Solr: Custom Request Handler
defaults: These simply establish default values for various request parameters. Parameters in the request will override them.
appends: For parameters that can be set multiple times, such as fq, this section specifies values that will be set in addition to any that may be specified by the request.
invariants: This sets defaults that cannot be overridden. It is useful for security purposes. It can also be used to override what the client sends when you don't have control over the client application; for instance, if the application is deployed and you can't easily re-deploy a new client.