------------------------------------------------------ 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 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 Mnemonic Enum / Comments A eAuthor 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 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 rFc822 address erFc822Addr For Internet mail, this is the delivery address of the message author. H eHumanNode Human node name eHumanNode Human-readable name of system message originated on. I emessageId Message ID emessageId An RFC822-compatible message ID for this message. J eJournal 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 Reply-To eReplyTo the Reply-To header for mailinglist outbound messages L eListID List-ID eListID Mailing list identification, as per RFC 2919 M eMesageText Message Text eMesageText Normal ASCII, newlines seperated by CR's or LF's, null terminated as always. N eNodeName Nodename eNodeName Contains node name of system message originated on. O eOriginalRoom Room eOriginalRoom - Room of origin. P eMessagePath 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 Recipient eRecipient - Only present in Mail messages. S eSpecialField 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 date/Time eTimestamp Unix timestamp containing the creation date/time of the message. U eMsgSubject sUbject eMsgSubject - Optional. Developers may choose whether they wish to generate or display subject fields. V eenVelopeTo enVelope-to eenVelopeTo The recipient specified in incoming SMTP messages. W eWeferences 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 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 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... <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: 1. 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. 2. 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

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

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.)