Papers in PDF format

4.6.1 Access times
To reduce access times there are two mechanisms currently used: mirroring and caching.
Mirroring is quite common with FTP archives. To make optimal use of the mirrors, ARCHIE
allows to locate the closest copy of a file. However, the Web cannot directly be compared to
FTP because: a) files tend to be much smaller and b) it is not common to use a search machine
to locate just one file and most Web users don't even know or obey netiquette. Thus, for best
usage the lookup of the closest file must be somehow automated, e.g. by resolving names depending
on the location of the user.
Advantages and disadvantages of replication (mirroring) are as follows:
+ Mirroring guarantees fast access, even in bad connected countries
+ The update frequency can (and should) be selected as required
- Where is the master source?
- Is the information up to date?
- Cache servers may be filled with copies of the same information
Caching is very helpful if some information is required by many people using one common
proxy/cache server. Thus caching works best if one server is used by a group with common
interests.
Advantages and disadvantages of caching are as follows:
+ Transparent to user
+ Fine for teams and frequently requested information
+ Easy to implement
- Delays every request
- Usually used for diverse groups
- Average hit rates of only 30%
- Slow/no access to information not in cache
4.6.2 Consistent view
Another problem with distributed data is to offer a consistent view to the user. Distributed
databases provide good mechanisms, but can be only used if very close cooperation exists between
the involved partners. Usually this will not be the case for organizations which would
like to combine their data for access through the Web.
Fortunately, there are mechanisms to allow common search interfaces without a distributed
database, think of Lycos which is, in the widest sense, a common query interface for (nearly)
all documents on the Web. However, these search engines are not a) focused on user interests,
and b) capable of indexing databases because there is the problem of an infinite query space.
The best way to deal with this problem is to generate a dynamic or static HTML-tree from
your database. This is especially helpful if your data is structured in a simple way, for example,
if you have stored contact information for your employees you may generate a company phone
book structured by departments. This will also allow full text indexing, e.g. with WAIS. An
update can be easily done, just generate the structure again and delete the old one.
-23-