Latest revision as of 11:25, 3 October 2011

The Content Manager service gives uniform access to content served by a variety of back-ends, both inside and outside the system. It is the central component of the gCube subsystem which deals with the organisation of content and related data.

We overview here the architecture, content model, and interfaces of the service . Related libraries, such as the Content Manager Library (CML) and the gCube Document Library (gDL), are discussed separately.

Architecture

The Content Manager is designed as an OCMA service. In OCMA terms, it classifies as a multi-type, 1-N adapter service:

• it is a multi-type service because it supports two front types for, respectively, reading and writing content modelled as labelled trees.

Collectively, the front types and the tree content model form the gDoc access type of the service.

• it is an adapter service because it adapts the gDoc access type to multiple back types, where each back type corresponds to the interface and content model of a class of remote repositories.

For this, the service employes an open architecture of type-specific plugins to which it delegates the creation and operation of its collection managers.

Plugins are dynamically deployed within service instances, and different instances may host different plugins. In addition, some plugins may support both service front types, i.e. grant read and write access to the corresponding class of repositories. Others may instead support read-only access or, less commonly, write-only access.

The figure below overviews the design and use of the service in the context of one its instances.

The instance exposes three stateful port-types:

• the ReadManager port-type serves as the interface of collection managers that offer read-only operations over the content of the bound collection.

The operations of ReadManager resources and their Resource Properties are discussed here.

• the WriteManager port-type serves as the interface of collection managers that offer write-only operations over the content of the bound collection.

The operations of WriteManager resources and their Resource Properties are discussed here.

• the Factory port-type serves as the front-end of a single WS Resource that creates ReadManager and WriteManager resources .

The operations of the Factory resource and its Resource Properties are discussed here.

Internally ReadManagers and WriteManagers share common state in a CollectionResource, a stateful resource that acts as an internal model of a remote collection accessed through the collection managers. The CollectionResource remains private to the service is not published within the system.

All the stateful resources of the service, whether public or private, are logically extended by components provided by service plugins, the resource delegates. The interactions between service and plugins are discussed in detail here, but we can summarise them here as follows:

• the factory delegate extends the Factory in order to handle requests which are specifically addressed to the plugin;
• at each such request, the factory delegate processes plugin-specific parameters to create one ore more "collection delegates", one for each collection that should be exposed through the service in order to satisfy the client request.
• the Factory receives such delegates and performs three key tasks:

• it initialises the delegates, a process during which each collection delegate may create and stage read delegates and write delegates, i.e. logical extensions of ReadManagers and WriteManagers.
• its creates CollectionResources, ReadManagers, and WriteManagers to match the available delegates, injecting collection delegates into CollectionResources so as to form the web of relationships shown in the figure above.
• it publishes collection profiles, one for each collection modelled within this process.

• later requests to ReadManager and WriteManager resources are handled by their read and write delegates, respectively, which translate the requests against the back-end repository that exposes the collection bound to the managers.

Finally, note that all stateful resources of the service are persistent and may thus be re-activated across restarts of the gCube Hosting Node:

• the Factory persists the history of its activations, i.e. the activation records that it published and/or processed (so as to not process them again).
• the CollectionResource persists its collection delegate and, with it, the read and write delegates.

Content Model

Architectural considerations aside, the most distinguished element in the design of the Content Manager is its content model. Rather than settle for a fixed set of document structures, the service adopts a generic structure that can act as a 'carrier' for an arbitrary number of concrete document models. In particular, the service deals with edge-labelled and node-attributed trees, the gDoc trees.

The expectation here is that producers (service plugins) and consumers (service clients) will convene on concrete document models and exchange gDoc trees with an agreed shape. The agreement may be bilateral or involve any number of parties, and it may apply to the entire document or to distinguished parts of it (e.g. document metadata, annotations, raw content packaging, etc). For maximum decoupling between consumers and producers, the agreement may reflect system-wide conventions and result in canonical tree forms, most noticeably those that define the gCube Document Model.

gDoc Trees

A gDoc tree has the following properties:

• its nodes may have an identifier and a number of uniquely named attributes;

• its edges have a label;

• its leaf nodes may have a value;

• its root may identify the collection of the corresponding document.

