GCube Document Library (2.0)

The gCube Document Library (gDL) is a client library for storing, updating, deleting and retrieving document description in a gCube infrastructure.

The gDL is a high-level component of the subsystem of gCube Information Services and it interacts with lower-level components of the subsystem to support document management processes within the infrastructure:

• the gCube Document Model (gDM) defines the basic notion of document and the gCube Model Library (gML) implements that notion into objects;
• the objects of the gML can be exchanged in the infrastructure as edge-labelled trees, and the Content Manager Library (CML) can model such trees as objects and dispatch them to the read and write operations of the Content Manager (CM) service;
• the CM implements these operations by translating trees to and from the content models of diverse repository back-ends.

The gDL builds on the gML and the CML to implement a local interface of CRUD operations that lift those of the CM to the domain of documents, efficiently and effectively.

Preliminaries

The core functionality of the gDL lies in its operations to read and write documents. The operations trigger interactions with remote services and the movement of potentially large volumes of data across the infrastructure. This may have a non-trivial and combined impact on the responsiveness of clients and the overall load of the infrastructure. The operations have been designed to minimise this impact. In particular:

• when reading, clients can qualify the documents that are relevant to their queries, and indeed what properties of relevant documents should be actually retrieved. These retrieval directives are captured in the gDL by the notion of document projections.

• when reading and writing, clients can move large numbers of documents across the infrastructure. The gDL streams this I/O movements so as to make efficient use of local and remote resources. It then defines a facilities with which clients can conveniently consume input streams, produce output streams, and more generally filter one stream into an other regardless of their origin. The facilities are collected into the stream DSL, an embedded domain-specific language for stream processing.

Understanding document projections and the stream DSL is key to reading and writing documents effectively. We discuss these preliminary concepts first, and then consider their use as input and outputs of the operations of the gDL.

Projections

A projection is a set of constraints over the properties of documents in the gDM. It can be used to match documents, i.e. identify documents whose properties satisfy the constraints of the projection.
Projections and matching are used in the read operations of the gDL:

• as a means to characterise relevant documents (projections as types);
• as a means to specify what parts of relevant documents should be retrieved (projections as retrieval directives).

The constraints of a projection take accordingly two forms:

• include constraints apply to properties that must be matched and retrieved;
• filter constraints apply to properties that must be matched but not retrieved.

note: in both cases, the constraints take the form of 'predicates' of the Content Manager Library] (CML). The projection itself converts into a complex predicate which is amenable for processing by the Content Manager service in the execution of retrieval operations. In this sense, projections are a key part of the document-oriented layer that the gDL defines over lower-level components of the service subsystem for content management.

As a simple example, a projection may define an include constraint over the name of metadata elements and a filter constraint over the time of their last update.
It may then be used to:

• characterise documents with metadata elements that match both constraints;
• retrieve of those documents only the name of matching metadata elements, excluding any other document property, including other inner elements and their properties.

All projections in the gDL have the Projection interface, which can be used in element-generic computations to access their constraints. To build projections, however, clients deal with one of the following implementation of the interface:

• DocumentProjection
• MetadataProjection
• AnnotationProjection
• PartProjection
• AlternativeProjection

A further implementation of the interface:

• PropertyProjection

allows clients to express constraints on the generic properties of any of the elements of the gDM.

Simple Projections

Clients create projections with the factory methods of the Projections companion class (a static import improves legibility and is recommended):

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;...
DocumentProjection dp = document();
 
MetadataProjection mp = metadata();
 
AnnotationProjection annp = annotation();
 
PartProjection pp = part();
 
AlternativeProjection altp = alteranative();

The projections above do not specify any include or filter constraints on the elements of the corresponding type. For example, dp matches all documents, regardless of their properties, inner elements, and properties of their inner elements. Similarly, mp matches all metadata elements of any document, regardless of their properties, and pp matches all the parts of any document, regardless of their properties. Thus the factory methods of the Projections class return empty projections.

