--- /dev/null
+The totally incomplete guide to Citadel internals
+-----------------------------------------------------
+-----------------------------------------------------
+
+Citadel has evolved quite a bit since its early days, and the data structures
+have evolved with it. This document provides a rough overview of how the
+system works internally. For details you're going to have to dig through the
+code, but this'll get you started.
+
+
+DATABASE TABLES
+---------------
+As you probably already know by now, Citadel uses a group of tables stored
+with a record manager (usually Berkeley DB). Since we're using a record
+manager rather than a relational database, all record structures are managed
+by Citadel. Here are some of the tables we keep on disk:
+
+
+USER RECORDS
+------------
+This table contains all user records. It's indexed by
+user name (translated to lower case for indexing purposes). The records in
+this file look something like this:
+
+struct ctdluser { /* User record */
+ int version; /* Cit vers. which created this rec */
+ uid_t uid; /* Associate with a unix account? */
+ char password[32]; /* password (for Citadel-only users)*/
+ unsigned flags; /* See US_ flags below */
+ long timescalled; /* Total number of logins */
+ long posted; /* Number of messages posted (ever) */
+ CIT_UBYTE axlevel; /* Access level */
+ long usernum; /* User number (never recycled) */
+ time_t lastcall; /* Last time the user called */
+ int USuserpurge; /* Purge time (in days) for user */
+ char fullname[64]; /* Name for Citadel messages & mail */
+};
+
+ Most fields here should be fairly self-explanatory. The ones that might
+deserve some attention are:
+
+ - *uid* -- if uid is not the same as the *unix uid* Citadel is running as, then the
+ account is assumed to belong to the user on the underlying Unix system with
+ that uid. This allows us to require the user's OS password instead of having
+ a separate Citadel password.
+
+ - *usernum* -- these are assigned sequentially, and **NEVER REUSED**. This is
+ important because it allows us to use this number in other data structures
+ without having to worry about users being added/removed later on, as you'll
+ see later in this document.
+
+
+ROOM RECORDS
+------------
+These are room records. There is a room record for every room on the
+system, public or private or mailbox. It's indexed by room name (also in
+lower case for easy indexing) and it contains records which look like this:
+
+ struct ctdlroom {
+ char QRname[ROOMNAMELEN]; /* Name of room */
+ char QRpasswd[10]; /* Only valid if it's a private rm */
+ long QRroomaide; /* User number of room aide */
+ long QRhighest; /* Highest message NUMBER in room */
+ time_t QRgen; /* Generation number of room */
+ unsigned QRflags; /* See flag values below */
+ char QRdirname[15]; /* Directory name, if applicable */
+ long QRinfo; /* Info file update relative to msgs*/
+ char QRfloor; /* Which floor this room is on */
+ time_t QRmtime; /* Date/time of last post */
+ struct ExpirePolicy QRep; /* Message expiration policy */
+ long QRnumber; /* Globally unique room number */
+ char QRorder; /* Sort key for room listing order */
+ unsigned QRflags2; /* Additional flags */
+ int QRdefaultview; /* How to display the contents */
+ };
+
+Again, mostly self-explanatory. Here are the interesting ones:
+
+*QRnumber* is a globally unique room ID, while QRgen is the "generation number"
+of the room (it's actually a timestamp). The two combined produce a unique
+value which identifies the room. The reason for two separate fields will be
+explained below when we discuss the visit table. For now just remember that
+*QRnumber* remains the same for the duration of the room's existence, and QRgen
+is timestamped once during room creation but may be restamped later on when
+certain circumstances exist.
+
+FLOORTAB
+--------
+Floors. This is so simplistic it's not worth going into detail about, except
+to note that we keep a reference count of the number of rooms on each floor.
+
+MSGLISTS
+--------
+Each record in this table consists of a bunch of message numbers
+which represent the contents of a room. A message can exist in more than one
+room (for example, a mail message with multiple recipients -- 'single instance
+store'). This table is never, ever traversed in its entirety. When you do
+any type of read operation, it fetches the msglist for the room you're in
+(using the room's ID as the index key) and then you can go ahead and read
+those messages one by one.
