--- a/model/ProvenanceModel.html Wed Nov 23 08:21:37 2011 +0000
+++ b/model/ProvenanceModel.html Wed Nov 23 12:55:47 2011 +0000
@@ -23,6 +23,12 @@
berjon.biblio[k] = extraReferences[k];
};
var extraReferences = {
+ "CLOCK":
+ "Lamport, L. "+
+ "<a href=\"http://research.microsoft.com/users/lamport/pubs/time-clocks.pdf\"><cite>Time, clocks, and the ordering of events in a distributed system</cite></a>."+
+ "Communications of the ACM 21 (7): 558–565. 1978"+
+ "URL: <a href=\"http://research.microsoft.com/users/lamport/pubs/time-clocks.pdf\">http://research.microsoft.com/users/lamport/pubs/time-clocks.pdf</a>" +
+ "DOI: doi:10.1145/359545.359563.",
"Logic":
"W. E. Johnson"+
"<a href=\"http://www.ditext.com/johnson/intro-3.html\"><cite>Logic: Part III</cite></a>."+
@@ -334,30 +340,83 @@
<section id='section-time-event'>
<h4>Time and Event</h4>
-
-
-<p>In our conceptualization of the world, instantaneous events, or <dfn id="concept-event">events</dfn> for short, happen in the world, which mark changes in the world, in its activities, and in its entities. This specification assumes that a partial order exists between events. How practically such order is realized is beyond the scope of this specification. Possible implementations of that ordering include a single global notion of time and Lamport's style clocks.</p>
-
+<p>Time is critical in the context of provenance, since it can help corroborate provenance claims. If an entity is claimed to be obtained by transforming another, then the latter must have existed before the former. If it is not the case, then there is something wrong in such a provenance claim. </p>
+
+<p> Time is critical, but we should also recognize that provenance can be used in many different contexts: in a single system, across the Web, or in spatial applications, to name a few. Hence, it is a design objective of PROV-DM to minimize the assumptions about time, so that PROV-DM can be used in varied contexts. </p>
+
+<p>Furthermore, consider two activities that started at the same time
+instant. Just by referring to that instant, we cannot distinguish
+which activity start we refer to. This is particularly relevant if we
+try to explain that the start of these activities had different
+reasons. We need to be able to refer to the start of an activity as a
+first class concept, so that we can talk about it and about its
+relation with respect to other similar starts. </p>
+
+
+<p>Hence, in our conceptualization of the world, an instantaneous event, or <dfn id="dfn-event">event</dfn> for short, happens in the world and marks a change in the world, in its activities and in its entities.
+The term "event" is commonly used in process algebra with a similar meaning. For instance, in CSP, events represent communications or interactions; they are assumed to be indivisible and instantaneous. [Better citation than Wikipedia to appear here].</p>
<section>
<h4>Temporal Events</h4>
-Four kinds of discrete events underpin the PROV-DM data model. They are:
-<ol>
-<li>Generation of an entity by an activity: identifies the final instant of an entity's creation timespan, after which it becomes available for use.</li>
-<li>Usage of an entity by an activity: identifies the first instant of an entity's consumption timespan.</li>
-<li>Start of an activity: identifies the instant an activity, represented by an activity, starts;</li>
-<li>End of an activity: identifies the instant an activity, represented by an activity, ends.</li>
-</ol>
+Four kinds of events underpin the PROV-DM data model. They are defined as follows.
+
+<p>An <dfn id="dfn-usage-event">entity generation event</dfn> is the <a>event</a> that marks the final instant of an entity's creation timespan, after which it becomes available for use.</p>
+
+<p>An <dfn id="dfn-usage-event">entity usage event</dfn> is the <a>event</a> that marks the first instant of an entity's consumption timespan by an activity.</p>
+
+<p>An <dfn id="dfn-start-event">activity start event</dfn> is the <a>event</a> that marks the instant an activity starts.</p>
+
+<p>An <dfn id="dfn-end-event">activity end event</dfn> is the <a>event</a> that marks the instant an activity ends.</p>
+
</section>
<section>
<h4>Event Ordering</h4>
-
-<p><dfn id="dfn-follows">Follows</dfn> is a partial order between events, indicating that an event occurs after another. For convenience, <dfn id="dfn-precedes">precedes</dfn> is defined as the symmetric of follows. </p>
-
-<p>This specification introduces inference rules allowing such event ordering to be inferred from provenance constructs.</p>
+<p>To allow for minimalistic clock assumptions, like Lamport
+[[CLOCK]], PROV-DM relies on a notion of relative ordering of <a>event</a>s,
+without using physical clocks. This specification assumes that a partial order exists between <a>event</a>s.
+</p>
+
+
+<p>Specifically, <dfn id="dfn-follows">follows</dfn> is a partial
+order between <a>event</a>s, indicating that an <a>event</a> occurs after another.
+For convenience, <dfn id="dfn-precedes">precedes</dfn> is defined as
+the symmetric of follows. </p>
+
+
+<p> How such partial order is realized in practice is beyond the scope
+of this specification. This specification only assumes that
+each <a>event</a> can be mapped to an instant in some form of
+timeline. The actual mapping is not in scope of this
+specification. Likewise, whether this timeline is formed of a single
+global timeline or whether it consists of multiple Lamport's style
+clocks is also beyond this specification. It is anticipated
+that <a>follows</a> and <a>precedes</a> correspond to some ordering
+over this timeline.
+</p>
+
+
+<p>This specification introduces a set of "temporal interpretation"
+rules allowing to derive <a>event</a> ordering constraints from
+provenance records. According to such temporal interpretation,
+provenance records MUST satisfy such constraints. We note that the
+actual verification of such temporal constraints is also outside the
+scope of this specification. </p>
+
+<p>PROV-DM also allows for time observations to be inserted in
+provenance records, for each of the recognized <a>event</a> introduced
+in this specification. The presence of a time observation for a
+given <a>event</a> fixes the mapping of this <a>event</a> to the
+timeline. It can also help with the verification of associated
+temporal constraints (though, again, this verification is outside the
+scope of this specfication).
+
+</p>
+
+
+
</section>
</section>
@@ -886,10 +945,10 @@
<p>identified by identifier <span class="name">id</span>, states the existence of an activity with recipe link <span class="name">add-crime-in-london</span>, start time <span class="name">2011-11-16T16:05:00</span>, and end time <span class="name">2011-11-16T16:06:00</span>, running on host <span class="name">server.example.org</span>, and of type <span class="name">edit</span> (declared in some namespace with prefix <span class="name">app</span>). The attribute <span class="name">host</span> is application specific, but MUST hold for the duration of activity. The attribute <span class="name">type</span> is a reserved attribute of PROV-DM, allowing for subtyping to be expressed.</p>
</div>
-<p>The mere existence of an activity assertion entails some event ordering in the world, since the start event <a>precedes</a> the end event. This is expressed by constraint <a href="#start-precedes-end">start-precedes-end</a>.</p>
+<p>The mere existence of an activity assertion entails some event ordering in the world, since the <a>activity start event</a> <a>precedes</a> the <a>activity end event</a>. This is expressed by constraint <a href="#start-precedes-end">start-precedes-end</a>.</p>
<div class='interpretation' id='start-precedes-end'> From an activity record, one can infer that the
-start event <a>precedes</a> the end event of the represented activity.</div>
+<a title="activity start event">start event</a> <a>precedes</a> the <a title="activity end event">end event</a> of the represented activity.</div>
<p>An activity record is not an entity record.
Indeed, an entity record represents an entity that exists in full at