Information Technology Reference
In-Depth Information
systems. The discussions earlier in the topic about digital preservation all apply to
PID name resolvers.
However a number of additional factors come into play more immediately,
namely that things which are pointed to do move. One can imagine a number of
general scenarios based on the movement of digital objects - which may be either
something in a name resolver or something in a “normal” repository. As will be
argued below, it is important to distinguish between:
•
whether the whole collection of information moves and the repository (which
may be a name server) ceases to exist, or alternatively only part of those holdings
move and the repository continues to exist.
•
whether or not the repository knows who is pointing to it - this is particularly
important for intermediate name resolvers. The basic function of such a name
resolver is to point forwards to the next in the chain; backward pointers, i.e.
knowing who is pointing to you, are not so common.
With these in mind we can imagine various scenarios:
1. A particular piece of information (or collection of information) moves but the
repository/name resolver continues to exist.
a.
If the repository has “backward pointers” then special arrangements could
be made with its predecessor in the look-up chain - for example “instead of
pointing to me, look over there when you get certain lookup names”
b.
If there are no backward pointers then the repository itself can act as a name
resolver for that piece of information and when that piece of information is
sought it redirects to the new location.
2. A repository/name resolver ceases to exist and its entire holding moves to
another repository/name resolver.
a.
If the repository has “backward pointers” then the repository should inform
the ones pointing to it and let them know the new location
b.
If there are no backward pointers then the repository must hand over its
location information, for example its DNS entry, to its chosen successor.
Following these one can ensure that the PID name resolution continues to work
despite these kinds of changes.
10.3.2.2 Alternative: Application of DNS Concepts
The DNS is very familiar to users of the internet and allows users to connect to
billions of internet nodes. An important concept it employs is that of “Time-To-
Live” (TTL) which is a hint to the name resolver about how long the lookup entry is
going to be valid for. Beyond this time the name resolver could, for example, seek
to verify whether or not the lookup entry remains valid. If an internet node ceases
to exist then, without any further action, after the TTL time, the DNS will cease to
point to the old address.