+
+Each room is basically just a list of message numbers. Each time
+we enter a new message in a room, its message number is appended to the end
+of the list. If an old message is to be expired, we must delete it from the
+message base. Reading a room is just a matter of looking up the messages
+one by one and sending them to the client for display, printing, or whatever.
+
+
+VISIT
+-----
+This is the tough one. Put on your thinking cap and grab a fresh cup of
+coffee before attempting to grok the visit table.
+
+This table contains records which establish the relationship between users
+and rooms. Its index is a hash of the user and room combination in question.
+When looking for such a relationship, the record in this table can tell the
+server things like "this user has zapped this room," "this user has access to
+this private room," etc. It's also where we keep track of which messages
+the user has marked as "old" and which are "new" (which are not necessarily
+contiguous; contrast with older Citadel implementations which simply kept a
+"last read" pointer).
+
+
+Here's what the records look like:
+
+ struct visit {
+ long v_roomnum;
+ long v_roomgen;
+ long v_usernum;
+ long v_lastseen;
+ unsigned int v_flags;
+ char v_seen[SIZ];
+ int v_view;
+ };
+
+ #define V_FORGET 1 /* User has zapped this room */
+ #define V_LOCKOUT 2 /* User is locked out of this room */
+ #define V_ACCESS 4 /* Access is granted to this room */
+
+This table is indexed by a concatenation of the first three fields. Whenever
+we want to learn the relationship between a user and a room, we feed that
+data to a function which looks up the corresponding record. The record is
+designed in such a way that an "all zeroes" record (which is what you get if
+the record isn't found) represents the default relationship.
+
+With this data, we now know which private rooms we're allowed to visit: if
+the *V_ACCESS* bit is set, the room is one which the user knows, and it may
+appear in his/her known rooms list. Conversely, we also know which rooms the
+user has zapped: if the *V_FORGET* flag is set, we relegate the room to the
+zapped list and don't bring it up during new message searches. It's also
+worth noting that the *V_LOCKOUT* flag works in a similar way to administratively
+lock users out of rooms.
+
+Implementing the "cause all users to forget room" command, then, becomes very
+simple: we simply change the generation number of the room by putting a new
+timestamp in the *QRgen* field. This causes all relevant visit records to
+become irrelevant, because they appear to point to a different room. At the
+same time, we don't lose the messages in the room, because the msglists table
+is indexed by the room number (*QRnumber*), which never changes.
+
+*v_seen* contains a string which represents the set of messages in this room
+which the user has read (marked as 'seen' or 'old'). It follows the same
+syntax used by IMAP and NNTP. When we search for new messages, we simply
+return any messages that are in the room that are **not** represented by this
+set. Naturally, when we do want to mark more messages as seen (or unmark
+them), we change this string. Citadel BBS client implementations are naive
+and think linearly in terms of "everything is old up to this point," but IMAP
+clients want to have more granularity.
+
+
+DIRECTORY
+---------
+This table simply maps Internet e-mail addresses to Citadel network addresses
+for quick lookup. It is generated from data in the Global Address Book room.
+
+USETABLE
+--------
+This table keeps track of message ID's of messages arriving over a network,
+to prevent duplicates from being posted if someone misconfigures the network
+and a loop is created. This table goes unused on a non-networked Citadel.
+
+THE MESSAGE STORE
+-----------------
+This is where all message text is stored. It's indexed by message number:
+give it a number, get back a message. Messages are numbered sequentially, and
+the message numbers are never reused.
+
+We also keep a "metadata" record for each message. This record is also stored
+in the msgmain table, using the index (0 - msgnum). We keep in the metadata
+record, among other things, a reference count for each message. Since a
+message may exist in more than one room, it's important to keep this reference
+count up to date, and to delete the message from disk when the reference count
+reaches zero.
+
+#Here's the format for the message itself:
+
+ - Each message begins with an 0xFF 'start of message' byte.
