# Re: Definitions and provenance and invariance

From: Stian Soiland-Reyes <soiland-reyes@cs.manchester.ac.uk>
Date: Fri, 24 Jun 2011 17:03:26 +0100
Message-ID: <BANLkTi=xigL=QRwQ8n2zceGYiATbQPOYJg@mail.gmail.com>

```On Fri, Jun 24, 2011 at 14:33, Paolo Missier <Paolo.Missier@ncl.ac.uk> wrote:

> I initially thought "invariant" was relative to Time, but it is not the
> case. so what is invariance relative to, here? all possible views of this
> particular rectangle A? but then we are back to "properties that are
> integral to identity", which I thought we had dropped

Invariant *is* relative to time, or immune to changes if you like.

See http://www.w3.org/2011/prov/wiki/FileExample :

i0: A file, for which we have a property name (/home/towns.txt) and a
property creator (Alice), which are invariant in the interval [t,t+4[
i1: A file (i0) with added property content which is empty; it exists
in the interval [t,t+1[
i2: A file (i0) with added property content with value London and
Edinburgh; it exists in the interval [t+1,t+3[
i3: A file (i0) with added property content with value London,
Edinburgh, NY, LA; it exists in the interval [t+3,t+4[

So invariant properties are properties which can not change over the
lifetime that for *that* IVPT. In the example here the content of i0
is changing over its lifetime, but its name and creator are immutable.
The name might or might not be integral to identity, but we are not
saying anything about that.  In i2 the content is immutable as well
(but perhaps not the "last accessed time" property).

When we want to say that Charles is emailing the file content, we want
to say "the file with this particular content". So we'll say that the
email i4 is derived from i2 - which is a view of i0 for the period
when the content was London/Edinburgh. It might not be important
exactly when that was, or what was before or after, as long as we are
able to distinguish i2 from i1 and i3. (In fact, in a distributed
setting, Charles might well attach the i2 content after David has
added NY and LA - perhaps his Dropbox had not synchronized yet)

>From this, if B is an invariant of A - then properties they share must
have corresponding values - so over that time period all the immutable
properties of B should correspond to (possibly mutable) properties in
A.

I guess it depends on the 'corresponding' if this means that those
properties are unchanged in A during B's life span - and (just thought