XML Background from HTML to XML HTML Example in XML XML As A

XML Background CS 186 Fall 2002 • eXtensible Markup Language • Roots are HTML and SGML XML – HTML mixes formatting and semantics Databases – SGML is cumbersome • XML is focused on content – Designers (or others) can create their own sets of tags. – These tag definitions can be exchanged and shared “The reason that so many among various groups (DTDs, XSchema). people are excited about XML is – XSL is a companion language to specify presentation. that so many people are excited • XML is ugly about XML.” – Intended to be generated and consumed by applications --- not people! ANON

Bibliography

Example in XML XML as a Wire Format • People quickly figured out that XML is a convenient way to exchange data among applications. Foundations… – E.g. Ford’s purchasing app generates a purchase order Abiteboul in XML format, e-mails it to a billing app at Firestone. Hull – Firestone’s billing app ingests the email, generates a bill Vianu in XML format, and e-mails it to Ford’s bank. Addison Wesley • Emerging standards to get the “e-mail” out of the picture: SOAP, WSDL, UDDI… 1995 • The basis of “Web Services” --- potential impact is tremendous. • Why is it catching on? … It’s just text, so… •Platform, Language, Vendor agnostic •Easy to understand, manipulate and extend. XML describes the content •Compare this to data trapped in an RDBMS.

1 What’s this got to do with Databases? XML – Basic Structure • Given that apps will communicate by exchanging XML – Ingest XML formatted data – Publish their own data in XML format

•Preamble hasXML declaration, E.F.Codd, root element, ref to “DTD” • Thinking a bit harder: A Relational Model •Elements have start and end –XML is kind of a <a href="/tags/Data_model/" rel="tag">data model</a>. of Data for Large Shared Data Banks. Tags – Why convert to/from relational if everyone wants XML? , •Well Formed: has root, proper • More cosmically: 377-387, nesting, … 1970, – Like evolution from spoken language to written language! •Valid: Conforms to DTD 13, •Note that order matters (i.e. no • The (multi-) Billion Dollar Question: CACM, 6, sets, only lists) – Will people really want to store XML data directly? db/journals/cacm/cacm13.html#Codd70 – Current opinion: ORACLE, IBM, INFORMIX say no, other db/journals/cacm/Codd70.html DB vendors say Yes, or at least, “Maybe” CACMs1/CACM13/P377.pdf

Another (partial) Example Can View XML Document as a Tree ABC Corp. Invoice as a tree

Mapping to Relational New splinters from XML Very expensive to store variable • Relational systems handle highly structured document types data

• Question: What is a relational schema for • Can leverage Schema (or DTD) information to storing XML data? create relational schema. • Answer – Depends on how “Structured” it is… • Sometimes called “shredding” • If unstructured – use an “Edge Map” • For semi-structured data use hybrid with edge map for overflow. ParentLabel ID 0 NULL article 0 article STORED table Overflow buckets 0 author 1 (author, year, journal, …) article 1 E.F. Codd NULL 1 2 3 4 5 … 0 pages 2 author pages year journal number … 2 377-387 NULL author pages year journal cdrom … … … E.F. 377- 1970 CACM 6 Codd 387 E.F. 377- 1970 CACM P377.pdf Codd 387

Other XML features Document Type Definitions (DTDs) • Elements can have “attributes” (not clear why). • Grammar for describing the allowed structure of XML Documents. 1.50 • Specify what elements can appear and in what order, nesting, etc. • XML docs can have IDs and IDREFs, URIs • DTDs are optional (!) – reference to another document or document element • Many “standard” DTDs have been developed • Two APIs for interacting with/parsing XML Docs: for all sorts of industries, groups, etc. –Document Object Model (DOM) – e.g. NITF for news article dissemination. • A tree “object” API for traversing an XML doc • Typically for Java –SAX • Event-Driven: Fire an event for each tag encountered during parse. • May not need to parse the entire document.

