[Couchdb Wiki] Update of "Technical Overview" by JanLehnardt

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The "Technical Overview" page has been changed by JanLehnardt.
The comment on this change is: No doc-level reader access.
http://wiki.apache.org/couchdb/Technical%20Overview?action=diff&rev1=6&rev2=7

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  CouchDB database instances have administrator accounts. Administrator accounts can create other administrator accounts and update design documents. Design documents are special documents containing view definitions and other special formulas, as well as regular fields and blobs.
  
- === Reader Access ===
- 
- To protect document contents, CouchDB documents can have a reader list. This is an optional list of reader-names allowed to read the document. When a reader list is used, protected documents are only viewable by listed users.
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- When a user accesses a database, then his/her credentials (name and password) are used to dynamically determine his reader names. The user credentials are input to a javascript function and the function returns a list of names for the user, or an error if the user credentials are wrong.
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- When a document is protected by reader access lists, any user attempting to read the document must be listed. Reader lists are enforced in views too. Documents that are not allowed to be read by the user are dynamically filtered out of views, keeping the document row and extracted information invisible to non-readers.
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  === Update Validation ===
  
  As documents written to disk, they can be validated dynamically by javascript functions for both security and data validation. When the document passes all the formula validation criteria, the update is allowed to continue. If the validation fails, the update is aborted and the user client gets an error response.