/// <summary>
/// The idea for making sure that there is no more than one instance
/// running in an HDFS is to create a file in the HDFS, writes the hostname
/// of the machine on which the instance is running to the file, but did not
/// close the file until it exits.
/// </summary>
/// <remarks>
/// The idea for making sure that there is no more than one instance
/// running in an HDFS is to create a file in the HDFS, writes the hostname
/// of the machine on which the instance is running to the file, but did not
/// close the file until it exits.
/// This prevents the second instance from running because it can not
/// creates the file while the first one is running.
/// This method checks if there is any running instance. If no, mark yes.
/// Note that this is an atomic operation.
/// </remarks>
/// <returns>
/// null if there is a running instance;
/// otherwise, the output stream to the newly created file.
/// </returns>
/// <exception cref="System.IO.IOException"/>
private OutputStream CheckAndMarkRunning()
{
try
{
if (fs.Exists(idPath))
{
// try appending to it so that it will fail fast if another balancer is
// running.
IOUtils.CloseStream(fs.Append(idPath));
fs.Delete(idPath, true);
}
FSDataOutputStream fsout = fs.Create(idPath, false);
// mark balancer idPath to be deleted during filesystem closure
fs.DeleteOnExit(idPath);
if (write2IdFile)
{
fsout.WriteBytes(Sharpen.Runtime.GetLocalHost().GetHostName());
fsout.Hflush();
}
return(fsout);
}
catch (RemoteException e)
{
if (typeof(AlreadyBeingCreatedException).FullName.Equals(e.GetClassName()))
{
return(null);
}
else
{
throw;
}
}
}
