[Mulgara-general] Two Database instances in the same process?

[Karsten Huneycutt]

Hello --

First:  Does Mulgara 2.1.3 allow one to have two distinct Database  
instances (and therefore two distinct XA1.1 StringPools, etc) in the  
same process?

If it doesn't, then I'll stop trying to do that.   If it does, then I  
must be doing something incorrectly...

A bit of background as to why I'm trying to do this:  I need to have  
something that applies rules to a set of graphs (that the consuming  
application accesses via a view) in the background that doesn't stop  
modification of the original graphs while the rules are being  
applied.  Each modification of the original graphs triggers a  
background reasoning run, so any entailments that are invalidated by  
the modifications will be corrected on the next pass of the reasoner;  
for me, this temporary inaccuracy is an acceptable trade-off for the  
responsiveness of modification that I require.

I first tried to copy the sub-graphs over which I wanted to reason  
into a separate graph in the same database, do the reasoning into a  
separate entailments graph, and then copy the results back into an  
entailments graph in the view.  That didn't work, of course -- as I  
learned, the "apply" command write-locks the entire database, not just  
the graphs on which it's applying, which holds up modification of the  
original graphs for the three to five minutes while the entailments  
are being calculated.

So, I went down the road of copying the graphs to a separate database  
in the same process.

The following example code shows what I'm trying to do:

public class TwoDatabases {

     public static void main(String[] args) {
	Database db1 = null;
	Database db2 = null;
	try {
	    URL l4j = TwoDatabases.class.getClassLoader().getResource("conf/ 
log4j-mulgara.xml");
	    org.apache.log4j.xml.DOMConfigurator.configure(l4j);
	
	    MulgaraUserConfig cfg1 = new MulgaraUserConfig("file:///tmp/mulgara-config1.xml 
");
	    MulgaraUserConfig cfg2 = new MulgaraUserConfig("file:///tmp/mulgara-config2.xml 
");
	
	    File path1 = new File("/tmp/path1");
	    File path2 = new File("/tmp/path2");
	
	    URI uri1 = new URI("local:db1");
	    URI uri2 = new URI("local:db2");
	
	    db1 = DatabaseFactory.newDatabase(uri1, path1, cfg1);
	
	    db2 = DatabaseFactory.newDatabase(uri2, path2, cfg2);
	
	} catch (Exception e) {
	    e.printStackTrace();
	} finally {
	    if (db1 != null) db1.close();
	    if (db2 != null) db2.close();
	}
     }

}

/tmp/mulgara-config1.xml is:

<MulgaraConfig>
   <PersistencePath>/tmp/path1</PersistencePath>
</MulgaraConfig>

and /tmp/mulgara-config2.xml is

<MulgaraConfig>
   <PersistencePath>/tmp/path2</PersistencePath>
</MulgaraConfig>


db1 is created normally, and all is well with it.  However, when  
DatabaseFactory attempts to create db2, I get the following exception:

java.io.IOException: Lock file busy (internal): /tmp/path1/ 
xaStringPool/xa11.sp.lock
	at org.mulgara.store.xa.LockFile.lockInternal(LockFile.java:151)
	at org.mulgara.store.xa.LockFile.<init>(LockFile.java:95)
	at org.mulgara.store.xa.LockFile.createLockFile(LockFile.java:137)
	at org.mulgara.store.xa.LockFile.createLockFile(LockFile.java:123)
	at org.mulgara.store.stringpool.xa11.XA11StringPoolImpl.<init> 
(XA11StringPoolImpl.java:166)
	at  
org.mulgara.store.stringpool.xa11.XA11StringPoolFactory.newStringPool 
(XA11StringPoolFactory.java:91)
	at org.mulgara.resolver.StringPoolSessionFactory.<init> 
(StringPoolSessionFactory.java:134)
	at org.mulgara.resolver.Database.<init>(Database.java:867)
	at org.mulgara.resolver.Database.<init>(Database.java:629)
	at org.mulgara.resolver.DatabaseFactory.newDatabase 
(DatabaseFactory.java:97)
	at TwoDatabases.main(TwoDatabases.java:27)
org.mulgara.store.stringpool.StringPoolException: Couldn't construct  
string pool
	at  
org.mulgara.store.stringpool.xa11.XA11StringPoolFactory.newStringPool 
(XA11StringPoolFactory.java:95)
	at org.mulgara.resolver.StringPoolSessionFactory.<init> 
(StringPoolSessionFactory.java:134)
	at org.mulgara.resolver.Database.<init>(Database.java:867)
	at org.mulgara.resolver.Database.<init>(Database.java:629)
	at org.mulgara.resolver.DatabaseFactory.newDatabase 
(DatabaseFactory.java:97)
	at TwoDatabases.main(TwoDatabases.java:27)
Caused by: java.io.IOException: Lock file busy (internal): /tmp/path1/ 
xaStringPool/xa11.sp.lock
	at org.mulgara.store.xa.LockFile.lockInternal(LockFile.java:151)
	at org.mulgara.store.xa.LockFile.<init>(LockFile.java:95)
	at org.mulgara.store.xa.LockFile.createLockFile(LockFile.java:137)
	at org.mulgara.store.xa.LockFile.createLockFile(LockFile.java:123)
	at org.mulgara.store.stringpool.xa11.XA11StringPoolImpl.<init> 
(XA11StringPoolImpl.java:166)
	at  
org.mulgara.store.stringpool.xa11.XA11StringPoolFactory.newStringPool 
(XA11StringPoolFactory.java:91)
	... 5 more

Note in particular that it says "/tmp/path1/...", which was correct  
for db1 but not for db2!  What am I doing incorrectly?

I'm currently shipping the reasoning off to a separate Mulgara  
process, using temporary files and RMI, but that seems inefficient,  
and I'd like to keep it all in the same process.

Alternately, is there a better way to do what I described than using a  
separate database (be it in the same process or a separate process)?   
(Note that, for now, I don't care so much about memory use or disk use.)

My apologies if these questions have already been asked and answered;  
I couldn't find the answer in the archives of the lists or in the wiki.

Thanks!

KH

-- 
Karsten Huneycutt
kph at skuld.us