What placement strategies are employed in implementing the associated services?
Answer:
We first consider the mapping to multiple machines (covering partitioning and replication strategies):
Web: Web page masters are held in a file system at a single server. The information on the web as a whole is therefore partitioned amongst many web servers. Replication is not a part of the web protocols, but a heavily-used web site may provide several servers with identical copies of the relevant file system using one of the well-known means for replicating slowlychanging data (Chapter 15). HTTP requests can be multiplexed amongst the identical servers using the (fairly basic) DNS load sharing mechanism described on page 169. In addition, web proxy servers support replication through the use of cached replicas of recently-used pages and browsers support replication by maintaining a local cache of recently accessed pages.
Mail: Messages are stored only at their destinations. That is, the mail service is based mainly on partitioning, although a message to multiple recipients is replicated at several destinations.
Netnews: Each group is replicated only at sites requiring it.
In terms of caching (and proxies), web servers cooperate with Proxy servers to minimize network traffic and latency. Responsibility for consistency is taken by the proxy servers - they check the modification dates of pages frequently with the originating web server.
The web also features significant use of mobile code, for example through applets where code is downloaded and run on the browser machine.
