WEEK 3 READING NOTES

"Security is an important consideration for retrieval systems in corporations. A low-level employee should not be able to find the salary roster of the corporation, but authorized managers need to be able to search for it. Users' results lists must not contain documents they are barred from opening; the very existence of a document can be sensitive information.
User authorization is often mediated through access control lists or ACLs. ACLs can be dealt with in an information retrieval system by representing each document as the set of users that can access them (Figure 4.8 ) and then inverting the resulting user-document matrix. The inverted ACL index has, for each user, a ``postings list'' of documents they can access - the user's access list. Search results are then intersected with this list. However, such an index is difficult to maintain when access permissions change - we discussed these difficulties in the context of incremental indexing for regular postings lists in Section 4.5. It also requires the processing of very long postings lists for users with access to large document subsets. User membership is therefore often verified by retrieving access information directly from the file system at query time - even though this slows down retrieval. "

In any kind of information related system, secruity is always one the most important parts. Especially people emphasize about information privacy much more than ever before. How to protect the information and improve the effiency of retrieval at the same time becomes a big challenge for nowaday's development of information retrieval technology. In many management information systems, different staffs may have different anthorization to get different information. Thus, different information may have different access frequency. It's difficult to define which information could be most accessable. It makes harder to develop the proper retrieval model to increase the effiency.