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The familiar model of the data request path is shown in the diagram below. When a data center sees a request arrive, the system knows it is a user on the other end sending the request. When the data is prepared, it is sent back directly to the user and the request is satisfied.

The next figure illustrates the same concept, except that now the user is replaced by another data center. For all intents and purposes, the method of processing the request is much the same as before. The requesting data center (the client) will receive data from the processing data center (the server).

Extending this example, we can make data from one data center available through another data center by simply propagating the user request from one data center to the next and having the data return along the same route.

Following this, we can expand the networked request model by having a number of data centers be possible destinations for a user's request.

By looking at Fig. 1.4, it is apparent that a user interested in data will contact only one site when making a request. The site that receives the request will have the responsibility of routing appropriate portions of the request to other data centers, should it be appropriate. In addition, this site is the central receiving point of completed data before it is sent to the user. In order to distinguish this data center’s role from others, we choose to call it the hub site for the request. At the same time that a data center is serving as a hub site for one request, it could also be servicing a request routed to it from another site. When it serves the latter role, it is termed a delegate site for that request.

The general flow of the NetDC protocol is this:

• The user submits a request for data to a data center. This data center becomes the hub site.
• The hub site then scans each line of the request and decides whether to route any of the lines to delegate sites.
• Individual request forms are sent to the selected delegate sites with information on how to return the data back to the hub site once processing is completed.
• The hub site processes its local request and collects data from the delegate sites as they come in.
• When data collection is completed, the user is notified of the data shipment and the data is made available to the user through FTP or email.

NetDC requests are always emailed to participating sites through the account name "netdc". All NetDC processing functions at that site will also operate under that username. Since each data center will have their own unique domain address, it is easy to distinguish NetDC sites, even as they share the same username. An example email address to which a user would submit his request would be:

netdc@somehost.org

From there, the request is processed and within a reasonable period of time the user can expect a data return that is either packaged at the hub site or received in separate pieces from each of the delegate sites. This relates to the merging feature of NetDC, which will be discussed further in Chapter 9.0.

introduction •• overall concept •• request format •• request reception and delegation
datagrams •• local request processing •• inventory requests •• response requests
waveform requests •• product shipment •• installation and setup •• writing interface code
troubleshooting •• future implementations •• conclusion •• appendix A - summary of NetDC datagrams

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