McIDAS User's Guide
Version 2015.1

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Updating Clients to Use Compressed Data Transfers

Remote servers running McIDAS-X versions 2003 or later can transfer data to McIDAS-X clients using three different data transfer methods. The methods are gzip-compressed, compressed, and uncompressed.

The client determines which method is used by setting a flag in the request header. If the client is running McIDAS-X 2003, requests for gzip-compressed data are sent to port 112, compressed to port 503, and uncompressed to port 500. If the client is running McIDAS-X 2004 or later, all requests, regardless of the data transfer method, are sent to port 112. The remote server then gathers the requested data and uses the appropriate compression method to package the data, then sends it to the client through the same port.

If the remote server is running version 2003 or 2004, ports 112, 500 and 503 are open so that requests from clients running version 2003 or older are accepted. If the remote server is running version 2005 or later, only port 112 is open (ports 500 and 503 are NOT open). Therefore, remote servers running 2005 or later will respond only to data requests from clients running version 2004 or later (and also 2003 IF the MCCOMPRESS environment variable is set to gzip). Thus, commands sent to server port 500 or 503 from clients running older versions will fail on the client (eventually timeout because of no response).

Sites with firewalls or other screening mechanisms must open the needed ports on their remote servers (port 112 on servers running 2005, and ports 112, 500 and 503 on servers running 2003 or 2004) if they want to allow their clients to access the server's data.

The gzip-compressed and compressed methods are most beneficial to clients who transfer large data files or those with slow data connections. However, they only reduce the amount of data during the transfer; they do not change the resultant file size or accounting (the byte counts remain the same). For example, the destination file size from an IMGCOPY command is the same no matter which of the three methods is used.

For a client to use the gzip-compressed or compressed methods, the user may set the environment variable MCCOMPRESS in their account or use the MCCOMPRESS global keyword. See Using Compressed Data Transfers for instructions. Other than firewall precautions, no special actions are required on the remote server. The three data compression options are automatically implemented when you run the script as part of the normal McIDAS-X installation. See Installing the McIDAS-X ADDE Remote Server.

To allow your McIDAS-X users to take advantage of all three data transfer methods, first install McIDAS-X 2003 or later on ALL your remote servers. Notify the users that your remote servers are updated and that they can configure their McIDAS-X sessions to request gzip-compressed or compressed data transfers by completing the instructions in Using Compressed Data Transfers. Warn them that they should NOT make those changes unless their client workstation and ALL of the remote servers they access are running McIDAS-X 2003 or later. Before updating your remote servers to version 2005, you should give your users sufficient notice that they need to upgrade their client workstations to version 2004 or later (or 2003 IF their MCCOMPRESS environment variable is set to gzip) in order to access the data on your servers.

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