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InterNetNews...[Salz92]: Innd structure

When innd starts up it reads the active file into memory. An array of NEWSGROUP structures is created, one for each newsgroup, that contains the following elements:
 char *Name; /* "comp.sources.unix */ 
 char *Dir; /* "comp/sources/u nix/" */ 
 long Last; /* 0211 */ 
 int LastWidth; /* 5 */ 
 char *LastString; /* "00211..." */ 
 char *Rest; /* "m\n..." */ 
 int SiteCount; /* 1 */ 
 SITE **Sites; /* defined below */ 

The C comments above show the data that would generated for the following line in the active file:
comp.sources.unix 00211 00202 m
The Last field specifies the name to be given to the next article in the group. The LastString element points into the in -memory copy of the file. This number is carefully formatted so that the file can be memory -mapped, or updated with a single write. A hash table into the structure array is built, using a function provided by Chris Torek [Torek91]. The hash calculation is very simple, yet empirically it gives near -uniform distribution. The secondary key is the highest article number, so groups with the most traffic tend to be at the top of the bucket. The INN equivalent of the sys file read next. An array of SITE structures is created, one for each site, that contains the following elements:
 BOOL Sendit; 
 char FileFlags[10]; 

The FileFlags array specifies what information should be written to the site's data stream when it receives an article. The subscription list for the site is then parsed, and for all the newsgroup that it recives, the matching NEWSGROUP structure will contain a pointer to the SITE structure. Using these two structures it is easy to step through how an article is propagated:

 extern ARTDATA *art; 
 extern SITE *Sites, *LastSite; 
 extern int nSites; 
 char **pp; 
 SITE *sp; 
 int i; 
 while (*pp) { 
   ng = HashNewsgroup(* pp++); 
   if (ng == NULL) 
   AssignName(ng) ; 
   for (i = 0; i < ng ->nSites; i++) { 
     if (MeetsSiteCrite ra(ng ->Sites[i], art)) 
       ng ->Sites[i] ->Sendit = TRUE; 
 for (sp = Sites; sp < LastSite; sp++) { 
   if (!sp ->Sendit) 
   for (p = sp ->FileFlags; *p; p++) 
     switch (*p) { 
     case 'm': 
       /* Write Message -ID */ 
     case 'n': 
       /* Write filename */
The ARTDATA structure contains information about the current article such as its size, the host that sent it, and so on. The MeetsSiteCriteria function is an abstraction for the in -line tests that are done to see if an article really should be propagated to a site (e.g., checking the Path header as described above). AssignName is described below. At its core, innd is an I/O scheduler that makes callbacks when select (2) has determined that there is activity on a descriptor. This is encapsulated in the CHANNEL structure, which has the following elements:
 enum TYPE Type; 
 enum STATE State; 
 int fd; 
 FUNCPTR Reader; 
 FUNCPTR WriteDone; 
The Type field is used for internal consistency checks. There four different types of channels local -accept, remote -accept, local -control (used by ctlinnd ) and NNTP connection. Each type is implemented in anywhere from 100 to 1200 lines of code. The Reader and WriteDone function pointers, and the State enumeration are used for protocol -specific data. For example, State field is used by the NNTP channel code to determine whether the site is sending an NNTP command or an article. The BUFFER datatype contains sized reusable I/O buffers that grow as needed.

At start time innd calls getdtablesize (2) to create an array of channels that can be directly indexed by descriptor. The code to listen on the NNTP port is show in Figure 3. When a host connects to the NNTP port, select (2) will report activity on the descriptor and call RemoteReader which will accept the connection and possibly create fill in a new CHANNEL out of the resultant descriptor.

It took a bit of effort to write the callback loop so that it was fair i.e., so that the lowest descriptors did not get priority treatment. The problem was complicated because other parts of the server can add and remove themselves from the select (2) read and write mask, as needed.

Once the NNTP channel has been created for a site, the server is ready to accept articles from that site. The reader function for NNTP channels reads as much data as is available from the descriptor. If it is in ``get a command'' state, it looks for a simple \r\n terminator; if it is in ``reading an article'' state, it looks for a ``\r\n.\r\n'' terminator. If not found, it just returns; the data will become available at some point. If the terminator is found, it processes the data in the buffer. For filing an article, this means cleaning up the NNTP text escapes, and calling the article abstraction to process the article.

Processing the article is the largest single routine in the server. The AssignName shown above increments the high -water mark for the newsgroup. If the article has already been written to disk, a link is made under the new name. (Symbolic links, if available, can be used if the spool area spans partitions.) If the article has not been written, a struct iovec array is set up as shown below, where vertical bars separate each iovec element:
 First headers... 
 Path: |LOCAL_PATH_PREFIX|rest of path... 
 Second headers... 
 |Article body. 
This is a very fast way of writing the article to disk; it avoids extra memory copies, and is only possible because the entire article is kept in memory until the last moment.

 RemoteReader(cp ) 
     CHANNEL *cp; 
     int newfd; 
     struct sockadr_in remote; 
     int remsize; 
     newfd = accept(cp ->fd, &remote, &remsize); 
     if (InAccessFile(r emsocket)) 
         CHANcreate(new fd, TYPEnntp, STATEgetcmd, NCreader, NCwritedone); 
     else { 
        ForkAndExec("n nrpd", newfd); 

     int fd; 
     fd = GetSocketBoundToNNTPPort(); 
     CHANcreate(fd, TYPEremote, STATElisten, RemoteReader, NULL); 
Figure 3 : Listening on an NNTP port

InterNetNews(Salz) [Source:"InterNetNews: Usenet transport for Internet sites"] [Copyright: 1992 Rich Salz]
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