Clients may add include constraints to a projection with the method with() declared by all projection classes. For document projections, for example:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().with(NAME);

With the above, the client adds the simplest form of constraint, an existence constraint that requires the target elements to have given properties, here the document to have name. Since this is an include constraint, the client is expressing an interest only in this property, regardless of the existence and values of other properties. Used as a parameter in the read operations of the gDL, this projection is translated into a directive to retrieve only the names of document(s) that have one.

note: properties are conveniently represented by constants in the Projections class. The constants are not strings, however, but dedicated Property objects that are specific to the type of projection. Trying to use properties that are undefined for the type of elements targeted by the projection is illegal and the error is detected statically.

Existence constraints may be expressed at once on multiple properties, e.g.:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().with(NAME,LANGUAGE,BYTESTREAM);

Besides inclusion constraints, clients may specify filter constraints with the method where() on projections, e.g:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().where(NAME,LANGUAGE);

Now, the client still requires documents to have a name and a language but he retains an interest in the other properties of matching documents. Used as a parameter in the read operations of the gDL, this projection is translated into a directive to retrieve all the properties of documents with a name.

Include and filter constraints can be combined, and the projections classes follow a builder pattern to add readability to the combinations. In particular, with() and where() return the very projection on which they are invoked. They may then be used as follows:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().with(NAME,SCHEMA_URI)
                                  .where(BYTESTREAM);

Here, the client requires documents to have a name and embed a bytestream that conforms to a schema, but he has an interest in processing only document names and schema URIs (e.g. for display purposes). Used as a parameter in the read operations of the gDL, this projection retrieves the requested information but avoids the transmission of bytestreams.

Optional Modifiers

Moving now beyond the simple existence of properties, another common requirement is to indicate the optionality of properties. Clients may wish to include certain properties, or equivalently filter by certain properties, if and only if these actually exists. In this case, clients can use the opt() of the Projections class as a constraint modifier, as this example illustrates:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().with(NAME,opt(SCHEMA_URI))
                                  .where(BYTESTREAM);

This projection differs from the previous one only because of the optionality constraint on the existence of a schema for the document's bytestream. Used as a parameter in the read operations of the gDL, this projection retrieves the name all documents that include a bytestream, but also their schema URI if they happen to have one.

A common use of optional modifier is with bytestream, which clients may wish either to find included in the document or else referred to with a URL:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().with(opt(BYTESTREAM),opt(URL));

Used as a parameter in the read operations of the gDL, this projection retrieves at most the bytestream and its URL for those documents that have both, only one of the two if the other is missing, and nothing at all if they are both missing.

note: The API allows optional modifiers in filter constraints too, but their application is rather pointless in this context (they will never elements from retrieval).

Deep Projections

In the examples above, we have considered existence constraints on simple element properties. The examples generalise easily to repeated structured properties, such as generic properties for all elements and inner element properties for documents.

Consider the following example:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
DocumentProjection dp = document().with(PART, opt(METADATA), PROPERTY);

Here the client adds three include constraints to the projection, all three for the existence of repeated properties. Documents that match this projection have at least one part, at least one generic property, and zero or more metadata elements. Used as a parameter in the read operations of the gDL, this projection retrieves all' the parts and all the generic properties of documents that have at least one of each, as well as all of their the metadata elements if they happen to have some.

Repeated properties such as generic properties and inner elements are also structured, i.e. have properties of their own. Clients that wish to constrain those properties too can use deep projections, i.e. embed within the projection of a given type one or more projections built for the structured properties of elements of that type. The following example illustrates the concept for metadata elements:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
MetadataProjection mp = meatadata().with(LANGUAGE).where(BYTESTREAM);
 
DocumentProjection dp = document().with(NAME, PART)
                                  .with(METADATA,mp);

The first projection constraints the existence of language and bytestream for metadata elements. The second projection constraints the existence of name and parts for document, as well as the existence of metadata elements that match the constraints of the first projection. The usual implications of include constraints and filter constraints apply. Used as a parameter in the read operations of the gDL, this projection retrieves the name, parts, and metadata elements of documents that have a name, at least one part, and at least one metadata element that includes a bystream. For the metadata elements, in particular, it retrieves only the language property.

