Difference between revisions of "EntityId Requirements"

Revision as of 08:34, 10 April 2009

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About

This page is for working out the requirements and design decisions for any changes to Higgins EntityIds in the migration from the Context Data Model 1.0 to the Context Data Model 1.1.

Current EntityId Definition in Context Data Model 1.0

1. Is of type [need info here].
2. Has cardinality 0..1
3. MUST be Context-unique; MAY be globally unique.
4. Is always exposed as an Attribute.
5. Exposes no information about mutability.

EntityId 1.1

An Entity is a node in the graph described by the Higgins Context Data Model.

1. An Entity is identified by 0..n EntityIds (vs. 0..1 in Higgins 1.0)
2. An EntityId MUST be locally unique within the Context.
3. An EntityId MAY be globally unique (GUID)
4. An EntityId MAY be exposed as an Attribute.
5. An Entity MAY have a single cannonical EntityId that MUST be immutable
6. At least one EntityId of an Entity SHOULD be immutable, i.e., serve as a persistent reference to the Entity within that Context (forever). However because Higgins does not control Contexts or Context policies, the CDM must be prepared that an identifier for an Entity MAY be mutable, i.e., may be reassigned in that Context to reference a different Entity.

IdAS API Requirements

Following are the key design decisions we need to make. We are posting votes as they are made in email. Feel free post your votes/comments directly (with your wiki signature).

Q1: Unique Identifier vs. Attribute Set

Must a Higgins EntityId be a single-part CUID or GUID, or could it be a multi-part key consisting of a set of Attributes?

• Jim: Yes - it must be a CUID or GUID.
• David: No - I prefer a multi-part key where the parts of the key might also be unique in a context. An example is a EntityID made up of a uniqueName, uniqueId, nativeName, nativeId. Any part of the of the Entity ID could be used to identify the object.
• Drummond: Abstain - Single-part IDs are easier, but multi-part keys are useful too.
• Tony: No.
• Tom: Yes - it must be a CUID or GUID.
• Paul: Yes - it must be a CUID or a GUID. With the synonyms proposal (see below) we can give David the multi-part keys he needs (each key-part is a synonym)

Q2: Representation of an EntityId as a Unique Identifier

If an EntityId is a unique identifier, should this be represented as:

1. A type of Attribute?
2. An inherent property of an Entity that MAY be exposed as an Attribute?

• Jim: #2
• David: #2
• Drummond: #2
• Tony: #2
• Tom: #2
• Paul: roughly #2

Q3: Cardinality

What is the cardinality of EntityId? (The answer may depend on the answer to #2.)

1. 0..n?
2. 0..1?
3. 1 (whose value may be null)?
4. None of the above?

• Jim: Abstain - I tend to want simple.
• David: #1 or #2 - 0..1 if the EntityId is mutlipart as in Q1. 0..n if it is a string, and then it needs a type.
• Drummond: #2 or #3 - For comparison's sake, you need to always get the same identifier value. But there should also be a way to get all synonyms.
• Tony: #1
• Tom: Abstain - +1 to Jim's feedback.
• Paul: #2 unless I see a real world use case that requires #1. Presuming such a use-case exists, I can't see any alternative to having 0..1 "canonical" EntityId AND 0..n synonyms. There must be a way to link the synonyms together--the (preferably immutable) canonical EntityId is the way to do this. I don't see how you can have a data model 0..n ids that are all perfectly equal. The most natural thing in the CDM model would be to have 0..1 EntityId and then define an Attribute type in CDM called "synonym" and have all these "other" ids be higgins:synonyms or Context-defined sub-attributes of this

Q4: Mutability

Is the EntityID of an Entity immutable?

1. Yes?
2. No?
3. Depends?

• Jim: Yes - I believe it must be as soon as we start tying policy to EntityIDs. Either that, or we need to require a way to ensure referential integrity for places where EntityIDs are stored in policy statements.
• David: Depends - My vote on Q1 was multipart where the decomposition could contain both mutable (uniqueName) and immutable (uniqueId) parts. They both have their use cases. If the EntityID is a string, then 1..n is needed to accomodate mutable, immutable types and if the id can be used in other protocols (compatability with legacy systems).
• Drummond: Depends - Both immutable and mutable SHOULD be possible. Best practice is to assign 1 immutable in any context and then allow 0..n synonyms (mutable or immutable). But Higgins does not control contexts so it seems like it must be open to either. However there should be a way to ask for an immutable identifier, or ask if an given identifier is mutable.
• Tony: No position yet.
• Tom: Yes - +1 to Jim's position.
• Paul: In the case where we have 0..1 EntityId I'd say #1 (yes). If we have 0..1 EntityId plus 0..n synonyms then I'd say only the entityID must be immutable, the 0..n synonyms may be mutable

