Question

Hello i am trying to implement a database-object(“connection”) pooler for BerkeleyDB…
I decided to use a singleton EJB propably or ENUM singleton implementation for this..

A final concurrenthash map would store database objects with a timestamp…

the method getConnection() would use double check locking as long as the value from map is volatile. – No performance issues i believe..(Java Connection Pooler getConnection is synchronized!!)
The database is spread into 100 files + the daily ones.. (application designed in mid seventies 1976)..

So far everything is fine… But i want to close daily unused handles.
So i decided to use a Timer to run every 24 hours a cleanup routine..
The problem is that how can i ensure that during cleanup a connection to be closed isnt requested ?
Pseudo algorithm

    cleanup(){
      for(Database db in map){
         if(db.getLastAccess - now >24hours) {
               res=map.remove("key",db);
               db.close();
         }
      }
    }

i know that the above isnt thread safe..How could i block getconnection ? Because many things could go wrong… “If condition” may be true but before removing db obj getLastAccess could be changed! Cleanup would be called by single thread though..
Is there any solution to block getconnection somehow so cleanup to work or anyother solution?

Answer 1

I am not sure if you currently do this, but if you have a way to determine if a connection is in use this would make this slightly easier. One thing that you can do, is iterate over the connections in your pool. When you find one that matches your criteria for being closed, try to mark it as being in use (assuming that a connection that is in use will not be returned as a open connection). If you succeed, close it. Otherwise, check it until it becomes free and you are able to mark it as being in use. Once you have been able to do this, you should be able to close it.