[dna-issues] [JBoss JIRA] Commented: (DNA-374) Multiplicity of some properties may change when saved and reloaded from the graph

Tue Apr 21 11:34:22 EDT 2009

    [ https://jira.jboss.org/jira/browse/DNA-374?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12463463#action_12463463 ] 

Randall Hauch commented on DNA-374:
-----------------------------------

There are two things that conspire to cause this problem.  The first is that JCR distinguishes between single-valued and multi-valued properties and does not allow a single-valued property to be changed with the multi-valued methods (e.g., the Property.setValues(...) methods or the Node.setProperty(...) that takes multiple values) - or vice versa.  The multiplicity of a property has a huge impact on which API methods may be used.

The second is that the DNA graph layer does not have this distinction.  The DNA graph API is significantly smaller and easier to use without having this distinction, and it allows any property to be set to any values.  After all, the DNA graph API was designed as a lower-level and more simplistic graph model that didn't come with a lot of overhead or baggage.

Therefore, since the DNA graph layer doesn't maintain this distinction, the JCR implementation currently determines when a node is read in the most applicable property definition for each property.  Many times there is only one applicable property definition.  For example, if the property has no values or has more than one value, then it is obvious that a multi-valued property definition must be chosen.  (The property types may also affect which property definition is selected.)

However, there are times when multiple property definitions apply.  One frequent case is "nt:unstructured", which has both a single-valued residual property definition and a multi-valued residual property definition.  Multiple applicable property definitions complicate the determination and selection of an appropriate property definition, and sometimes results in a property definition being selected (when a node is read in) that may not match the definition that was used to create the property.  

This is the case when a property is created to be multi-valued (via using JCR's setters for multiple values) but only has one single value, and both a single-valued and multi-valued property definition are available.  When the parent node is saved, the property definition is not saved, and when reading the node back in our JCR implementation (specifically SessionCache) reads this property and attempts to determine the most appropriate property definition.  Because there is one value and single- and multi-valued property definitions, SessionCache does not know which property definition to choose.  Currently, the single-valued property definition is used when the property has only one value.  

Note that we'd have the same problem even if we'd change SessionCache to pick the multi-valued property definition, and the client created a single-valued property.  In this case, our implementation would read the graph and assign the multi-valued property definition.  Therefore, it is not sufficient to change how SessionCache determines the most appropriate property definition.  The problem is that we're losing information that is not persisted in the graph and that cannot always be recovered accurately.

There are a few options:
1) Change the DNA graph layer to distinguish between single-valued and multi-valued properties.  This would be a big change to the graph API and connectors, because it adds a lot of subtle semantics.  Plus, it may snowball to needing property types and therefore node types.  Since the DNA graph layer was intended to be a lower-level and more generic graph model, adding all this complexity can hopefully be avoided.
2) Change the DNA graph layer to support "metadata" on each property, but to not define in the graph layer what this metadata might be.  For example, the JCR layer could use this metadata to say whether the property is single-valued or multi-valued.  This approach is far less intrusive, but it does add a "meta-level" to the existing graph model, and this "meta-level" would not be handled the same by the connectors or the graph API as the existing nodes and properties.  Essentially we'd be duplicating some of the functionality by allowing properties to have "meta-properties".
3) Do not change the DNA graph layer at all, but change the JCR implementation to use other properties on the node to store this additional information. We're already doing that with "dna:nodeDefinition", which stores the name of the child node definition in the parent's node type (primary or mixins) that was used to create the node.  This "dna:nodeDefinition" property is defined on DNA's node type for "nt:base" (actually, a mixin that "nt:base" extends), and is marked as "protected" meaning clients may not set that property through the JCR API.  But we're also hiding that property by not including it in the set of properties exposed by the JCR API.  So this option just extends this already-used approach to be used to capture enough information to be able to properly determine the property definition for each property.

So, option #3 appears to be the most appealing.  The only question is: what information should be stored?  Should we store the ID of the property definition for each property?  Or can we store only the names of the multi-valued properties that have exactly one value?  Or, maybe we store only the names of the multi-valued properties that have exactly one value and that have a single-valued property definition?

I don't think it's worth storing the ID of the property definitions for all properties.  That's just overkill, and in many situations it is not required.  But at the other extreme, a change in the node types might mean that we cannot always recover a property definition if we only store the names of the multi-valued properties with one value that use a multi-valued property definition that also have a similarly-named single-valued property definition.

Therefore, I think the correct approach is to store the names of the multi-valued properties that have a single value.  This information, coupled with the existing value(s) for each property, can then allow us to always figure out when should a property should be treated as a single-valued property or a multi-valued property.

> Multiplicity of some properties may change when saved and reloaded from the graph
> ---------------------------------------------------------------------------------
>
>                 Key: DNA-374
>                 URL: https://jira.jboss.org/jira/browse/DNA-374
>             Project: DNA
>          Issue Type: Bug
>          Components: JCR
>            Reporter: Randall Hauch
>            Assignee: Randall Hauch
>             Fix For: 0.4
>
>
> This problem arises when a client uses the multi-valued setter methods in the JCR API to create on a node a new property with a single value, to save the node, and then to continue working with the property (again, using the multi-valued setter or getter methods).  When the property is created, an appropriate multi-valued property definition will be chosen from those available on the node's primary type (and mixin types, if there are any).  However, after the save, the property may node have a single-valued property definition.  And since JCR does not allow a single-valued property to be changed via the multi-valued setters, the result is an exception when the client attempts to set the property using the multi-valued setters.

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