In particular:

• identifiers, attributes, and leaves have text values;
• attribute names and labels may be qualified with a namespace.

The figure below uses a graphical representation to show an example of a gDoc tree.

gDoc trees serialise to XML documents for exchange over the network. In particular:

• nodes serialise to elements and attributes serialise to element attributes
  • elements are named like the edges that enter the corresponding nodes, with the exception of the document element which is named http://gcube-system.org/namespaces/contentmanagement/gdoc:godc
• the elements that correspond to inner nodes contain the elements that correspond to their children
• the elements that correspond to leaves contain their value
• node identifiers serialise to attributes called http://gcube-system.org/namespaces/contentmanagement/gdoc:id
• collection identifiers serialise to attributes called http://gcube-system.org/namespaces/contentmanagement/gdoc:collID

For example, the gDoc tree above serialises as:

note gDoc trees inherit constraints from their XML serialisation. In particular, the names of edges, the names of attributes, the values of attributes, and the values of leaves are regulated by the definition of the format.

gDoc API

The XML serialisation of gDoc trees is 'natural', in that it does not employ dedicated element structures for the representations nodes, edges, attributes, etc. This streamlines its manipulation with standard XMl technologies (e.g. XPath, XSLT, XQuery, DOM, SAX, etc.) and does not inhibit object binding technologies (e.g. JAXB, XStream, etc).

As a native option, however, the service defines a bespoke object model and API for gDoc trees which offer:

• dedicated support for tree processing requirements associated with the use of the service;
• transparencies and optimisations for tree storage, construction, deconstruction, and input/output.

While the model is available to service clients, it also forms the basis of the interface between the service and its plugins. For this reason, its main features are overviewed here while its client-oriented features are discussed [[Content_Manager:_Stub_Distribution#Stub Distribution|elsewhere].

As the figure below illustrates, the model is defined in org.gcube.contentmanagement.contentmanager.stubs.model.trees in terms of the following components:

• Node: an abstract base for nodes with an identifier, a state, and a map of QName-ed attributes.
• State: an inner enumeration of Node for node states.
• Edge: A QName-ed edge to a target Node.
• InnerNode: a Node with a list of outgoing Edges.
• Leaf: a Node with a value.
• gDoc: an InnerNode with a collection identifier.
• Nodes: a collection of static utilities to generate Nodes and Edges.
• Bindings: a collection of static utilities to serialise and deserialise Nodess to and from DOM trees and/or character streams.
• NodeView: a base class for JAXB bindings to Nodes.
• GDocView: a NodeViewM for JAXB bindings to GDoc nodes.

The model API is illustrated by example in the rest of this Section. The full list of methods and their signatures can be found in the code documentation.

Building Trees

The first and obvious way to create gDoc trees is with the constructors of the concrete node classes (GDoc, InnerNode, Leaf). As a first example, the following code illustrates the creation of a tree with an attributed root and two leaf nodes:

GDoc doc = new GDoc("someid");
doc.setAttribute(new QName("x"), "1");
doc.setAttribute(new QName("someNS","y"), new Date().toString());
doc.collectionID("...");
 
Leaf leaf1 = new Leaf(null,"2"); //no identifier
Leaf leaf2 = new Leaf(null,"true");
 
Edge e1 = new Edge(new QName("a"),leaf1);
Edge e2 = new Edge(new QName("someNS","b"),leaf2);
 
doc.add(e1,e2);

While already more convenient than cross-language and format-oriented tree APIs (e.g. DOM), step-by-step construction is verbose, even in the case of small trees. For a first degree of improvement, the node classes offer rich suites of constructors and setter overloads that allow for more 'in-lined' tree constructions and absorb the creation of QNames:

GDoc doc2 = new GDoc("someid",
		new Edge("a", new Leaf(null,"2")),
		new Edge("someNS","b", new Leaf(null,"true")));
 
doc2.setAttribute("x", "1");
doc2.setAttribute("someNS","y", new Date().toString());
doc2.collectionID("somecollID");

For additional convenience, the Nodes class defines a large number of generators, i.e. factory methods that can be statically imported and then composed into an 'embedded expression language' for gDoc trees:

import static org.gcube.contentmanagement.contentmanager.stubs.model.trees.Nodes.*;
...
 
GDoc doc3 = attr(
		gdoc("somecollID","someid",
			e("a",2), 
			e("someNS","b",true)),
            a("x",1),a("someNS","y",new Date()));>

Here, gdoc, attr, e, a are examples of node, attribute, and edge generators. Besides allowing fully in-lined tree expressions without the use of the new operator, the generators offer QName creation transparencies and object-to-string conversion transparencies (cf. the int, boolean, and Date example above). The transparency of date conversions is particularly important here, as it ensures adherence to XML serialisation standards that are not natively adopted in Java (e.g. in the implementation of toString). See the code documentation for the full list of available generators, as well as for the additional examples that follow:

doc = gdoc();
doc = gdoc("someid");
doc = gdoc("collectionid","someid");
 
doc = gdoc("1",
              e("a", n("2",            //n() => inner node generator
              e("b",l("3",0)))));
 
doc = gdoc(    //no identifier
                e("a", attr(
                            n("2", 
                                e("b",l("3",0)),            //l()= explicit leaf generator for identity assignment
                                e("a",l("4",0))),
                          a("foo","0"))));
 
 
doc = attr(gdoc("1",
		e("a",l("2",5)),
		e("b",attr(
                             n("3",e("c",4)),
			  a("foo",0))),
		e("c",5)),
      a("x",0));
 
 
doc = attr(gdoc("1",
               e("a", n("2",
               e("b",n("$2")))),
                   e("a",n("a1",
                               e("c",n(
                                          e("d","..."),
                                           e("d",attr(                                       //l()= explicit leaf generator for attribute assignments
                                                        l("<xml>..</xml>"),
                                                     a("w",".."))))))),
               e("b",attr(
                            n("1:/2"),
                       a("w","...")))),
         a("x","http://org.acme:8080"),a("y","<a>...</a>"));

The literal construction of trees is particularly convenient in during testing, though it composes well with the programmatic construction in the development of production code:

Edge edge = ....
InnerNode node = ....;
 
attr( 
   node.add(e("before","..."), edge, e("after","..."))
), a("newattr","...");

note: the node classes override equals for equivalence-based comparisons, and hashCode for their correct use as keys within hash-based data structures, and toString for convenience of debugging.

Serialising and Deserialising Trees

The Bindings class offers static facilities to transform native models of gDoc trees into XML-based models. Two representations are supported natively, based on which other XML-based representation can be produced using standard platform facilities (e.g. TRAX):

• Bindings.toElement(GDoc) converts native models of gDoc trees into equivalent DOM models.
• Bindings.fromElement(Element) converts DOM models of gDoc trees into equivalent native models.
• Bindings.toXML(GDoc, Writer, boolean?) converts the native model into XML document streams, optionally excluding document declarations.
• Bindings.fromXML(Reader) converts XML document streams into gDoc trees.

note: DOM conversions of native models are implemented directly, as they are most commonly required for interactions with the Content Manager service. Stream conversions are instead derived from DOM conversions via TRAX, at an additional processing cost.

note: conversions from native models to XML-based models assign the conventional name http://gcube-system.org/namespaces/contentmanagement/gdoc:gdoc (cf. Bindings.GDOC_NS, and Bindings.GDOC_NAME constants) to the document element. Vice versa, conversion from XML-based representations to native models discard the name of the document element.

Here is a usage example, which shows that equivalence of native models is preserved under round-trip conversions.

import static org.gcube.contentmanagement.contentmanager.stubs.model.trees.Bindings.*;
...
GDoc doc = ....
 
//DOM conversion
GDOc doc2 = fromElement(toElement(doc));
assert doc.equals(doc2); //true!
 
//stream conversion
StringWriter w = new StringWriter();
toXML(doc,w);
GDOC doc3 = fromXML(w.toString());
assert doc.equals(doc3);   //true!

note: due to the treatment of root element names, equivalence of XML-based representations is not necessarily preserved after round-trip conversion. It is preserved only if the XML-based representations have been previously produced with the conversion routines.

note: in all the conversions above, null values in attribute and leaf values are serialised using a special constant (exposed programmatically as Node.NULL).

note: the conversions are also available at arbitrary inner nodes, not only roots (cf. Bindings.nodeToElement(Node, QName?), Bindings.nodeFromElement(Element),Bindings.nodeToXML(Node, Writer, QName), and Bindings.nodeFromXML(Reader).

Consuming Trees

The gDoc API offers simple means of procedural tree navigation. For declarative queries, clients can convert the model into an XML-based representation and leverage platform standards and popular offerings (e.g. XPath, XQuery, or XSLT implementations). If required, the gDoc API can then be reasserted on query outputs.

The Node class defines methods to expose the state common to all nodes of a gDoc tree:

• id(): returns the identifier.
• parent(): returns the parent.
• ancestors(): returns the list of all nodes from the parent to the root.
• ancestorsAndSelf(): behaves ancestors but the returned list includes and starts with the recipient node.
• attributes(): returns a copy of the attributes, indexed by name.
• attribute(QName): returns the value of an attribute with the given name (or fails).
• hasAttribute(QName): checks for the existence of an attribute with a given name.

note: identifiers can only be set at node creation time. Attributes can be added, modified, and removed at any point (cf. Node.state(Node.State), setAttribute(QName,String), removeAttribute(QName)).

note: for convenience, all methods that take QNames are overloaded to accept local names as well as (namespace,local name) pairs.

note: invoking methods that take node types is simplified by statically importing the class constants defined in the Nodes class (cf. Nodes.N for InnerNode.class and Nodes.L for Leaf.class).

The Leaf class adds methods to read and set the value (cf. value(), value(String)).

The InnerNode class adds methods to navigate along edges and or identifiers:

• children(): returns the list of children.
• children(QName): returns the list of children under edges whose label matches a given label.
• <T extends Node> children(class<T>): returns the list of children of a given node type.
• <T extends Node> children(class<T>, QName): returns the list of children of a given node type under edges whose label matches a given label.
• child(QName): returns the child under an edge whose label matches a given label (or fails if there are zero o more such children).
• <T extends Node> child(Class<T>, QName):returns the child of a given node type under an edge whose label matches a given label (or fails if there are zero o more such children).
• descendants(QName*): returns the list of descendants that can be reached following edges whose labels match a given label.
• <T extends Node> descendants(Class<T>,QName*): returns the list of descendants of a given type that can be reached following edges whose labels match a given label.
• edges(): returns the list of all the edges.
• edges(QName): returns the list of edges whose label matches a given label.
• edge(QName): returns the list of edges whose labels match a given label (o fails if there are zero or more such edges).
• hasEdge(QName): checks for the existence of an edge whose label matches a given label.
• labels(): returns the list of all edge labels.
• labels(QName): returns the list of labels that match a given label.

note: edges can be added or more removed at any time (cf. add(Edge*), removeEdge(Edge*), removeEdge(QName)).

note: all matches on qualified names are based on arbitrary regular expressions, both on the namespace and the local name of the label.

note: as above, methods that take QNames have overloads that accepts local names and, where appropriate, overloads that accept (namespace,local name) pairs.

note: as above, invoking methods that take node types is simplified by statically importing the corresponding constants in Nodes (cf. Nodes.N, Nodes.L).

The GDoc class adds a method to read and set the collection identifier (cf. collectionID, collectionID(String)).

Finally, the Edge class exposes its label and target (cf. label(), target()).

The following example illustrates some of the supported idioms, do check the code documentation for detailed information about method signatures:

import static org.gcube.contentmanagement.contentmanager.stubs.model.trees.Nodes.*;
 
GDoc doc = attr(gdoc("1",
		e("a",l("2",5)),
		e("b",attr(
				n("3",e("c",4)),
			  a("foo",0))),
		e("c",5)),
           a("x",0));
 
//typed children
String val = doc.child(L,"a").value();
 
//typed descendant
String val2 = doc.descendant(N,"3").child(L,"c").value();
 
for (InnerNode node : doc.children(N))
	for (QName l : node.labels())
		//process label
 
for (Node d : doc.descendants("b","e"))
	for (Edge siblingEdge : d.parent().edges()) 
		if (siblingEdge.target()!=d)
		//process sibling of descendant

Binding Trees

Clients that expect gDoc trees of a given form may wish to bind them to objects. The API offers two classes to streamline JAXB object bindings in the package org.gcube.contentmanagement.contentmanager.stubs.model.views. In particular, it includes two base classes for node and document bindings to XML serialisations of gDoc trees:

• NodeView is a base class for node bindings. The view binds and exposes the identifier of the node as well as the URL of the node, if one exists (cf. getID(), getURL()). Node URLs are discussed later.

• GDocView extends NodeView as a base class for document bindings. The view binds and exposes the collection identifier of the root node (cf. getCollID()), in addition to what already bound and exposed via its superclass.

Clients can extend these classes and the corresponding bindings. The following example illustrates:

@XmlRootElement(name=Bindings.GDOC_NAME,namespace=Bindings.GDOC_NS)
class MyDocView extends GDocView {
	@XmlElement(namespace="http://acme.org") int i;
	@XmlElement(namespace="http://acme.org") MyDocComponent c;
 
 
class MyDocComponent extends MyNodeView {
	@XmlElement Date date;
}

MyDocView and MyDocComponent are toy examples of user-defined views over gDoc trees and tree nodes, and they should be familiar to JAXB users.

MyDocView extends GDocView and uses JAXB annotations to specify the qualified name of the document elements to which it will be bound. Here we have chosen a name that aligns with the serialisations produced by the Bindings class, as shown above, but different names may be specified if the binding target serialisations produced through different means.MyDocView then includes two fields in its own namespace, an integer field and a MyDocComponent field, both of which are bound to XML elements. MyDocComponent extends NodeView, specifies a single Date, and uses JAXB annotations to bind it to an XML element. In both classes, we have chosen simple JAXB annotations. For example, we have assumed that the gDoc trees that will come to binding have labels that match the field names. The full range of JAXB facilities is of course available to customise bindings to less aligned trees.

Suppose now MyDocView is to be bound to the gDoc tree below. Wee use the generators of the gDoc API to denote the tree, but this is just for convenience of exposition; the tree may have been generated through any suitable means.

GDoc doc = gdoc("collID","123",
                   e(NS,"i",3),
                   e(NS,"c", n("789",
                              e("d",l("1",new Date())),
                              e("b",l("2",15)))),
                   e(NS,"d",new Date()), 
                   e(NS,"b",n("456")));

Clearly, the tree contains a subset that matches the binding expectations of the classes above. As with all JAXB clients, the binding would require steps similar to the following:

JAXBContext context = JAXBContext.newInstance(MyDocView.class);
...
 
//assuming a DOM binding to the tree has already occurred (other JAXB inputs could have been used instead, e.g. character streams)
Element docElement = ....
 
//bind
MyDocView mv = (MyDocView) context.createUnmarshaller().unmarshal(docElement);
 
 
...mv.id()...
...mv.collID()...
...mv.url()...
...mv.i...
...mv.c...
...mv.id()...
...mv.url()...
...mv.c.d...
 
//serialiase (again to DOM)
Document dom = ....;
Marshaller m = context.createMarshaller();
m.marshal(mv,dom);

gDoc Predicates

The gDoc model is untyped, in that neither the topology of trees nor the values of their attributes or leaves are subjected to constraints (beside those dictated by the XML serialisation). Types are reintroduced later, under the view that they can be projected on gDoc trees at the point of consumption.

Type projections serve two main purposes in the context of the Content Manager:

• to validate the content of gDoc trees.

The main use case for validation is at the point of content ingestion through the write operations of the Content Manager. In particular, a plugin may project a type on incoming gDoc trees, with a view to rejecting those that fail the projection.

• to identify the data of interest within gDoc trees.

The main use case for content identification is at the point of content retrieval through the read operations of the Content Manager. Through the service, in particular, a client may ask plugins to return only the portion of the data that succeeds the projection, and to discard the rest. Content pruning results in minimal bandwidth consumption and delivers content to client in forms which are optimal for their own object bindings.

Accordingly, support for type projections requires:

• a language of tree types with which clients and plugins can capture the required shape and content of gDoc trees.
• the ability to project such types over gDoc trees with both validation and pruning semantics.