A Query Algebra for Fragmented XML Stream Data

1
A Query Algebra for FragmentedXML Stream Data

• Sujoe Bose
• Leonidas Fegaras
• David Levine
• Vamsi Chaluvadi
• University of Texas at Arlington

2
Processing Streamed XML Data

• Most web servers are pull-based
• A client submits a request, the server returns
  the requested data.
• This doesnt scale very well for large number of
  clients and large query results.
• Alternative method pushed-based dissemination
• The server broadcasts/multicasts data in a
  continuous stream
• The client connects to multiple streams and
  evaluates queries locally
• No handshaking, no error-correction
• All processing is done at the client side
• The only task performed by the server is slicing,
  scheduling, and broadcasting data
• Critical data may be repeated more often than
  no-critical data
• Invalid data may be revoked
• New updates may be broadcast as soon as they
  become available.

3
A Framework for Processing XML Streams

• The server slices an XML data source into XML
  fragments. Each fragment
• is a filler that fills a hole
• may contain holes which can be filled by other
  fragments
• is wrapped with control information, such as its
  unique hole ID, the path that reaches this
  fragment, etc.
• The client opens connections to streams and
  evaluates XQueries against these streams
• For large streams, its a bad idea to reconstruct
  the streamed data in clients memory
• need to process fragments as soon they become
  available from the server
• There are blocking operators that require
  unbounded memory
• Sorting
• Joins between two streams or self-joins
• Group-by with aggregation.

4
The Fragmented Hole-Filler Model
ltcommoditiesgt ltvendorgt ltnamegt Wal-Mart
lt/namegt ltitemsgt ltstreamhole
id"10" tsid"5"/gt ltstreamhole id"20"
tsid"5"/gt ... lt/vendorgt
... lt/commoditiesgt
ltstreamfiller id"10" tsid"5"gt ltitemgt
ltnamegt PDA lt/namegt ltmakegt HP lt/makegt
ltmodelgt PalmPilot lt/modelgt ltprice
currency"USD"gt315.25ltpricegt
lt/itemgt lt/streamfillergt
ltstreamfiller id"20" tsid"5"gt ltitemgt
ltnamegt Calculator lt/namegt ltmakegt Casio
lt/makegt ltmodelgt FX-100 lt/modelgt ltprice
currency"USD"gt50.25ltpricegt lt/itemgt lt/streamfi
llergt
5
An Algebra for Stored XML Data

• Based on the nested-relational algebra
• ?v(T) access the XML data source T using v
• ?pred(X) select fragments from X that satisfy
  pred
• ?v1,.,vn(X) project
• X ? Y merge
• X predY join
• ?predv,path (X) unnest (retrieve descendents of
  elements)
• ?pred?,h (X) apply h and reduce by ?
• ?gs,predv,?,h(X) group-by gs, apply h to each
  group,
• and reduce each group by ?

6
Semantics

• ?v(T) lt v T gt
• ?pred(X) t t ? X, pred(t)
• ?v1,.,vn(X) ltv1t.v1,,vnt.vngt t ? X
• X ? Y X Y
• X predY tx ? ty tx ? X, ty ? Y,
  pred(tx,ty)
• ?predv,path(X) t ? ltvwgt t ? X, w ?
  PATH(t,path), pred(t,w)
• ?pred?,h (X) ?/ h(t) t ? X, pred(t)
• ?gs,predv,?,h (X)

7
Example 1
where
8
Example 1 (cont.)
??,element(book,b/title)
?
b/publisherAddison-Wesley and b/_at_year gt 1991
b
?
v/bib/book
v
?
document(http//www.bn.com)
9
Example 2

• for u in document(users.xml)//user_tuple
• return ltusergt u/name
• for b in document(bids.xml
  )//bid_tupleuseridu/userid/itemno
• i in document(items.xml)//ite
  m_tupleitemnob
• return ltbidgt i/description/text()
  lt/bidgt
• sortby(.)
• lt/usergt
• sortby(name)

?
sort, elem(bid,i/description/text())
i/itemnob
sort(u/name), elem(user,u/name