McIDAS User's Guide
Version 2017.1

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Updating Clients to Use Compressed Data Transfers

Remote servers 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. All client requests to remote servers, regardless of the data transfer method, are sent to port 112. So sites with firewalls or other screening mechanisms must open port 112 on their remote servers if they want to allow their clients to access the servers' data. After successfully receiving a request from a client, the remote server then gathers the requested data and uses the appropriate compression method to package the data, then sends it to the client back through port 112.

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, tell them 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.

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