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-Author: Bertrand Guiheneuf <Bertrand.Guiheneuf@aful.org>
-Date: August 9th 1999
-Version: 0.1 
-
-The last version of this document is always available in gnome CVS in
-the gnome-mailer module: devel-docs/misc/ref_and_id_proposition.txt
-
-
-
-A) Identifying messages within folders
---------------------------------------
-
-Currently, in Camel there is only one way to retrieve a message from a
-mail store:
-     CamelMimeMessage * 
-     get_message (CamelFolder *folder, gint number)
-
-where number is an integer representing the message rank within its
-parent folder.
-
-This is a traditional method (JavaMail, MAPI) and it is very useful
-because this is often the only way to get a identify message in a
-classical store (pop3 for example).
-
-Moreover, various documents ([1], [2]) proposed to generalize the URL
-scheme used in Camel ([3]) to access mail stores in order to identify
-messages. Such an URL would be, for instance:
-
-pop3://po.myisp.com:1
-
-Meaning: "Access message 1 on Pop3 server po.myisp.com"
-
- 
-However, referencing a message with its number within a folder is a
-very unreliable method:
-
-1) Message order in a folder can change during a session: 
-   
-   The user can move or remove messages from the folder, thus
-   completely changing message numbers.  We could however imagine to
-   follow message operations in order to keep camel in a coherent
-   state at each time instant. This could be quite complex but may
-   be feasible using gtk signal system.
-
-2) Message order can change between sessions:
-
-   Gnome-mailer was designed from the begining to allow messages to be
-   stored in classical mailboxes (mbox, maildir, MH, IMAP ...), in
-   order to allow users to run other MUA on their mailboxes if
-   necessary.  These other MUA can change message order within folders
-   without any chance for Camel to trace the operations.
-   
-These two scenarii show that it is quite impossible to use reliable
-folder caching or message referencing if messages are referenced only
-by their position within their parent folder.
-   
-
-We thus have to find a general way to identify and retreive a message
-within its folder. One thing is sure, however: all folders
-implementation won't allow this method. Pop3 stores will always access
-messages using their rank on the server. MUA using Camel will thus
-have to be prepared to access some stores providing only the old
-fashionned message number access method.
-
-Basically, we have two choices:
-
-1) Accessing messages using (mailbox) Unique ID (UID)
-
-   A UID is a string identifier associated to a message, which is
-   guaranteed to be unique within its parent folder and which will not
-   change between sessions.
-
-2) Accessing messages using Message ID
-
-   A Message ID is a string identifier associated to a messages which
-   is guaranteed to be unique in the world, that is, no other message
-   can have the same Message ID. The message ID is defined in RFC 822,
-   and is stored as the message header "Message-id"
-   
-Method (1) already exists in IMAP.  
-It is quite simple to define on local stores (MH, mbox, ....) but it
-may not resist to message modification by other MUA.
-Methods based on Message-id matching or message content-checksum seem
-to be the best one. Using an "X-" header is another possibility on
-non-read only headers. A combination of these three methods may be the
-most reliable solution.
-The UID is impossible to implement in a POP3 store provider.
-
-(2) Can be used with IMAP, but would be very ineficient. 
-The main issue with this method is its dependancy upon other MUAs and
-MTAs. Message-id is set before or during message transport. Moreover,
-some rfc822 compliant messages may not even have any Message-id
-header.
-These are major issues when accessing read-only stores.  
-The M-ID is also impossible to implement in a POP3 store provider.
-   
-
-We may not rely on external MUA and MTA to guarentee the uniqueness of
-the identifier . We may lose messages by never being able to read them
-if two had the same uid. It would be possible to find workarounds, but
-it could make Camel use a bit tricky.
-
-Given that most users will use IMAP or a database based store as their
-main mail store, and given that this stores allow UID very
-easily, I suggest that we use method (1). Discussion is still open,
-though.
-
-Here are the public methods I propose to add to CamelFolder:
-
-gboolean camel_folder_supports_uid (CamelFolder *folder)
-    returns true if the folder can get messages 
-    by their uid.
-
-gchar * camel_folder_get_uid_by_number (CamelFolder *folder, gint message_number)
-    return the uid of message which number in the folder
-    is %message_number.