Note that optionality constraints apply to deep projections as well as they apply to flat projections, as the following example shows:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
MetadataProjection mp = meatadata().with(LANGUAGE).where(BYTESTREAM);
 
DocumentProjection dp = document().with(NAME, PART)
                                  .with(opt(METADATA,mp));

This projection differs from the previous one only because the existence of on metadata elements that match the inner projection is optional. Documents that have a name and at least one part match the outer projection even if the have no metadata elements that match the inner projection (or no metadata elements at all).

Projections over Generic Properties

Generic properties are repeated and structured properties common to all elements. As for other properties with these characteristics, clients may wish to build deep projections that constraints their inner properties. For this purpose, the class Projections includes a dedicated factory method property(), as well as as specialised methods to express constraints. The following example illustrates the approach:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
...
 
PropertyProjection pp = property().withKey("somekey").with(PROPERTY_TYPE);
 
DocumentProjection dp = document().with(NAME, PART)
                                  .with(PROPERTY,pp);

Here, the client creates a document projection and embeds in it an inner projection that constrains its generic properties. The inner projection uses the method with() to add an include constraint for the existence of a type for the generic property, as usual. It also adds an include constraint to specify an exact value for the key of a generic property of interest. This relies on a method withKey() which is specific to projection over generic properties of elements. The reason for this specific construct is that, differently from other constrainable properties of elements, they key of a generic property serves as its identifier.

For the rest, property projections behave like other projections (e.g. can be used with optional modifiers). Used as a parameter in the read operations of the gDL, the projection above matches documents with a name, at least one part, and a property with key somekey and some type.

Advanced Projections

In more advanced forms of projections, clients may wish to specify constraints on properties other than mere existence. In these cases, they can use overloads of with() and where() that take as parameters Predicates that capture the desired constraints. As mentioned above, predicates are defined in the CML and gDL clients need to become acquainted with the range of available predicates and how to build them.

note: Deep projections already make use of this customisability. When clients embed a projection into another, they constrain the corresponding structured property with the predicate into which the inner projection translates.

Commonly, clients may wish to constrain the value of a property, as in the following example:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
import static org.gcube.contentmanagement.contentmanager.stubs.model.constraints.Constraints.*;import static org.gcube.contentmanagement.contentmanager.stubs.model.predicates.Predicates.*;...
DocumentProjection p = document().with(LANGUAGE,text(is("it"));

The client uses here the predicate text(is("it")) to constrain the language of documents to match the ISO639 code for the Italian language. As documented in the CML, the client builds the predicate with the static methods of the Predicates and Constraints classes, which he previously imports.

note: in building predicate expressions with the API of the CML, clients take responsibility for associating properties with predicates that are compatible with their type. In the example above, the language of an element is a textual property and thus only text()-based predicates can successfully match it. The gDL relinquishes the ability to ensure the correct construction of projections so as to allow clients to use the full expressiveness of the predicate language of the CML.

The type of constraints that can be expressed on properties is thus bound by the expressiveness of the predicate language of the CML. We include here another example to illustrate some of the possibilities:

import static org.gcube.contentmanagement.gcubedocumentlibrary.projections.Projections.*;
import static org.gcube.contentmanagement.contentmanager.stubs.model.constraints.Constraints.*;
import static org.gcube.contentmanagement.contentmanager.stubs.model.predicates.Predicates.*;
...
Calendar from = ...
Calendar until = ....
DocumentProjection p = document().with(URL,uri(matches("^ftp.*")));
                                 .where(CREATION_TIME,date(all(after(from),before(to))));

This projection is matched by documents that have been created at some point in between two dates, and with a bytestream available at some ftp server. Used as a parameter in the read operations of the gDL, the projection would retrieve only the URL of (the bytestream of) matching documents.