+
+ - The next byte denotes whether this is an anonymous message. The codes
+ available are *MES_NORMAL*, *MES_ANON*, or *MES_AN2* (defined in citadel.h).
+
+ - The third byte is a "message type" code. The following codes are defined:
+ - 0 - "Traditional" Citadel format. Message is to be displayed "formatted."
+ - 1 - Plain pre-formatted ASCII text (otherwise known as text/plain)
+ - 4 - MIME formatted message. The text of the message which follows is
+ expected to begin with a "Content-type:" header.
+
+ - After these three opening bytes, the remainder of
+ the message consists of a sequence of character strings. Each string
+ begins with a type byte indicating the meaning of the string and is
+ ended with a null. All strings are printable ASCII: in particular,
+ all numbers are in ASCII rather than binary. This is for simplicity,
+ both in implementing the system and in implementing other code to
+ work with the system. For instance, a database driven off Citadel archives
+ can do wildcard matching without worrying about unpacking binary data such
+ as message ID's first. To provide later downward compatability
+ all software should be written to IGNORE fields not currently defined.
+
+
+#The type bytes currently defined are:
+
+
+| BYTE | Enum | NW | Mnemonic | Enum / Comments
+|-------|-------------------|------|----------------|---------------------------------------------------------
+| A | eAuthor | from | Author | *eAuthor*
+| | | | | Name of originator of message.
+| B | eBig\_message | | Big message | *eBig\_message*
+| | | | | This is a flag which indicates that the message is
+| | | | | big, and Citadel is storing the body in a separate
+| | | | | record. You will never see this field because the
+| | | | | internal API handles it.
+| C | eRemoteRoom | | RemoteRoom | *eRemoteRoom*
+| | | | | when sent via Citadel Networking, this is the room
+| | | | | its going to be put on the remote site.
+| D | eDestination | | Destination | *eDestination*
+| | | | | Contains name of the system this message should
+| | | | | be sent to, for mail routing (private mail only).
+| E | eExclusiveID | exti | Exclusive ID | *eExclusiveID*
+| | | | | A persistent alphanumeric Message ID used for
+| | | | | network replication. When a message arrives that
+| | | | | contains an Exclusive ID, any existing messages which
+| | | | | contain the same Exclusive ID and are *older* than this
+| | | | | message should be deleted. If there exist any messages
+| | | | | with the same Exclusive ID that are *newer*, then this
+| | | | | message should be dropped.
+| F | erFc822Addr | rfca | rFc822 address | *erFc822Addr*
+| | | | | For Internet mail, this is the delivery address of the
+| | | | | message author.
+| H | eHumanNode | hnod | Human node name| *eHumanNode*
+| | | | | Human-readable name of system message originated on.
+| I | emessageId | msgn | Message ID | *emessageId*
+| | | | | An RFC822-compatible message ID for this message.
+| J | eJournal | jrnl | Journal | *eJournal*
+| | | | | The presence of this field indicates that the message
+| | | | | is disqualified from being journaled, perhaps because
+| | | | | it is itself a journalized message and we wish to
+| | | | | avoid double journaling.
+| K | eReplyTo | rep2 | Reply-To | *eReplyTo*
+| | | | | the Reply-To header for mailinglist outbound messages
+| L | eListID | list | List-ID | *eListID*
+| | | | | Mailing list identification, as per RFC 2919
+| M | eMesageText | text | Message Text | *eMesageText*
+| | | | | Normal ASCII, newlines seperated by CR's or LF's,
+| | | | | null terminated as always.
+| N | eNodeName | node | Nodename | *eNodeName*
+| | | | | Contains node name of system message originated on.
+| O | eOriginalRoom | room | Room | *eOriginalRoom* - Room of origin.
+| P | eMessagePath | path | Path | *eMessagePath*
+| | | | | Complete path of message, as in the UseNet news
+| | | | | standard. A user should be able to send Internet mail
+| | | | | to this path. (Note that your system name will not be
+| | | | | tacked onto this until you're sending the message to
+| | | | | someone else)
+| R | eRecipient | rcpt | Recipient | *eRecipient* - Only present in Mail messages.
