I took the action item on the Higgins call
today to parse the key questions being raised about EntityId on this thread
into a decision analysis page on the Higgins wiki. I have posted this page at:
http://wiki.eclipse.org/EntityId_Requirements
…and placed a link to it on http://wiki.eclipse.org/Context_Data_Model_1.1_Open_Issues#Open_Issues.
The ideal way to proceed is for folks to
post opinions to the page and then ping the list with a pointer. However, for
those who prefer responding in email, following is the wikitext version of the
page to which you can respond directly.
=Drummond
== About ==
This page is for discussing/documenting
the terminology, requirements, and design decisions for [[EntityId]]s.
== Content Data Model Requirements ==
In terms of the underlying graph model,
following is a summary of the abstract requirements derived in a recent
(2008-09-11) thread on the email list. The first step is determining if there
is consensus about these requirements. '''Please post a note with your wiki
signature if you disagree with any of the following:'''
# An [[Entity]] is a node in the graph
described by the Higgins [[Context Data Model]]. The CDM needs a consistent way
of representing arcs referencing that node.
# There MAY be 0..n such arcs referencing
the node. (0 is possible for blank nodes.)
# An arc MAY theoretically be represented
as either:
## A unique identifier.
## A set of [[Attribute]]s of that
[[Entity]], none of which itself is required to be a unique identifier.
# If the arc is represented as a unique
identifier:
## It MUST be locally unique within the
[[Context]], and it MAY be globally unique across all [[Context]]s).
== Higgins API Requirements ==
The second step, based on the above requirements,
is answering the following questions with respect to the Higgins API. '''Please
post your votes/answers (with your wiki signature).'''
=== #1: Unique Identifier vs. Attribute
Set ===
Should the Higgins API constrain an
[[EntityId]] to be a unique identifier, or can it be a set of [[Attribute]]s?
=== #2: Representation of an EntityId as a
Unique Identifier ===
If an [[EntityId]] is a unique identifier,
should this be represented as:
# A type of [[Attribute]]?
# A separate property of an [[Entity]]
that MAY be exposed as an [[Attribute]]?
# Inherent in the definition of an
[[Entity]]?
=== #3: Cardinality ===
What is the cardinality of [[EntityId]]?
(The answer may depend on the answer to #2.)
# 0..n?
# 0..1?
# 1 (whose value may be null)?
From:
higgins-dev-bounces@xxxxxxxxxxx [mailto:higgins-dev-bounces@xxxxxxxxxxx] On Behalf Of Anthony Nadalin
Sent: Thursday, September 11, 2008
7:36 AM
To: Higgins (Trust Framework)
Project developer discussions
Cc: higgins-dev;
higgins-dev-bounces@xxxxxxxxxxx
Subject: Re: [higgins-dev]
entityID not an attribute?
So there are a couple of things here, we have always
talked about the EntityID as being a reference to the Entity and not the unique
identifier. There are many ways to reference an Entity, so I don't believe that
this is limited to 0..1. I also believe that the EntityID encapsulates a given
set of attributes.The unique identifier is only has to be unique within a
context. So I believe that the unique identifier is an attribute, not a way to
reference the Entity.
Anthony Nadalin | Work 512.838.0085 | Cell 512.289.4122
Paul Trevithick ---09/10/2008
05:51:39 PM---Raj has suggested the need to clarify the language here. So here
is a restatement. Additions in red. Substitutions in blue. All
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Raj
has suggested the need to clarify the language here. So here is a restatement.
Additions in red.
Substitutions in blue.
All defined terms in initial caps.
Background: We remain committed to these two principles:
- An Entity has 0..1 unique identifier (called an EntityId) (...and we expect
almost all Entities will have an EntityId).
- [Raj: you asked about why this
EntityId is optional. The answers are (1) that our
“complex” Attributes have values that are themselves
Entities and we didn’t want to require developers to explicitly
“name” these values (especially in situations where there
was no need for N>1 Entities to share (link to the same)
value/Entity and (2) we need this in order that our model remain a pure
super-set of RDF/OWL (and thus allows IdAS to losslessly
“adapt” the Semantic Web (including all Linked Data).]
