Pure message-based systems provide a mechanism to append messages to users' private mailboxes. Pure artifact-based systems allow multiple users to share an artifact. News combines features of these two kinds systems: As in the former, users send requests to append messages to a mail box, but the mailbox, as in the latter, is a shared object accessible to multiple users. In fact, news bulletin-boards have to be far more scalable than other shard artifacts we have seen so far, both because of the number of user sharing them (including potentially all of internet users) and the number of news messages generated (around 100MB per day).
How do we build such scalable objects? Fortunately, the problem is not as difficult as in file/database for two reasons. First,news messages are immutable, Second, and perhaps more importantly, there is no need to ensure that at any time users see the same state of the shared artifact, it is enough to ensure that a news item be eventually accessible to all users. Put another way, when users starts a news session, they are not expecting to receive all items posted before the session was started. They are happy if these news items are received in later sessions. critical dependencies among their actions,
News uses a simple approach forming the basis of Lotus Notes, which we will discuss later. Each user accesses then news system using a news client such as the NN and Gnus clients. The news client, in turn, contacts a news server, which keeps a repository of all news items. The news client downloads these items for its user from the server and sends all news items posted by the user to the server. There are several replicated news servers in the internet which are connected either directly to each other or indirectly through other news servers. They keep their repositories consistent by propagating the postings they receive to other news servers to which they are directly connected. This approach allows a news server to receive duplicate postings along different paths, but the duplicates can be easily detected since each news posting has a unique id.
A news server may delete items to save disk space, which interferes with the guarantee that a posted news item should be received by all users.