computer science

XML catalog

XML catalog is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand XML catalog rather than just read about it. In short: XML documents typically refer to external entities, for example the public and/or system ID for the Document Type Definition. These external relationships are expressed using URIs, typically as URLs.

Key takeaways

XML catalog belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect XML catalog to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of XML catalog from memory before moving on to harder problems.

Reference excerpt

XML documents typically refer to external entities, for example the public and/or system ID for the Document Type Definition. These external relationships are expressed using URIs, typically as URLs. However absolute URLs only work when the network can reach them. Relying on remote resources makes XML processing susceptible to both planned and unplanned network downtime. Relative URLs are only useful in the context where they were initially created. For example, the URL "../../xml/dtd/docbookx.xml" will usually only be useful in very limited circumstances. One way to avoid these problems is to use an entity resolver (a standard part of SAX) or a URI Resolver (a standard part of JAXP). A resolver can examine the URIs of the resources being requested and determine how best to satisfy those requests. The XML catalog is a document describing a mapping between external entity references and locally cached equivalents.

Example Catalog.xml The following simple catalog shows how one might provide locally cached DTDs for an XHTML page validation tool, for example.

This catalog makes it possible to resolve -//W3C//DTD XHTML 1.0 Strict//EN to the local URI dtd/xhtml1/xhtml1-strict.dtd. Similarly, it provides local URIs for two other public IDs. Note that the document above includes a DOCTYPE – this may cause the parser to attempt to access the system ID URL for the DOCTYPE (i.e. http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd) before the catalog resolver is fully functioning, which is probably undesirable. To prevent this, simply remove the DOCTYPE declaration. The following example shows this, and also shows the equivalent <system/> declarations as an alternative to <public/> declarations.

Using a catalog – Java SAX example Catalog resolvers are available for various programming languages. The following example shows how, in Java, a SAX parser may be created to parse some input source in which the org.apache.xml.resolver.tools.CatalogResolver is used to resolve external entities to locally cached instances. This resolver originates from Apache Xerces but is now included with the Sun Java runtime. It is necessary to create a SAXParser in the standard way by using factories. The XML reader entity resolver should be set to the default or to a customly-made one.

It is important to call the parse method on the reader, not on the SAX parser.

External references XML Catalogs, OASIS Standard V1.1, 7 October 2005 XML Entity and URI Resolvers, Sun XML Catalog Manager project on SourceForge

Worked examples

Example 1 — a first encounter with XML catalog

Start with the simplest possible case. Write down what XML catalog claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to XML catalog before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about XML catalog ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of XML catalog

In research: XML catalog appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses XML catalog in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: XML catalog is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer file formats, Markup languages, XML, so understanding it makes those chapters shorter.
In everyday life: Look for XML catalog outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study XML catalog in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what XML catalog means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain XML catalog out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is XML catalog in simple terms?

XML documents typically refer to external entities, for example the public and/or system ID for the Document Type Definition. These external relationships are expressed using URIs, typically as URLs.

Why does XML catalog matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study XML catalog?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on XML catalog.