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 and Description
void	close()
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 Detail

getDDS

ServerDDS getDDS()
          throws DAP2Exception,
                 ParseException

Get the DDS for this Dataset.

Returns:

the ServerDDS

Throws:

DAP2Exception

ParseException

getDAS

DAS getDAS()
    throws DAP2Exception,
           ParseException

Get the DAS for this Dataset.

Returns:

the DAS

Throws:

DAP2Exception

ParseException

release

void release()

Release the lock, if any, on this dataset.

close

void close()