Proposed Changes in Context Data Model 1.1

WARNING: we had a conversation about this section and the next two on the Higgins call 9/18 and these sections need to be revised/updated based on that conversation

#1: Not Require EntityId to be Exposed as an Attribute

The proposed change is to make it OPTIONAL to expose EntityId as some kind of Attribute. Contexts that do not want to expose the EntityId can omit it from the list of Attributes for an Entity. Note: if the EntityId is mutable, it SHOULD be exposed as an Attribute so it can be modified.

#2: Add a higgins:synonym Attribute to higgins.owl

For those Context Provider developers who wish to explicitly tag certain Attributes as being capable of being used as an alternative identifier for this Entity (i.e. it uniquely at LEAST within the containing Context identifies this Entity).

For example, if the developer wished to declare a "mobile" telephone number attribute as being a synonym to whatever kind of identifier getEntityId() returns, they would, in their Attribute Definition define their new mobile attribute as a sub-property of higgins:synonym. For example:

:mobile
     a       owl:DatatypeProperty ;
     rdfs:range xsd:string ;
     rdfs:subPropertyOf higgins:synonym .

[well, in practice the range would likely be a syntax restriction on xsd:string, not a plain old xsd:string, but fixing that would complicate the example]

#3: Changes to EntityID definition

• The canonical EntityId (if it exists) is immutable

Proposed Changes to IdAS API for Higgins 1.1

public Object[] IEntity.getEntityIds();

This method returns an array of EntityIds that uniquely identify the Entity within the Context. Each Object is either

• a String (if the EntityId is not an Attribute of the Entity)
• an IAttribute (if the EntityId is also an Attribute of the Entity)

public Object IEntity.getCanonicalEntityId();

This method returns the "canonical" EntityId, i.e. the preferred one. The returned object is either a String or an IAttribute. The context provider guarantees that this EntityId is immutable. Returns null if this Entity has no Cannonical EntityId

public IEntity IContext.getEntity(String);

This method already exists today. There is no change to it. It looks up an IEntity based on a String which is not an Attribute of the Entity.

public Iterator IContext.getEntities(IFilter);

This method already exists today. There is no change to it. It looks up IEntitys based on an IFilter, which can select them by Attribute Values.

public IAttributeModel.isEntityId();

Returns true, if IAttributes that use this IAttributeModel also act as EntityIds. These IAttributes may be returned by the above IEntity.getEntityIds() method.

public IAttributeModel.isMutable();

Returns true, if IAttributes that use this IAttributeModel are mutable, i.e. if its IAttributeValues can be changed/added/removed.

Example Using Proposed Changes

The following diagram shows three Entities: two ordinary Entities and one Entity Class (higgins:Person):

• The Context Provider developer has defined three simple Attributes: "ssn", "mobile", and "shoe-size".
• The developer chose to use the SSN as the canonical EntityId
• The developer chose the option to "repeat" the canonical EntityId value as the value of the "ssn" Attribute.
• Although it is not shown in the above diagram, the developer has defined two of these (SSN & mobile) as being Synonym Attributes.
• Just to show off to his boss, the developer defined a complex Attribute called "knows" and used it to link the entity on the left with the entity on the right by referring to the right-most entities canonical (and hopefully immutable) entityId.

Using IdAS...

• Calling IEntity.getCanonicalEntityId() on the Entity at the left will return the (canonical) EntityId, which is the IAttribute that represents the "ssn" Attribute. It has the single, simple value 033568888.
• Calling IEntity.getCanonicalEntityId() on the Entity at the right will return the (canonical) EntityId, which is the IAttribute that represents the "ssn" Attribute. It has the single, simple value 034898786.
• Calling IEntity.getEntityIds() on the Entity at the left will return a list of these two IAttributes:
  • mobile with value +16175137924
  • ssn with value 033568888
• Calling IEntity.getAttribute(<knows>) on the Entity at the left will return the Entity on the right [some liberties taken here for brevity]