[rules-users] Integration of Drools with Hadoop

[Mark Proctor]

On 31/10/2010 05:35, Roger Smith wrote:
>
> We are working on a project to integrate Drools with Apache Hadoop, 
> http://hadoop.apache.org/, and run into some road blocks. We would 
> very much appreciate any suggestions/help from this list.
>
> We have an app where we call drools rule engine inside the reducer 
> task of a Hadoop map reduce job. It throws a NullPointerException when 
> we the rule package resource to the knowledge builder. The same code 
> works fine when run as part of a stand alone app.
>
> Code:
>
>     private static Map<String, StatefulKnowledgeSession> sessions =
>             new HashMap<String, StatefulKnowledgeSession>();
>     private static final String RULE_PACK_DIR = 
> "file:///home/roger/Projects/gridx/ 
> <file:///%5C%5Chome%5Cpranab%5CProjects%5Cgridx%5C>";
>     private static final String RULE_PACK_EXT = ".drl";
>
>
>     public int process(String rulePackage, String dateTime, String 
> type) throws TException {
>         int rate = 0;
>         StatefulKnowledgeSession session = sessions.get(rulePackage);
>         if (null == session){
>             KnowledgeBuilder kbuilder = 
> KnowledgeBuilderFactory.newKnowledgeBuilder();
>             String rulePackPath = RULE_PACK_DIR + rulePackage + 
> RULE_PACK_EXT;
>             kbuilder.add( ResourceFactory.newFileResource(rulePackPath 
> ), ResourceType.DRL);
>             if ( kbuilder.hasErrors() ) {
>                 System.err.println( kbuilder.getErrors().toString() );
>             }
>             KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
>             kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());
>
>             session = kbase.newStatefulKnowledgeSession();
>             sessions.put(rulePackage, session);
>         }
>
>         ContractRule contractRule = new ContractRule();
>         contractRule.prepare(dateTime, type);;
>         FactHandle ruleHandle = session.insert(contractRule);
>         session.fireAllRules();
>         System.out.println("" + contractRule);
>         rate = contractRule.getRate();
>         session.retract(ruleHandle);
>
>         return rate;
>     }
>
> This line throws the exception:
> kbuilder.add( ResourceFactory.newFileResource(rulePackPath ), 
> ResourceType.DRL);
>
ResourceFactory.newFileResource just creates a file object but cleaning 
up the paths.
     public FileSystemResource(File file) {
         if ( file == null ) {
             throw new IllegalArgumentException( "File must not be null" );
         }
         this.file = new File( StringUtils.cleanPath(file.getPath()) );
     }
The code is borrowed from Spring and uses the sme cleanPath logic. Once 
the file is created it just creates the stream as so:
     public InputStream getInputStream() throws IOException {
         this.lastRead = getLastModified();
         return new FileInputStream(this.file);
     }

So it's pretty basic, the stack trace with line numbers may help. But I 
suspect you'll need to do a debug and step into the drools code at the 
point where you get the nullpointer and see what's going on.

Mark

>
> It works fine as a stand alone app, outside hadoop
>
> Roger Smith
>
>
>
>
>
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