Package opendap.servlet

Interface GuardedDataset

public interface GuardedDataset
A GuardedDataset allows us to handle multithreaded stateful processing.

In a multi-threaded environment such as a Servlet, multiple requests for the same dataset may happen at the same time. This is a problem only when there is some state that is being maintained. Caching strategies (eg the netcdf server caches open netcdf files) may need to maintain state information in order to manage resources efficiently.

All accesses to the DDS and DAS are made through the GuardedDataset. Typically the server puts a mutex lock on the resource when getDDS() or getDAS() is called. When the dataset processing is complete, release() is called, and the server releases the mutex.

Example use: 

 
 public void doGetDAS(HttpServletRequest request,
 HttpServletResponse response,
 ReqState rs)
 throws IOException, ServletException {
 

 response.setContentType("text/plain");
 response.setHeader("XDODS-Server",  getServerVersion() );
 response.setHeader("Content-Description", "dods-dds");
 OutputStream Out = new BufferedOutputStream(response.getOutputStream());
 

 GuardedDataset ds = null;
 try {
 ds = getDataset(rs);
 DAS myDAS = ds.getDAS();            // server would lock here
 myDAS.print(Out);
 response.setStatus(response.SC_OK);
 }
 catch (DAP2Exception de){
 dap2ExceptionHandler(de,response);
 }
 catch (ParseException pe) {
 parseExceptionHandler(pe,response);
 }
 finally {                           // release lock if needed
 if (ds != null) ds.release();
 }
 }

Its important that the DDS or DAS not be used after release() is called.

See opendap.servers.netcdf.NcDataset for example of implementing a locking GuardedDataset. If a server is not keeping state, it can simply pass the DDS and DAS without locking, and implement a dummy release() method.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    close()
     
    opendap.dap.DAS
    getDAS()
    Get the DAS for this Dataset.
    ServerDDS
    getDDS()
    Get the DDS for this Dataset.
    void
    release()
    Release the lock, if any, on this dataset.

  • Method Details

    • getDDS

      ServerDDS getDDS() throws opendap.dap.DAP2Exception, opendap.dap.parsers.ParseException
      Get the DDS for this Dataset.
      Returns:
      the ServerDDS
      Throws:
      opendap.dap.DAP2Exception
      opendap.dap.parsers.ParseException

    • getDAS

      opendap.dap.DAS getDAS() throws opendap.dap.DAP2Exception, opendap.dap.parsers.ParseException
      Get the DAS for this Dataset.
      Returns:
      the DAS
      Throws:
      opendap.dap.DAP2Exception
      opendap.dap.parsers.ParseException

    • release

      void release()
      Release the lock, if any, on this dataset.

    • close

      void close()