CODEWITHSUNDEEP
javamultithreading
GorillaApe
GorillaApe

Reputation: 3641

Database Pooler

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?

Upvotes: 2

Views: 597

Answers (2)

iruediger
iruediger

Reputation: 953

Each connection would have a lock associated with it, in order for the connection to be returned by the getConnection method, the correct lock would have to be acquired. The cleanup method would also need to acquire the lock before closing a connection. Take a look at the java.util.concurrent.lock package.

Maybe a Semaphore is a better solution. Follow the link for an example.

I've never worked with BerkeleyDB, but I assume it has a JDBC interface. Can't you use an out of the box solution like DBCP or c3p0? Also check the Pool Component, it is a generic pool interface.

Upvotes: 1

John Kane
John Kane

Reputation: 4453

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.

Upvotes: 1

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