Here’s a DTD for a Contract • XML Schema is a proposal to replace/augment DTDs – Has a notion of types and typechecking – Quite complicated, controversial ... not really ? = 0 or 1 * = 0 or more adopted yet Namespaces segmentKey %string; #IMPLIED > – Can import tag names from others – Disambiguate by prefixing the namespace name

• Xpath – Navigation is defined by “paths” – A single-document language for “path expressions” – Used by other standards: XSLT, XQuery, XPointer,XLink • / = root node or separator between steps in path • XSLT – XPath plus a language for formatting output • * matches any one element name • @ references attributes of the current node • XQuery • // references any descendant of the current node – An SQL-like proposal with XPath as a sub-language – Supports aggregates, duplicates, … • [] allows specification of a filter (predicate) at a – Data model is lists, not sets step – “reference implementations” have appeared, but language is • [n] picks the nth occurrence from a list of elements. still not widely accepted.

• SQL/XML • The fun part: – the SQL standards community fights back Filters can themselves contain paths

XPath Examples XQuery • Parent/Child (‘/’) and Ancestor/Descendant (‘//’): /catalog/product//msrp • Wildcards (match any single element): FOR $x in /bib/book WHERE $x/year > 1995 /catalog/*/msrp RETURN • Element Node Filters to further refine the nodes: $x/title – Filters can contain nested path expressions //product[price/msrp < 300]/name //product[price/msrp < /dept/@budget]/name – Note, this last one is a kind of “join”

Main Construct (replaces SELECT-FROM-WHERE): • FOR $x in expr -- binds $x to each value in the • FLWR Expression: FOR-LET-WHERE-RETURN list expr

FOR/LET Clauses • LET $x = expr -- binds $x to the entire list Ordered List of tuples expr – Useful for common subexpressions and for WHERE Clause aggregations Filtered list of tuples

• ASCII makes things easy FOR $p IN distinct(document("bib.xml")//publisher) –Easy to parse LET $b := document("bib.xml")/book[publisher = $p] – Easy to ship (e.g. across firewall, via email, etc.) • Self-documenting WHERE count($b) > 100 – Metadata (tag names) come with the data RETURN $p • Nested – Can bundle lots of related data into one message – (Note: object-relational allows this) • Can be sloppy –don’t have to define a schema in advance distinct = a function that eliminates duplicates • Standard count = a (aggregate) function that returns the number of elms – Lots of free Java tools for parsing and munging XML • Expect lots of Microsoft tools (C#) for same • Tremendous Momentum!

What XML does not solve Reminder: Benefits of Relational • XML doesn’t standardize metadata – It only standardizes the metadata language • Data independence buys you: • Not that much better than agreeing on an alphabet – Evolution of storage -- vs. XML? – E.g. my tag vs. your tag – Evolution of schema (via views) – vs. XML? • Mine includes shipping and federal tax, and is in $US • Database design theory • Yours is manufacturer’s list price in ¥Japan – IC’s, dependency theory, lots of nice tools for ER – XML Schema is a proposal to help with some of this • Remember, databases are long-lived and reused • XML doesn’t help with data modeling – Today’s “nesting” might need to be inverted tomorrow! – No notions of IC’s, FD’s, etc. • Issues: – In fact, encourages non-first-normal form! – XML is good for transient data (e.g. messages) • You will probably have to translate to/from XML (at – XML is fine for data that will not get reused in a different least in the short term) way (e.g. Shakespeare, database output like reports) – Relational vendors will help with this ASAP – Relational is far cleaner for persistent data (we learned this – XML “features” (nesting, ordering, etc.) make this a pain with OODBs) – Flatten the XML if you want data independence (?) • Will benefits of XML outweigh these issues?????

• 100s of books published – Each seems to be 1000 pages • Try some websites – xml.org provides a business software view of XML – xml.apache.org has lots of useful shareware for XML – www.ibm.com/developerworks/xml/ has shareware, tutorials, reference info – xml.com is the O’Reilly resource site – www.w3.org/XML/ is the official XML standard site – the most standardized XML dialects are: • Ariba’s Commerce XML (“cxml”, see cxml.org) • RosettaNet (see rosettanet.org) • Microsoft trying to enter this arena (BizTalk, now .NET)