- An Entity has 0..N
Attributes some of which may be used
singly or in combination to identify an Entity or a set of Entities
within a Context.
- [Raj: To date
we have decided not to define an explicit “Identifier”
Attribute type. The reason for not defining it is twofold: First, the distinction between an
Identifier and an Attribute has so far proved impossible to agree on. Second, Context Provider developers
are free to create their own Attribute Definitions and thus a developer
could define their own “Identifier” sub-Attribute]
The proposal remains:
- To no longer consider
the one, optional EntityId as an Attribute.
- To have an IdAS
getEntityId() method to return this EntityId (or return null if it
doesn’t exist) whereas other getAttribute methods return
Attributes/values
- NOTE: CP developers
remain free to present the EntityId value as the value of some Attribute
type that they define and use within their Context
With the above clarified and annotated definitions, I’m interested to
hear Tony’s, Raj’s and anyone else’s reactions.
-Paul
On 9/9/08 1:20 PM, "Nataraj Nagaratnam" <natarajn@xxxxxxxxxx> wrote:
Yea, there seems to be
disconnect here with usage of the term 'identifier' (or Id).
The statement "An
Entity has 0..N Attributes some of which may be used as identifiers" tells
me that there is more than one identifier, and then the statement "An
Entity has 0..1 EntityId" says that there is one identifier (as i think
"EntityId" means "Entity Identifier"). This seems to be
contradicting statements in some sense, and maybe the cause of disconnect here.
So how about this..
· An Entity has 0..N
Attributes
· An Entity has 1
UniqueIdentifier within a given context.
Then it makes the calculation of uniqueIdentifier to be relevant to the Entity
within a given context; this way, we leave attributes as they are - if we end
up using those attributes to identify/search/lookup an entity, then fine but
uniqueness is not guaranteed. Wrt those attributes that are used to
search/lookup,.. maybe we don't need to designate those attributes to be
identifiers in a formal manner in the data model?
So proposal can be
To have an IdAS getUniqueEntityId() method to return a unique identifier within
the context of that entity, whereas other getAttribute methods return
Attributes/values
another comment - do we really want entities without unique identifiers at all?
Regards,
Raj
Anthony
Nadalin---09/09/2008 12:40:37 PM---OK, So not sure I agree
From:
Anthony Nadalin/Austin/IBM@IBMUS
To:
"Higgins \(Trust Framework\) Project developer discussions" <higgins-dev@xxxxxxxxxxx>
Cc:
higgins-dev <higgins-dev@xxxxxxxxxxx>, higgins-dev-bounces@xxxxxxxxxxx
Date:
09/09/2008 12:40 PM
Subject:
Re: [higgins-dev] entityID not an attribute?
OK, So not sure I agree
I believe that there are 0..N EntityIDs and the EnitityID job is to encapsulate
the referenced attributes, thus there may be multiple EntityIDs.
Anthony Nadalin | Work 512.838.0085 | Cell 512.289.4122
Paul
Trevithick ---09/09/2008 10:56:26 AM---Just to make sure we’re all
discussing the right proposal. Let me back up a bit here and restate it:
From:
Paul Trevithick <paul@xxxxxxxxxxxxxxxxx>
To:
higgins-dev <higgins-dev@xxxxxxxxxxx>
Date:
09/09/2008 10:56 AM
Subject:
Re: [higgins-dev] entityID not an attribute?
Just to make sure we’re all discussing the right proposal. Let me back up
a bit here and restate it:
Background: We remain committed to these two principles:
· An Entity has 0..1
EntityId (...and we expect almost all Entities will have an EntityId)
· An Entity has 0..N
Attributes some of which may be used as identifiers (that is, these attributes
may singly or in combination uniquely identify an Entity within its Context)
The proposal is:
· To no longer
consider the one, optional EntityId as an Attribute.
· To have an IdAS
getEntityId() method to return this EntityId (or return null if it
doesn’t exist) whereas other getAttribute methods return Attributes/values
· NOTE: CP
developers remain free to present the EntityId value as the value of some
Attribute type that they define and use within their Context
-Paul
