Google Search Appliance Connectors Administration Guide - Manual (Page 6)

The Lister/Retriever model
Connectors 4.0 are based on the lister/retriever model. In this model, the lister notifies the
search appliance of the names of documents in the repository and the retriever sends
URLs and ACLs in feeds to the search appliance. The search appliance uses the URLs to
crawl documents in the repository over HTTP/HTTPS. Each document in a repository is
identified by a unique document identifier (DocId).
The search appliance performs the task of determining which DocIds the user can access
and handles authentication requests between the search appliance and the repository.
The following diagram shows how the connectors interact with the search appliance and
the repository.