-
-gchar * camel_folder_get_message_uid (CamelFolder *folder, CamelMimeMessage *message)
-    return the uid of the message within the folder.
-
-CamelMimeMessage *camel_folder_get_message_by_uid  (CamelFolder *folder, gchar *uid)
-    return the message which uid is %uid
-
-In addition, the CamelFolder Class will have a new public method
-
-gchar * camel_mime_message_get_uid (CamelMimeMessage *message) 
-    return the uid associated to the message in its physical parent
-    folder.
-
-
-
-B) Handling message references in (v)folders.
----------------------------------------------
- 
-
-We want the future Gnome mailer to be able to build (virtual) folders
-holding references to messages located physically in other
-folders. More generally, we would like folders to be able to hold:
-
-1) messages
-2) subfolders
-3) references to messages
-
-(1) and (2) are already implemented in Camel because most mail stores
-can hold messages and/or subfolders.
-
-(3) is a different issue, because no existing mail store can currently
-hold within folders references to messages in other folders.
-It will thus be a specific gnome-mailer extension. 
-
-
-One of the main issue is to determine what kind of behaviour we expect
-from folders holding references. Here is a possible API.
-
-( the world (v)folder is used to distinguish between the physical
-parent folder and the folder holding a reference to the message, when
-a confusion may arise)
-
-Addition to CamelFolder:
-
-gboolean camel_folder_can_hold_references (CamelFolder *folder)
-    return true if the folder can contain references
-
-void camel_folder_add_reference_by_uid (CamelFolder *folder, gchar *folder_url, gchar *message_uid)
-    add a reference into a folder. %folder_url is the url of 
-    the folder, %message_uid is the uid of the message within 
-    its physical parent folder.
- 
-void camel_folder_add_reference_by_message (CamelFolder *folder, CamelMessage *message)
-    add a reference. The place where the reference points 
-    to is found using CamelMessage methods
-
-void camel_folder_remove_reference_by_uid (CamelFolder *folder, gchar *uid)
-    remove a message reference form a folder. Reference 
-    is identified using its uid within the folder.
-
-gboolean camel_folder_uid_is_reference (CamelFolder *folder, gchar *uid)
-    return true if the message corresponding to the uid is a reference.
-
-Then all usual operations on the folder act if the message was
-actually physically stored in this folder. For example, when the mailer
-uses camel_folder_get_message_by_uid onto the (v)folder, the actual
-message is retreived from its physical store.
-
-As you can see, the uid of the message within its physical parent
-folder is different than its uid within the (v)folder. This is because
-there is no way to guarantee that the uids of two messages in two
-different folders would be different. Using references on this two
-message in the same vfolder would break uniqueness of the uid in the
-(v)folder.
-
-A couple of other methods could be defined but all the basics are
-described here.  
-
-This draft API is far from complete nor perfect, and is described here
-only to stimulate discussions before the actual implementation.
-
-
-The question now is to know how we store references. There are basically 
-two ways:
-
-1) references are stored using the URL of the physical folder
-   and the uid of the message within the folder
-
-2) a list of reference is kept, and in this list, reference are stored
-as in (1). Folders would refer to the actual message using index in
-the list
-
-
-   
-The main problem with (1) is that references get lost as soon
-as the actual message is moved. There is no way to find in which
-folders references to the message exist. 
-
-(2) is a way to solve this issue. When messages are used, Camel looks
-in the list to see if the message is refered somewhere, and actualize 
-the URL and the uid with their new values. 
-
-The problem with (2) is that we need to keep this information in a file
-and libraries writing automatically to files are generally a bad idea.
-
-As in additional remark, it is clear that Camel will only be able to
-hold references to messages on stores supporting UIDs.
-
-
-Thanks in advance for your comments and ideas,
-
-
-       Bertrand <Bertrand.Guiheneuf@aful.org>
-
-
---
-
-[1] : http://www.selequa.com/%7epurp/gnomail/mail2db.html 
-[2] : http://www.selequa.com/%7epurp/gnomail/dbRecFmt.html 
-[3] : http://www.gnome.org/mailing-lists/archives/gnome-mailer-list/1999-April/0248.shtml