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From Thomas Mueller <muel...@adobe.com>
Subject Re: [jr3] Tree model
Date Tue, 28 Feb 2012 15:08:38 GMT
Hi,

What is your use case, meaning where would this interface be used and why?

Regards,
Thomas


On 2/28/12 3:59 PM, "Jukka Zitting" <jukka.zitting@gmail.com> wrote:

>Hi,
>
>[Here's me looking at lower-level details for a change.]
>
>I was going through the prototype codebases in the sandbox, trying to
>come up with some clean and simple lowest-common-denominator -style
>interface for representing content trees. Basically a replacement for
>the ItemState model in current Jackrabbit.
>
>The trouble I find with both the current Jackrabbit ItemState model
>and the efforts in the sandbox is that this key concept is modeled as
>concrete classes instead of as interfaces. Using an interface to
>describe and document the fundamental tree model gives us a lot of
>extra freedom on the implementation side (lazy loading, decoration,
>virtual content, etc.).
>
>So, how should we go about constructing such an interface? I started
>by laying some ground rules based on concepts from the sandbox and
>past jr3 discussions:
>
>  * A content tree is composed of a hierachy of items
>  * Tree items are either leaves or non-leaves
>  * Non-leaves contain zero or more named child items (*all* other
>data is stored at leaves)
>  * Each child item is *uniquely* named within its parent
>  * Names are just opaque strings
>  * Leaves contain typed data (strings, numbers, binaries, etc.)
>  * Content trees are *immutable* except in specific circumstances
>(transient changes)
>
>As a corollary of such a proposed design, the following features (that
>with a different tree model could be a part of the underlying storage
>model) would need to be handled as higher level constructs:
>
>  * Same-name-siblings (perhaps by name-mangling)
>  * Namespaces and other name semantics
>  * Ordering of child nodes (perhaps with a custom order property)
>  * Path handling
>  * Identifiers and references
>  * Node types
>  * Versioning
>  * etc., etc.
>
>As you can see, it's a very low-level interface I'm talking about.
>With that background, here's what I'm proposing:
>https://gist.github.com/1932695 (also included as text at the end of
>this message). Note that this proposal covers only the interfaces for
>accessing content (with a big TODO in the Leaf interface). A separate
>builder or factory interface will be needed for content changes in
>case this design is adopted.
>
>Please criticize, as this is just a quick draft and I'm likely to miss
>something fairly important. I'm hoping to evolve this to something we
>could use as a well-documented and thought-of internal abstraction for
>jr3. Or, if this idea is too broken, to provoke someone to provide a
>good counter-proposal. :-)
>
>BR,
>
>Jukka Zitting
>
>----
>
>import java.util.Map;
>
>/**
> * Trees are the key concept in a hierarchical content repository.
> * This interface is a low-level tree node representation that just
> * maps zero or more string names to corresponding child nodes.
> * Depending on context, a Tree instance can be interpreted as
> * representing just that tree node, the subtree starting at that node,
> * or an entire tree in case it's a root node.
> * <p>
> * For familiarity and easy integration with existing libraries this
> * interface extends the generic {@link Map} interface instead of
> * providing a custom alternative. Note also that this interface is
> * named Tree instead of something like Item or Node to avoid confusion
> * with the related JCR interfaces.
> * </p>
> *
> * <h2>Leaves and non-leaves</h2>
> * <p>
> * Normal tree nodes only contain structural information expressed as
> * the set of child nodes and their names. The content of a tree,
>expressed
> * in data types like strings, numbers and binaries, is stored in special
> * leaf nodes with no children. Such leaf nodes implement the {@link Leaf}
> * sub-interface and can be identified and accessed using the
> * {@link #isLeaf()} and {@link #asLeaf()} methods.
> * </p>
> * <p>
> * Note that even tough such leaf nodes are guaranteed to have no children
> * (i.e. {@link #isLeaf()} implies {@link #isEmpty()}), the reverse is not
> * necessarily true. It's possible for a non-leaf node to contain no
>children,
> * though such cases occur normally only transiently when new subtrees are
> * being constructed.
> * </p>
> *
> * <h2>Mutability and thread-safety</h2>
> * <p>
> * Tree objects are immutable by default and thus safe for concurrent
>access.
> * Using a mutator method like {@link #clear()} or {@link #put(String,
>Tree)}
> * results in an {@link UnsupportedOperationException exception}. A new
>Tree
> * instance is needed to express a modified content tree. As a result it's
> * safe to repeat operations like iterating over all child nodes of a Tree
> * instance and expect results to be the same.
> * </p>
> * <p>
> * In specific situations like when constructing new trees it's possible
>for
> * Tree instances to be mutable. Such cases need to be explicitly
>documented
> * and managed in a way that prevents thread-safety issues, for example by
> * keeping a reference to such a mutable Tree instance local to a single
> * thread.
> * </p>
> *
> * <h2>Persistence and error-handling</h2>
> * <p>
> * A Tree instance can be (and often is) backed by local files or network
> * resources. All IO operations or related concerns like caching should be
> * handled transparently below this interface. Potential IO problems and
> * recovery attempts like retrying a timed-out network access need to be
> * handled below this interface, and only hard errors should be thrown up
> * as {@link RuntimeException unchecked exceptions} that higher level code
> * is not expected to be able to recover from.
> * </p>
> * <p>
> * Since this interface exposes no higher level constructs like access
> * controls, locking, node types or even path parsing, there's no way
> * for content access to fail because of such concerns. Such functionality
> * and related checked exceptions or other control flow constructs should
> * be implemented on a higher level above this interface.
> * </p>
> *
> * <h2>Decoration and virtual content</h2>
> * <p>
> * Not all content exposed by Tree objects needs to be backed by actual
> * persisted data. An implementation may want to provide provide derived
> * data like for example the aggregate size of the entire subtree as an
> * extra virtual leaf node. A virtualization, sharding or caching layer
> * could provide a composite view over multiple underlying content trees.
> * Or a basic access control layer could decide to hide certain content
> * based on specific rules. All such features need to be implemented
> * according to the API contract of this interface. A separate higher
>level
> * interface needs to be used if an implementation can't for example
> * guarantee immutability of exposed content as discussed above.
> * </p>
> */
>interface Tree extends Map<String, Tree> {
>
>    /**
>     * Checks whether this is a {@link Leaf} instance. Can be used to
>     * control program flow without explicit <code>instanceof</code>
>checks
>     * for handling leaf content. See also the {@link #asLeaf()} method
>     * that can additionally take care of type casting.
>     *
>     * @return <code>true</code> if this is a {@link Leaf},
>     *         <code>false</code> if not
>     */
>    boolean isLeaf();
>
>    /**
>     * Returns this instance as a {@link Leaf} if possible. Can be used
>     * to access leaf nodes without <code>instanceof</code> checks or
>     * explicit type casting. A typical access pattern is:
>     * <pre>
>     * Leaf leaf = tree.asLeaf();
>     * if (leaf != null) {
>     *     // handle leaf content
>     * } else {
>     *     // handle non-leaf content
>     * }
>     * </pre>
>     *
>     * @return this instance as a {@link Leaf},
>     *         or <code>null</code> if this is a non-leaf node
>     */
>    Leaf asLeaf();
>
>}
>
>/**
> * Leaves are special {@link Tree} nodes contain typed data like strings,
> * numbers, binaries, etc. This interface extends {@link Tree} and thus
> * also {@link Map}, but all Leaf instances are guaranteed to contain zero
> * child nodes. Leaves are always immutable.
> */
>interface Leaf extends Tree {
>
>    // TODO: Add data access methods
>
>}


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