Release Manager routes new sessions on https://hollowverse.com to different versions of the website in order to test code changes in a production environment on a small percentage of sessions before the changes land on master.
It can also be used to preview code changes before they are ready for production.
Release Manager implements cookie-based traffic splitting.
Each environment, including master, is assigned a numerical value that indicates the weight of that environment, i.e. how many sessions are routed to that environment relative to the other ones.
The values are set in src/trafficSplitter/environments.ts.
A new session is assigned an environment picked at random, while still ensuring that the assignments respect the configured weight values.
The name of the new environment is stored in a cookie and read on subsequent requests, so once a user is routed to one environment, all subsequent pages are served from the same environment until the cookie expires.
When Release Manager receives a request, it looks in the URL for a query string field named branch. If the field exists, and has a value that matches one of the already-deployed AWS EB environments for the intended application, it routes the request to that environment. This can be used to preview branches that are not ready to be merged to a production environment.
A cookie is set to persist the routing for the user session so that subsequent requests do not have to set the branch field for the lifetime of the cookie. This cookie will always override the one that is used for traffic splitting. When that cookie expires or is deleted, the user is routed back to the environment specified in the traffic splitting cookie.