+| S | eSpecialField | spec | Special field | *eSpecialField*
+| | | | | Only meaningful for messages being spooled over a
+| | | | | network. Usually means that the message isn't really
+| | | | | a message, but rather some other network function:
+| | | | | -> "S" followed by "FILE" (followed by a null, of
+| | | | | course) means that the message text is actually an
+| | | | | IGnet/Open file transfer. (OBSOLETE)
+| | | | | -> "S" followed by "CANCEL" means that this message
+| | | | | should be deleted from the local message base once
+| | | | | it has been replicated to all network systems.
+| T | eTimestamp | time | date/Time | *eTimestamp*
+| | | | | Unix timestamp containing the creation date/time of
+| | | | | the message.
+| U | eMsgSubject | subj | sUbject | *eMsgSubject* - Optional.
+| | | | | Developers may choose whether they wish to
+| | | | | generate or display subject fields.
+| V | eenVelopeTo | nvto | enVelope-to | *eenVelopeTo*
+| | | | | The recipient specified in incoming SMTP messages.
+| W | eWeferences | wefw | Wefewences | *eWeferences*
+| | | | | Previous message ID's for conversation threading. When
+| | | | | converting from RFC822 we use References: if present, or
+| | | | | In-Reply-To: otherwise.
+| | | | | (Who in extnotify spool messages which don't need to know
+| | | | | other message ids)
+| Y | eCarbonCopY | cccc | carbon copY | *eCarbonCopY*
+| | | | | Optional, and only in Mail messages.
+| 0 | eErrorMsg | | Error | *eErrorMsg*
+| | | | | This field is typically never found in a message on
+| | | | | disk or in transit. Message scanning modules are
+| | | | | expected to fill in this field when rejecting a message
+| | | | | with an explanation as to what happened (virus found,
+| | | | | message looks like spam, etc.)
+| 1 | eSuppressIdx | | suppress index | *eSuppressIdx*
+| | | | | The presence of this field indicates that the message is
+| | | | | disqualified from being added to the full text index.
+| 2 | eExtnotify | | extnotify | *eExtnotify* - Used internally by the serv_extnotify module.
+| 3 | eVltMsgNum | | msgnum | *eVltMsgNum*
+| | | | | Used internally to pass the local message number in the
+| | | | | database to after-save hooks. Discarded afterwards.
+
+EXAMPLE
+-------
+Let *<FF>* be a *0xFF* byte, and *<0>* be a null *(0x00)* byte. Then a message
+which prints as...
+
+ Apr 12, 1988 23:16 From Test User In Network Test> @lifesys (Life Central)
+ Have a nice day!
+
+might be stored as...
+
+ <FF><40><0>I12345<0>Pneighbor!lifesys!test_user<0>T576918988<0> (continued)
+ -----------|Mesg ID#|--Message Path---------------|--Date------
+
+ AThe Test User<0>ONetwork Test<0>Nlifesys<0>HLife Central<0>MHave a nice day!<0>
+ |-----Author-----|-Room name-----|-nodename-|Human Name-|--Message text-----
+
+Weird things can happen if fields are missing, especially if you use the
+networker. But basically, the date, author, room, and nodename may be in any
+order. But the leading fields and the message text must remain in the same
+place. The H field looks better when it is placed immediately after the N
+field.
+
+
+EUID (EXCLUSIVE MESSAGE ID'S)
+-----------------------------
+This is where the groupware magic happens. Any message in any room may have
+a field called the Exclusive message *ID*, or *EUID*. We keep an index in the
+table *CDB_EUIDINDEX* which knows the message number of any item that has an
+*EUID*. This allows us to do two things:
+
+ - If a subsequent message arrives with the same *EUID*, it automatically
+ *deletes* the existing one, because the new one is considered a replacement
+ for the existing one.
+ - If we know the *EUID* of the item we're looking for, we can fetch it by *EUID*
+ and get the most up-to-date version, even if it's been updated several times.
