Error when using Bitsy in a WAR

[xlorepdarkhelm]

When I build a bitsy graph, and expose it to two different servlets, it works on the first servlet, but when I try the second servlet, I get the following error:

com.lambdazen.bitsy.BitsyException: INSTANCE_ALREADY_EXISTS: A BitsyGraph object with the same path has been registered with the MBeanServer. Creating multiple instances of BitsyGraph (without calling shutdown) will cause data corruption. Path C:\Users\QTB0065\AppData\Local\Temp\pampas\graph

I am not sure how I would go about solving this.

I am attempting to use Bitsy in a Glassfish 3.1 environment, through Jython, to handle a relatively small graph. I have successfully been able to create the graph, but I am now unable to continue and use the graph with other servlets.

My eventual goal would be to have a single process that is updating my graph (or builds the graph the first time), but multiple processes that can access the graph and view its contents read-only. The building/updating process might be a scheduled EJB, while the read-only functionality will be RESTful servlets.

Please help.

[xlorepdarkhelm]

This error implies that the BitsyGraph exists already, somewhere (JMX?). Is there a way I can find that instance from one servlet to the other?

[Sridhar Ramachandran]

You are right about JMX. Check out line 80 in https://bitbucket.org/lambdazen/bitsy/src/26997f2ac8c23c00e576f1d4eacf4205ae379b5b/src/main/java/com/lambdazen/bitsy/BitsyGraph.java?at=master&fileviewer=file-view-default

Every BitsyGraph object is like an instance of a database. Only one object can be created for a path, otherwise the file operations will overwrite each other. However, the database object is thread-safe and you can operate on the same object from any number of threads. In other words, your servlet container can hit the same BitsyGraph object from any number of servlets simultaneously.

Would you be able to store it in the ServletContext object as recommended at http://stackoverflow.com/questions/123657/how-can-i-share-a-variable-or-object-between-two-or-more-servlets

The best time to create the object is when the application is loaded. This avoids the first servlet hit from paying the price for the database startup. I think the there is a way to do this using the ContextListener -- not 100% sure. Check out http://docs.oracle.com/javaee/6/api/javax/servlet/ServletContextListener.html

Does this help?

[xlorepdarkhelm]

Just in case you are curious, the solution I had to do was to use a Singleton Session Enterprise Java Bean, which maintains my connection, and then I can connect to that bean in my code.

The fun on this is I am running inside of Glassfish 3.1, with JEE6 and JSE7. BUT, because I am partially a glutton for punishment, I am running my application within Jython 2.7, and am using Flask. so, my EJB looks like:

package pampas.ejb;

import java.nio.file.Path;
import java.nio.file.Paths;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.ConcurrencyManagement;
import javax.ejb.ConcurrencyManagementType;
import javax.ejb.LocalBean;
import javax.ejb.Singleton;
import javax.ejb.Startup;
import javax.ejb.TransactionManagement;
import javax.ejb.TransactionManagementType;

import com.lambdazen.bitsy.BitsyGraph;

/**
 * Session Bean implementation class GraphBean
 */
@Startup
@Singleton
@TransactionManagement(TransactionManagementType.BEAN)
@ConcurrencyManagement(ConcurrencyManagementType.BEAN)
@LocalBean
public class GraphBean {
    private BitsyGraph graph;

    @PostConstruct
    void init() {
        Path graph_path = Paths.get(System.getProperty("java.io.tmpdir"), "pampas", "graph");
        graph_path.toFile().mkdirs();
        BitsyGraph graph = new BitsyGraph(graph_path);
        setInst(graph);
    }

    @PreDestroy
    void destroy() {
        getInst().shutdown();
    }

    private void setInst(BitsyGraph graph) {
        this.graph = graph;
    }

    public BitsyGraph getInst() {
        return graph;
    }
}

My Jython code looks like:

from __future__ import division, absolute_import, print_function

import collections
import java.lang.Class
import java.lang.Runtime
import javax.naming.InitialContext

import concurrent.futures
import flask
import flask_restful
import werkzeug.local

import com.lambdazen.bitsy.BitsyException
import com.tinkerpop.blueprints.Direction
import contextlib2 as contextlib
import javax.persistence.Persistence as JPA
import org.joda.time.DateTime


app = flask.Flask(__name__.split('.')[0])
api = flask_restful.Api(app)


def get_graph():
    try:
        return flask.g._graph

    except AttributeError:
        jndi_context = javax.naming.InitialContext()
        graph = jndi_context.lookup(u'java:global/pampas-be/GraphBean')
        flask.g._graph = graph.inst
        return flask.g._graph


graph = werkzeug.local.LocalProxy(get_graph)


@contextlib.contextmanager
def graph_session():
    session = graph.newTransaction()
    try:
        yield session
    except:
        session.rollback()
    finally:
        session.commit()


class Indi(flask_restful.Resource):
    FIELDS = {
        u'indi_key',
        u'name',
        u'link',
        u'tooltip',
        u'status',
        u'last_updated',
        u'message',
        u'suppressed',
        u'key_value',
        u'edit_list',
    }

    def get(self, indi_key):
        def get_indi(indi):
            return {
                field: indi.getProperty(field)
                if field not in {u'last_updated'} or not indi.getProperty(field)
                else org.joda.time.DateTime(
                    indi.getProperty(field)
                ).toString()
                for field in self.FIELDS
            }
        with graph_session() as trans:
            indi = None
            for node in trans.getVertices(u'indi_key', indi_key):
                indi = node
                break
            ret = {u'current': get_indi(indi)}
            children = []
            for edge in indi.getEdges(com.tinkerpop.blueprints.Direction.IN, u'ancestor'):
                child = edge.getVertex(com.tinkerpop.blueprints.Direction.OUT)
                children.append(get_indi(child))
            if children:
                ret[u'indis'] = children
            return ret

api.add_resource(Indi, u'/indi/<string:indi_key>')

And this all works together beautifully.