request namely: timestamp, HTTP response headers, sha256 of response body, content-type sniff, content size and download time. If desirable, sentry can also store the response, currently to amazon S3. Sentry archives urls in two ways: