The Content Manager service provides its clients with 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 that deals with the organisation of content and related data.

Design Overview

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 access type of a whole 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 single instances of the services, 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 repository. 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 running instances. The instance exposes three stateful port-types:

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

The interface defines the gDocRead front type of the service.

The front type and the identifier of the bound collection are published as Resource Properties of the manager, in accordance with OCMA patterns for publication and discovery of service state. A third Resource Property is the name of the bound plugin, i.e. the plugin to which the manager delegates the resolution of its requests.

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

The interface defines the gDocWrite front type of the service.

Again, the type, the identifier of the bound collection, and the name of the bound plugin are published as Resource Properties of the manager.

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

The resource is created at the activation of the service instance in the gCube Hosting Node.

During its lifetime, it publishes creation requests as activation records. Conversely, it subscribes for the activation records that are published by other instances of the service, in line with OCMA patterns for replication of service state.

The resource also publishes as a Resource Property the list of summary descriptions of the plugins that are hosted at the service instance.

Service plugins logically extend factory and collection manager resources with corresponding resource delegates. In particular:

the factory delegate extends the Factory resource at plugin deployment time in order to handle requests that are specifically addressed to the plugin;
at each such request, the factory delegate processes plugin-specific parameters to create one ore more read delegates and/or write delegates, which the service instance uses to create and extend corresponding collection managers;
future requests to the managers are then handled by their delegates, which translate the requests against the back-end repository that exposes the collection bound to the managers.

Finally, note that factory and collection managers are persistent resources 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.
the collection managers persist the name and state of their 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.

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
- the document element is arbitrarily named
the elements of inner nodes contain the elements of their children
the elements of leaves contain their value
node identifiers serialise to attributes called http://gcube-system.org/namespaces/contentmanagement/gdoc:id
node states serialise to attributes called http://gcube-system.org/namespaces/contentmanagement/gdoc:state
collection identifiers serialise to attributes called http://gcube-system.org/namespaces/contentmanagement/gdoc:collID

For example, the gDoc tree above may serialise as:

<g:gdoc xmlns:g="http://gcube-system.org/namespaces/contentmanagement/gdoc"
	g:id="1" x="..." y="..." g:collID="...">
   <a g:id="2">
     <b g:id="5"/>
  </a>
  <a g:id="3">
     <c>
        <d >...</d>
        <d w="..">...</d>
     </c>
  </a>
   <b g:id="4" w="..." />
</g:gdoc>

Note that 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 later on.

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

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

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 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 subject 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.

the 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 processing or 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.

XML schema languages are natural candidates for the choice of tree types. However, the also introduce complexity - both conceptually and in terms of tooling - which is not required when working with the subset of XML that corresponds to the gDoc model. As importantly, schema languages are strongly associated with validation and there are no implementations that use them towards document pruning (or indeed content extraction).

Accordingly, the tree API includes a native language of tree types, the gDoc predicates, as well as support for projecting them over content for validation and pruning purposes. gDoc predicates, in particular, can be used to constrain:

the topology of gDoc trees, including the labels and cardinality of edges (e.g. the existence of at least one edge with a given label).
the values of leaves, so that they conform to the textual literal of a range of atomic types (e.g. numbers or boolean values) or simply verify some type-specific predicate.

note: Support for predicates on attributes and wildcard expressions on edge labels is forthcoming.

Predicate API

gDoc predicates are defined in the packages org.gcube.contentmanagement.contentmanager.stubs.model.predicates and org.gcube.contentmanagement.contentmanager.stubs.model.constraints, the main components of which are the following:

Predicate: the interface of all node predicates, defines match(Node) and prune(Node) methods for validation-based and pruning-based projection semantics.
- AnyPredicate: a Predicate that specifies no constraints on nodes, i.e. matches any node and prunes nothing from it.
- TreePredicate: a Predicate that specifies a list of EdgePredicates on inner nodes.
- LeafPredicate: a Predicate that specifies a Constraints on the value of leaf nodes.
  - Bool: an LeafPredicate that specifies a boolean Constraint on the value of leaf nodes.
  - Num: an LeafPredicate that specifies a numeric Constraint on the value of leaf nodes.
  - Text: an LeafPredicate that specifies a textual Constraint on the value of leaf nodes.
  - Date: an LeafPredicate that specifies a date Constraint on the value of leaf nodes.
  - URI: an LeafPredicate that specifies a URI Constraint on the value of leaf nodes.
EdgePredicate: a predicate that specifies a node Predicate on the targets of edges with a given label.
- - One: an EdgePredicate that specifies the existence of a single edge with a given label.
  - Opt: an EdgePredicate that specifies the existence of at most on edge with a given label.
  - AtLeast: an EdgePredicate that specifies the existence of one or more edges with a given label.
  - Many: an EdgePredicate that specifies the existence of zero or more edges with a given label.
  - ID: an EdgePredicate that specifies a given identifier for the source.
Predicates: a set of facilities to build tree predicates.

Constraint: the interface of all constraints over values of leaf nodes.
- Same: the Constraint that is satisfied by values that are equivalent to a given value.
- Match: the Constraint that is satisfied by values that match a given regular expression.
- More: the Constraint that is satisfied by values that are numbers strictly greater than a given number.
- Less: the Constraint that is satisfied by values that are number strictly smaller than a given number.
- Before: the Constraint that is satisfied by values that are earlier dates than a given date.
- After: the Constraint that is satisfied by values that are later dates than a given date.
- Not: the Constraint that is satisfied by values that do not satisfy another Constraint.
- Either: the Constraint that is satisfied by values that satisfy at least one of a number of other Constraints.
- All: the Constraint that is satisfied by values that satisfy a number of other Constraints.
Constraints: a set of facilities to build Constraints.

Building Predicates

Similarly to gDoc trees, gDoc predicates may be built with classic constructor-based idioms and/or else with predicate generators, a collection of factory methods in Predicates class and Constraints class which can be statically imported and then composed into a pseudo literal syntax for gDoc predicates. We concentrate here on predicate generators as the preferred way to build gDoc predicates. See the code documentation for the constructors available in predicate and constraint classes.

Consider this first example:

import static org.gcube.contentmanagement.contentmanager.stubs.model.constraints.Constraints.*;
import static org.gcube.contentmanagement.contentmanager.stubs.model.predicates.Predicates.*;
 
Predicate p = tree(
               one("a",
                   num(more(6))));

Here, tree() generates a TreePredicate which characterises trees which satisfy a single EdgePredicate. This latter predicate requires that the trees have exactly one outgoing edge with label a and with a leaf target. This leaf must in turn satisfy a Num predicate, i.e. its text value must represent a number and this number must satisfy a More constraint, which requires it to be greater than 6. In summary, we are characterising trees with a single a-edge that ends in a leaf with a number greater than 6.

The following example showcases a range of other predicates and constraints:

import static org.gcube.contentmanagement.contentmanager.stubs.model.constraints.Constraints.*;
import static org.gcube.contentmanagement.contentmanager.stubs.model.predicates.Predicates.*;
 
Predicate p = tree(
                  one("a",any),
                  one("b",text(either(is("abc"),is("efg")))),
                  atleast("c",bool(is(true))),
                  opt("d",tree()),
                  many("e",date(future())),,
                  one("f", uri(matches("^http.*"))),
                  many("g", num(all(less(5),more(10))),
                  one("h", text());
                  one("j",text(not(is("somestring")))),
                  one("k",tree(id("12345"))));

Here, the predicate characterises trees with:

a single a-edge that ends in any type of node, inner node or leaf, not characterised further;
one or more b-edges that end in leaves whose values are either one of two strings;
a single c-edge that ends in a leaf with a boolean value of true;
zero or one d-edges that end in inner nodes, not characterised further;
zero or more e-edges that end in leaves whose values are dates in the future;
a single f-edge that ends in a leaf whose value is an absolute http URI;
zero or more g-edges that end in leaves whose values are numbers between 5 and 10;
a single h-edge that ends in a leaf, not characterised further;
a single j-edge that ends in a leaf whose value differs from a given string;
a single k-edge that ends in an an inner node with an identifier of 12345;

Clearly, predicates can nest recursively to match the structure of trees. For a full list of available predicate and constraint generators, see the code documentation of the Predicates and Constraints classes.

Matching and Pruning

A gDoc predicate can be projected over a gDoc tree using the methods match() and prune() common to all Predicates. The first indicates whether tree satisfies the predicate, the second prunes it of all the nodes that remain uncharacterised by the predicate.

Consider this simple example:

import static org.gcube.contentmanagement.contentmanager.stubs.model.constraints.Constraints.*;
import static org.gcube.contentmanagement.contentmanager.stubs.model.predicates.Predicates.*;
 
Date d = new Date();
 
GDoc doc = gdoc(
         e("a",-1),
         e("a",1),
         e("a",2),
         e("b","..."),
         e("b",n(
             e("b1","..."))),
             e("c",n(
                  e("c1",d),
                  e("c2","..."))),
          e("d","..."));
 
Predicate p = tree(
          many("a",num(more(0))),
          atleast("b",tree()),
          one("c", tree(
                          one("c1",date()))));
 
assert p.matches(doc)==true;
 
GDoc pruned = gdoc(
              e("a",1),
              e("a",2),
              e("b",n(
                 e("b1","..."))),
              e("c",n(
                   e("c1",d))));
 
assert pruned.equals(p.prune(doc));

Here, it is easy to see that the gDoc tree satisfies the predicate. Accordingly, match() returns true and prune() successfully reduces the tree to include only the paths from the root which are directly described by the predicate (i.e. a tree equivalent to pruned in the example). If the tree had had a second c-edge, for example, match would have returned false and prune() would have failed with an exception. Note that, If the tree had had no b-edges, or if its b-edges had all ended in leaves, or in fact under any of a number of alternative assumptions, the outcome would have been equally negative.

note: match() does never fail, as a gDoc tree either satisfies the predicate or it does not. In contrast, prune fails whenever the tree does not match the predicate. In other words, prune subsumes match and reacts with a failure to mismatches.

Serialising and Deserialising Predicates

The predicate and constraint classes are ready for JAXB binding to XML (i.e. contain appropriate JAXB annotations). In addition, the Predicates class encapsulates a JAXB context and exposes javax.xml.bind.Marshallers and javax.xml.bind.Unmarshallers ready for client use (cf. getMarshaller(), getUnmarshaller).

For example, a client that needs works with character streams may operate as follows:

import static org.gcube.contentmanagement.contentmanager.stubs.model.predicates.Predicates.*;
 
//serialise predicate
Predicate p1 = ...;
Writer w = ...
getMarshaller().marshal(p,w);
 
//deserialise predicate
Reader r =...
Predicate  p2 = (Predicate) getUnmarshaller().unmarshal(r);

note: clients who use the Content Management libraries do not explicitly need to worry about conversion to and from DOM representations of predicates. The libraries perform conversions on their behalf.

Interfaces

As overviewed above, the interface of the Content Manager is distributed across three stateful port-types:

the Factory port-type, which is the interface of a single WS-Resource;
the ReadManager port-type, which is the read-only interface of many, collection-specific WS-Resources;
the WriteManager port-type, which is the write-only interface of many, collection-specific WS-Resources.

In this Section, we discuss in more detail the operations of the port-types and the Resource Properties of the corresponding WS-Resources. For clarity, we show and comment the signatures of operations in terms of the underlying Java implementation, which mirror the WSDL definitions. We point directly to the WSDL for the definition of auxiliary data structures for input and outputs (e.g. values of Resource Properties or records of ResultSets).

note: The service offers client-side libraries which operate at a higher level of abstraction than its public interface. The operations discussed below are of interest to clients that choose to bypass those facilities.

Factory

The Factory resource exposes two operations, both which are intended for clients that wish to create Collection Managers (ReadManagers and/or WriteManagers). The operations are not for generic use, in that clients are expected to target specifically the plugin to which the Factory resource ought to delegate the creation of Collection Managers. The operations, their inputs, and their outputs are defined in the WSDL of the port-type.

The first operation is synchronous, in that clients block waiting for the creation of the Collection Managers:

public EprList create(CMSCreateParameters parameters) throws GCUBEFault

the operation creates Collection Managers and returns a list of references to their endpoints, indexed by the name of the corresponding port-type. The creation of resource is paremetrised by:

the name of the plugin to which the service ought to dispatch the request, the target plugin.
the directive to the service to broadcast the request for state replication, in line with OCMA patterns.
an arbitrary DOM payload of parameters specific to the target plugin.

these may vary from plugin to plugin, but typically will contain sufficient information to directly or indirectly identify one or more collections and the repository that hosts them.

The second operation is instead asynchronous and it is indicated when target plugins declare long creation times in their documentation:

String createAsync(CMSCreateParameters parameters) throws GCUBEFault

the operation takes the same input as the synchronous version but returns immediately with the locator of a ResultSet which will be populated by the service only when the Collection Managers are created by the target plugin. Specifically, the ResultSet will contain a single CMSCreateOutcome, which captures the outcome of the operation (see WSDL definition). This can be a success or else a failure. In the first case, clients obtain the EprList described above. In the second case, they obtain the stack trace of the fault that occurred during the creation of the Collection Managers.

FInally, the Factory resource publishes a multi-valued Resource Property Plugin, which contains a PluginDescription for each of the plugins hosted by the running instance (see WSDL definition).

Read Managers

A ReadManager resource exposes three read-only operations over the content of the bound collection. The operations are intended for generic use, in that clients are not required to know the repository that hosts the collection or the service plugin that mediates access to that repository. The operations, their inputs, and their outputs are defined in the WSDL of the port-type.

AnyHolder getByID(GetByIDParams params) throws GCUBEFault

the operation returns (the DOM representation of) a gDoc tree. Invocations are parameterised by:

the identifier of the tree root;
[optional] (the DOM representation of) a tree predicate with respect to which the tree should be pruned before it is returned.

String getByIDs(GetByIDsParams params) throws GCUBEFault

the operation returns the locator of a ResultSet of (DOM representations of) gDoc trees. Invocations are parameterised by:

the locator of a ResultSet of (DOM representations of) tree root identifiers;
[optional] (the DOM representations of) a tree predicate with respect to which the trees should be pruned before they are returned.

note: tree predicates in input and gDoc trees in output are contained in an AnyHolder wrapper (see WSDL definition).

String get(GetParams params) throws GCUBEFault

the operation returns the locator of a ResultSet of (DOM representations of) gDoc trees. Invocations are parameterised by:

[optional] (the DOM representations of) a tree predicate with respect to which the trees should be pruned before they are returned.
[optional] (the DOM representations of) a tree predicate with respect to which the trees should be filtered before they are returned.

Filtering occurs before pruning and may be concern parts of the trees that are subsequently pruned.

note: invocations that specify no parameters effectively ask for the contents of the whole collection.

note: tree predicates in input and gDoc trees in output are contained in an AnyHolder wrapper (see WSDL definition).

A ReadManager resource publishes the following Resource Properties:

CollectionID: the identifier of the bound collection, as per OCMA requirements;
TypeID: the access type of the port-type, as per OCMA requirements. The value of this property is constant: gDocRead;
Plugin: the name of the bound plugin;

Write Managers

A WriteManager exposes four write-only operations over the content of a bound collection. The operations are intended for generic use, in that clients are not required to know the repository that hosts the collection or the service plugin that mediates access to that repository. The operations, their inputs, and their outputs are defined in the WSDL of the port-type.

String add(AnyHolder holder) throws InvalidDocumentFault, GCUBEFault

the operation adds a gDoc tree to the bound collection and returns the identifier assigned to the tree root as a result.

note: the operation assumes that all nodes of the input tree acquire identifiers as a result of a successful addition. Depending on the plugin that serves the request, the operation may fail if the nodes already have an identifier.

String addRS(String locator) throws GCUBEFault

the operation adds zero or more gDoc trees to the bound collection and returns a locator of a ResultSet of AddOutcomes, one for each tree in input (see WSDL definition). In the case of successful outcome, clients obtain the identifier assigned to the tree root. In the case of failure, they obtain the stack trace of the fault that prevented the addition of the tree.

note: again, the operation assumes that all nodes of the input tree acquire identifiers as a result of a successful addition. Depending on the plugin that serves the request, the operation may fail if the nodes already have an identifier.

VOID update(AnyHolder holder) throws UnknownDocumentFault, InvalidDocumentFault, GCUBEFault

the operation updates a gDoc tree in the bound collection (the target tree). Invocations are parameterised by (the DOM representation of) of a delta tree, a gDoc tree that captures all the updates to be applied to the target tree, including addition or removal of attributes, changes to attribute values, addition and removal of nodes, and changes to leaf values. In more detail, the delta tree is comprised of nodes marked with an attribute http://gcube-system.org/namespaces/contentmanagement/gdoc:status and includes:

all the nodes of the target tree which have changed, with the corresponding identifiers and with a status of MODIFIED or DELETED;

note: attributes of the target tree that have been removed occur in the delta tree with a value of _null;

note: DELETED nodes are either leaves with a value of _null_ or inner nodes with no children.

note: nodes that have no identifiers cannot be referred to in the delta tree, hence cannot be updated.

all the nodes that should be added to the target tree, without identifiers and with a status of NEW.

note: NEW nodes are leaves or inner nodes with NEW descendants.

note: depending on the bound plugin, NEW nodes may be assigned identifiers upon being added to the target tree.

As an example, consider the following gDoc tree, where status attributes are depicted as node marker:

Here, the delta tree captures a set of updates to the descendants of a gDoc tree:

the root has acquired or changed an attribute x;
node 2 has lost its child 5 and all its descendants;
node 3 has acquired a child and its descendants;
node 4 has lost the attribute z and the child 6, while the value of its child leaf 7 has changed.

note: all the nodes of the target tree that have not changed do not occur in the delta tree.

String updateRS(String locator) throws GCUBEFault

the operation takes the locator of a ResultSet with delta trees for zero or more (DOM representations of) target gDoc trees and returns the locator of a ResultSet of UpdateFailures (see WSDL definition), one for each delta tree that could not be processed into an update of the corresponding target tree. In particular, clients obtain the identifier of the root of the target tree and the stacktrace of the error that occurred during its update.

note: delta trees in input are contained in AnyHolder wrappers (see WSDL definition).

A WriteManager resource publishes the following Resource Properties:

CollectionID: the identifier of the bound collection, as per OCMA requirements;
TypeID: the access type of the port-type, as per OCMA requirements. The value of this property is constant: gDocWrite;
Plugin: the name of the bound plugin;

Plugins

...coming soon...

Client Libraries

The Content Manager service is developed alongside a number of client libraries that simplify the task of interacting with the service.

The Stub Distribution of the service is the first and foremost client library. Its design goal is to offer abstractions over the content model and interface of the service. In this sense, the distribution acts both as a client library and and a service-side library for plugin developments. In particular, it is a dependency of the service as well as a dependency of service clients.

The Content Management Library (CML) builds upon the stub distribution to support the gCube document model, a concrete document model made of canonical forms for metadata, annotations, document parts, and alternative representations.

Stub Distribution

The distribution includes:

the API for gDoc trees;
the API for gDoc tree predicates;
the stubs of the service automatically generated from the WSDL definition of its port-types;
the high-level calls, a set of abstractions over the service stubs;
a Java protocol handler and associated facilities for deriving and resolving content URLs, i.e. URLs with a service-specific scheme cms.

We have previously presented most of the APIs for gDoc trees and tree predicates. We concentrate here on high-level calls and content URLs, completing the presentation of the tree and tree predicate APIs in the process.

High-Level Calls

High-level calls are Java objects that model single-step or multi-step interactions with the Content Management service. The objects encapsulate stub-based interactions behind local object-oriented interfaces that offer transparencies over the remote interfaces of the service port-types.

The local interfaces are based on language features that are not found in the service stubs, including high-level models of inputs and outputs, method overloading, parametric types, asynchronous callbacks.

Behind these abstractions, the call objects engage in optimised and best-effort interactions with the WS-Resources of the services; in particular, they can hide from clients the complexity of resource discovery while keeping visible the remote nature of the interactions and the possibility of their failure.

High-level calls are defined in the package org.gcube.contentmanagement.contentmanager.stubs.calls. The main components are depicted below:

BaseCall: the base class for all high-level calls.
FactoryCall: a BaseCall that represents calls to the code>Factory</code> resource of the service.
FactoryParams: used in FactoryCall to model the input of operations to the code>Factory</code> resource of the service.
FactoryConsumer: used in FactoryCall to callback invokers of the asynchronous operation of the code>Factory</code> resource of the service.
ManagerCall: an abstract extension of BaseCall for calls to the Collection Managers of the service.
ReadManagerCall: a ManagerCall that represents calls to <core>ReadManager</code> resources of the service.
WriteManagerCall: a ManagerCall that represents calls to <core>WriteManager</code> resources of the service.
MappingRegistry: a central registry of type mappings for I/O.
Constants: a collection of service-specific constants.
Utils: a collection of utilities for I/O conversions.

In what follows, we exemplify the use of FactoryCalls, ReadManagerCalls, and WriteManager.

Factory Calls

A FactoryCall represents a call to the code>Factory</code> resource of the service.

Content Manager

Contents

Design Overview

Content Model

gDoc Trees

gDoc API

Building Trees

Serialising and Deserialising Trees

Consuming Trees

Binding Trees

gDoc Predicates

Predicate API

Building Predicates

Matching and Pruning

Serialising and Deserialising Predicates

Interfaces

Factory

Read Managers

Write Managers

Plugins

Client Libraries

Stub Distribution

High-Level Calls

Factory Calls

ReadManager Calls

WriteManager Calls

Content URLs

Content Management Library

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