What newsgroups are and how they work

In a nutshell, newsgroups are a means of "public discussion". Newsgroup articles (messages) look like e-mail, but they can be read (potentially) by millions of people all over the world.

This note describes "how newsgroups work" in the sense of what happens to articles after they are posted. To find out "how newsgroups work" in the sense of how to use your news-reading software, look for documentation for your software, or post a question to an appropriate newsgroup. If you're using Netscape or some other Web browser, try (
or comp.infosystems.www.browsers.mac ( news:comp.infosystems.www.browsers.mac).

Otherwise try (
or news.newusers.questions ( news:news.newusers.questions).

: Can someone give a brief, not-too-technical description of where
: a message goes when I post it into a newsgroup?
Newsgroup articles are distributed via "news servers", which contain databases of articles, and are operated by Internet service providers, schools, universities, and companies.

: Do all the messages travel to a central site and then all the sites
: that want it, pick it up there?
No, there is no "central" server on Usenet. A newsgroup article propagates from one server to another, starting from the server where it is first posted.

"Moderated" newsgroups are sort of an exception, in that all articles are first forwarded via e-mail to a "moderator" for approval. The moderator posts them on his/her news server, and from there they propagate as described below. But different moderators use different news servers.

: Do all the messages travel to all the sites?  
Ideally, all the article in a newsgroup travel to all sites (news servers) that carry the newsgroup. This means that when you post an article, the final result is tens of thousands of copies, all over the world.

More specifically, when you post an article, it goes first to your "local" news server (operated by your Internet service provider, company, or school). Your server then sends copies of the article to its "neighbors," that is, to servers with which it has agreed to exchange articles. Those servers in turn send copies to *their* neighbors. Eventually every server that carries the newsgroup has a copy.

: In what order?  
Most servers normally send articles to other servers more or less in the order of arrival. This sequence can get scrambled for various reasons, which is why you often see responses before the "original" article arrives.

: How do they know which sites want them?  
News server administrators make arrangements among themselves about which newsgroups they exchange. The "receiving" server's admin tells the "sending" server's admin which newsgroups he/she wants to receive. The "sending" server's admin configures his/her server to send only those newsgroups.

: How do they know which sites they've already visited?  How does a message 
: avoid the same site twice?
There are two methods. Servers usually use both of them, in sequence:
1. The "Path:" header line shows the sites that the article has traveled through, so far, between the originating server and the current server. If the "receiving" server appears in the "Path:" line, the "sending" server does not try to send the article because it knows that the "receiving" server has already received a copy.

2. The "Message-ID:" header line contains an identifying code which is different for every article. Before transmitting the article, the "sending" server asks the "receiving" server, in effect, "Do you have an article with Message-ID such-and-such?" The "receiving" server responds either "No, please send me a copy," or "Yes, I have it already," whereupon the "sending" server either sends the article or goes on to the next one.

: How long do messages stay posted?
Each news server "expires" (removes) old articles, usually once a day, to make space for new ones. Most servers do this based on the number of days an article has been on that server. The "expiration time" varies from one server to another, and can also vary from one newsgroup to another, on the same server. It might be a few days, or it might be two weeks or more. So, even after an article expires from your own server, it is probably still visible on many other servers.

Note, however, that even though you cannot see an article any more, it may still be present on your server. Most news-reading software keeps track of which articles you've read, and shows them to you only once. This way, you don't have to wade through the same articles over and over and over again. There should be a command or button or something which "shows all" or "shows previously-read" or "unmarks" articles, so that you can see all the articles that have not yet expired from your server.

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