+
+This functionality is made more useful by server-side hooks. For example,
+when we save a vCard to an address book room, or an iCalendar item to a
+calendar room, our server modules detect this condition, and automatically set
+the *EUID* of the message to the *UUID* of the *vCard* or *iCalendar* item.
+Therefore when you save an updated version of an address book entry or
+a calendar item, the old one is automatically deleted.
+
+NETWORKING (REPLICATION)
+------------------------
+Citadel nodes network by sharing one or more rooms. Any Citadel node
+can choose to share messages with any other Citadel node, through the sending
+of spool files. The sending system takes all messages it hasn't sent yet, and
+spools them to the recieving system, which posts them in the rooms.
+
+The *EUID* discussion above is extremely relevant, because *EUID* is carried over
+the network as well, and the replacement rules are followed over the network
+as well. Therefore, when a message containing an *EUID* is saved in a networked
+room, it replaces any existing message with the same *EUID* *on every node in
+the network*.
+
+Complexities arise primarily from the possibility of densely connected
+networks: one does not wish to accumulate multiple copies of a given
+message, which can easily happen. Nor does one want to see old messages
+percolating indefinitely through the system.
+
+This problem is handled by keeping track of the path a message has taken over
+the network, like the UseNet news system does. When a system sends out a
+message, it adds its own name to the bang-path in the *<P>* field of the
+message. If no path field is present, it generates one.
+
+With the path present, all the networker has to do to assure that it doesn't
+send another system a message it's already received is check the <P>ath field
+for that system's name somewhere in the bang path. If it's present, the system
+has already seen the message, so we don't send it.
+
+We also keep a small database, called the "use table," containing the ID's of
+all messages we've seen recently. If the same message arrives a second or
+subsequent time, we will find its ID in the use table, indicating that we
+already have a copy of that message. It will therefore be discarded.
+
+The above discussion should make the function of the fields reasonably clear:
+
+ o Travelling messages need to carry original message-id, system of origin,
+ date of origin, author, and path with them, to keep reproduction and
+ cycling under control.
+
+(Uncoincidentally) the format used to transmit messages for networking
+purposes is precisely that used on disk, serialized. The current
+distribution includes serv_network.c, which is basically a database replicator;
+please see network.txt on its operation and functionality (if any).
+
+PORTABILITY ISSUES
+------------------
+Citadel is 64-bit clean, architecture-independent, and Year 2000
+compliant. The software should compile on any POSIX compliant system with
+a full pthreads implementation and TCP/IP support. In the future we may
+try to port it to non-POSIX systems as well.
+
+On the client side, it's also POSIX compliant. The client even seems to
+build ok on non-POSIX systems with porting libraries (such as Cygwin).
+
+SUPPORTING PRIVATE MAIL
+-----------------------
+Can one have an elegant kludge? This must come pretty close.
+
+Private mail is sent and recieved in the *Mail>* room, which otherwise
+behaves pretty much as any other room. To make this work, we have a
+separate Mail> room for each user behind the scenes. The actual room name
+in the database looks like *"0000001234.Mail"* (where *'1234'* is the user
+number) and it's flagged with the *QR_MAILBOX* flag. The user number is
+stripped off by the server before the name is presented to the client. This
+provides the ability to give each user a separate namespace for mailboxes
+and personal rooms.
+
+This requires a little fiddling to get things just right. For example,
+*make_message()* has to be kludged to ask for the name of the recipient
+of the message whenever a message is entered in *Mail>*. But basically
+it works pretty well, keeping the code and user interface simple and
+regular.
+
+PASSWORDS AND NAME VALIDATION
+-----------------------------
+This has changed a couple of times over the course of Citadel's history. At
+this point it's very simple, again due to the fact that record managers are
+used for everything. The user file (user) is indexed using the user's
+name, converted to all lower-case. Searching for a user, then, is easy. We
+just lowercase the name we're looking for and query the database. If no
+match is found, it is assumed that the user does not exist.
+
+This makes it difficult to forge messages from an existing user. (Fine
+point: nonprinting characters are converted to printing characters, and
+leading, trailing, and double blanks are deleted.)
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