Dovetail CDN Viewer Request

CloudFront Function to verify and normalize all requests to the Dovetail CDN.

This function is tied to the CloudFront viewer-request event, verifies the request, redirects old/expired urls, and normalizes the requested paths.

Configuration

As this is a CloudFront Function, you must hardcode everything instead of any configurations. That said, we use a Custom::CodeFetcher and a bit of python in CloudFormation to replace values at deploy time.

<EXPIRED_REDIRECT_PREFIX> - the scheme, domain, and optional path to redirect requests back to if your url ?exp= is expired.

Usage

Inputs

The lambda is invoked with a CloudFront Function viewer-request event (which is NOT the same as an edge function viewer-request event!!!) of the structure:

{
  "request": {
    "method": "GET",
    "uri": "/<podcast_id>/<episode_guid>/<arrangement_digest>/filename.mp3",
    "querystring": {
      "exp": { "value": "99999999" },
      "force": { "value": "1" }
    }
  }
}

The uri may also optionally include a <feed_id> token, to indicate this came from a non-default feed (such as "adfree"):

"uri": "/<podcast_id>/<feed_id>/<episode_guid>/<arrangement_digest>/filename.mp3"

AND the uri may also also optionally start with an <behavior_prefix> token, to indicate which "stack" the Dovetail Router redirect originated from. This will tie the CloudFront behavior + realtime-logs to that specific dovetail stack within a region, for analytics processing. This token must start with a lowercase character, not a number.

The exp is a (currently optional) epoch seconds timestamp, when this url will no longer be valid and must be redirected back for a newer arrangement.

The force param optionally forces an edge-server cache miss, so we have to go all the way down through the origin-request and arranger lambdas to restitch.

Work

Check that the uri looks valid, and 404 right away on unknown paths.
Verify that the ?exp=<epoch_seconds> param is still valid. If the url has expired, redirect back to EXPIRED_REDIRECT_PREFIX for a new redirect.
Normalize uri so that we don't make a bunch of redundant requests for the same arrangement_digest. The dovetail-cdn-origin-request expects /<podcast_id>/<episode_guid>/<digest> ... so rewrite to that.
If we're ?force-ing, append a random /random-<random-stuff> string to the request uri. The CloudFront edge server will not have that full path cached, so the request will fall through to origin-requesting.
Return the rewritten request.

Error Handling

This function is very simple, and generally should never error. But if it did, the user would just get a 500.

Development

Tests can only be run locally. And since this is a CloudFront Function, it's not really modern javascript, so you can't do any real linting and your syntax is limited. But prettier is installed, so at least use that!

nvm use
yarn
npm test

# lint things up before committing
npm run lint

Deploying

This function's code is pulled directly from Github via our Infrastructure Custom::CodeFetcher. You will need to change the Cycle param and redeploy the stack to pick up any code changes here.

License

AGPL License

PRX / dovetail-cdn-viewer-request

readme