[Lucene-hadoop Wiki] Update of "Hbase/RDF" by udanax

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- [[TableOfContents(4)]]
- ----
- == HbaseRDF, a Planet-Scale RDF Data Store ==
  
- We have started to think about storing and querying RDF data in Hbase. But we'll jump into its implementation after prudence investigation. 
- 
- We introduce an Hbase subsystem for RDF, called HbaseRDF, which uses Hbase + !MapReduce to store RDF data and execute queries (e.g., SPARQL) on them.
- We can store very sparse RDF data in a single table in Hbase, with as many columns as 
- they need. For example, we might make a row for each RDF subject in a table and store all the properties and their values as columns in the table. 
- This reduces costly self-joins in answering queries asking questions on the same subject, which results in efficient processing of queries, although we still need self-joins to answer RDF path queries.
- 
- We can further accelerate query performance by using !MapReduce for 
- parallel, distributed query processing. 
- 
- === Related projects ===
- 
-  * [:Hbase/HbaseShell: Hbase Shell] provides a command line tool in which we can manipulate tables in Hbase. We are also planning to use !HbaseShell to manipulate and query RDF data stored in Hbase.
-  * [http://www.openrdf.org/forum/mvnforum/viewthread?thread=1423 A forum at Aduna/Sesame] would be interested in working with this group.
-  
- === Initial Contributors ===
- 
-  * [:udanax:Edward Yoon] (R&D center, NHN corp.)
-  * [:InchulSong: Inchul Song] (Database Lab, KAIST) 
- 
- ----
- == Some Ideas ==
- When we store RDF data in a single Hbase table and process queries on them, an important issue we have to consider is how to efficiently perform costly self-joins needed to process RDF path queries. 
- 
- To speed up these costly self-joins, it is natural to think about using 
- the !MapReduce framework we already have. However, in the Sawzall paper from Google, the authors say that the !MapReduce framework is 
- not good, or inappropriate for performing table joins. 
- It is possible, but while we are reading one table in map 
- or reduce functions, we have to read other tables on the fly, which
- results in less parallelized join processing.
- 
- There is a paper on this subject written by Yang et al., from Yahoo (SIGMOD 07). 
- The paper provides Map-Reduce-Merge, which is an extended version of the !MapReduce framework, 
- that implements several relational operators, including joins. They have extended the 
- !MapReduce framework with an additional Merge phase to implement efficient data relationship processing.
- See the Paper section below for more information. -- Thanks stack.
- (Edward is now implementing join operators using the !MapReduce framework.)
- 
- But the problem is that there is an initial delay in executing !MapReduce jobs due to 
- the time spent in assigning the computations to multiple machines. This 
- might take far more time than necessary, thus hurt query response time. So, parallelism obtained by using !MapReduce is best enjoyable for queries over huge amount of RDF data, where it takes much time to process them. 
- We might consider a selective parallelism where 
- people can decide whether to use !MapReduce or not to process their queries, as in 
- "select ... '''in parallel'''".
- 
- Now that we have two sets of join algorithms, non-parallel versions and parallel versions with !MapReduceMerge,
- we are ready to do some massive parallel query processing on tremendous amount of RDF data.
- Currently, C-Store shows the best query performance on RDF data.
- However, we, armed with Hbase and !MapReduceMerge, can do even better.
- ----
- == Resources ==
-  * http://www.w3.org/TR/rdf-sparql-query/ - The SPARQL RDF Query Language, a candidate recommendation of W3C as of 14 June 2007.
-  * A test suit for SPARQL can be found at http://www.w3.org/2001/sw/DataAccess/tests/r2. The web page provides test RDF data, SPARQL queries, and expected results.
-  * [https://jena.svn.sourceforge.net/svnroot/jena/ARQ/trunk/Grammar/sparql.jj SPARQL Grammer in JavaCC] - from Jena ARQ
-  * [http://esw.w3.org/topic/LargeTripleStores Large triple stores]
-  * [http://web.mit.edu/dna/www/abadirdf.pdf Scalable Semantic Web Data Management Using Vertical Partitioning] Good summary of techniques storing RDF in RDBMS.
- 
- == Architecture Sketch ==
- 
- === HbaseRDF Data Loader ===
- HbaseRDF Data Loader (HDL) reads RDF data from a file, and organizes the data 
- into a Hbase table in such a way that efficient query processing is possible. In Hbase, we can store everything in a single table.
- The sparsicy of RDF data is not a problem, because Hbase, which is 
- a column-based storage and adopts various compression techniques, 
- is very good at dealing with nulls in the table
- 
- === HbaseRDF Query Processor ===
- HbaseRDF Query Processor (HQP) executes RDF queries on RDF data stored in a Hbase table. 
- It translates RDF queries into API calls to Hbase, or !MapReduce jobs, gathers and returns the results
- to the user. 
- 
- Query processing steps are as follows:
- 
- {{{
- SPARQL query -> Parse tree -> Logical operator tree 
- -> Physical operator tree -> Execution
- }}}
- 
- Implemenation of each step may proceed as an individual issue. 
- 
- === HbaseRDF Data Materializer ===
- HbaseRDF Data Materializer (HDM) pre-computes RDF path queries and stores the results
- into a Hbase table. Later, HQP uses those materialized data for efficient processing of 
- RDF path queries. 
- 
- === Hbase Shell Extension ===
- 
- {{{
- Hbase > rdf;
- 
- Hbase RDF version 0.0.1
- Type 'help;' for help.
- 
- Hbase.RDF > SELECT ?title
-         --> FROM rdf_table
-         --> WHERE { ?book author ‘‘Fox, Joe’’
-         -->         ?book copyright ‘‘2001’’
-         -->         ?book title ?title }
- 
- results here.
- 
- Hbase.RDF > exit;
- 
- Hbase > 
- }}}
- ----
- == Alternatives For RDF Storage ==
-  * A triples table stores RDF triples in a single table with three attributes, subject, property, and object.
-  * A property table. Put properties frequently queried togather into a single table to reduce costly self-joins. Used in Jena and Oracle. 
-  * A dicomposed storage model (DSM), one table for each property, sorted by the subject. Used in C-Store.
- ----
- == Papers ==
- 
-  * OSDI 2004, ''!MapReduce: Simplified Data Processing on Large Clusters'', proposes a very simple, but powerfull, and highly parallelized data processing technique.
-  * CIDR 2007, ''[http://db.lcs.mit.edu/projects/cstore/abadicidr07.pdf Column-Stores For Wide and Sparse Data]'', discusses the benefits of using C-Store to store RDF and XML data.
-  * VLDB 2007, ''[http://db.lcs.mit.edu/projects/cstore/abadirdf.pdf Scalable Semantic Web Data Management Using Vertical Partitoning]'', proposes an efficient method to store RDF data in table projections (i.e., columns) and executes queries on them.
-  * SIGMOD 2007, ''Map-Reduce-Merge: Simplified Relational Data Processing on Large Clusters'', !MapReduce implementation of several relational operators.
-