The __rest expansion directive allows mongod or mongos to start with additional directives, or an entire configuration block, from an external REST request. Using parameters supplied with the directive, the server will issue an HTTP GET request to the REST application programming interface (API) endpoint specified and will receive its configuration in turn. The advantage of this approach is that it enables the centralization of startup directives across a large distributed MongoDB system.
An obvious disadvantage when making requests to remote resources is that it opens a potentially huge security vulnerability. Thus, at a minimum, requests to any remote endpoint must use HTTPS. In addition, operating system permissions must restrict read access of the configuration file containing the __rest expansion directive to only the mongod or mongos user.
Here is a summary of parameters needed in conjunction with the __rest expansion directive: