This paper describes the design of the data delivery components of the Distributed Oceanographic Data System. This design uses HTTPD and the CGI interface to provide access to remotely stored data. User programs, which were never intentionally written with access to remote data in mind, can be re-linked with DODS client libraries and access data using HTTP. The DODS client libraries are direct call-for-call replacements for existing APIs, thus any user program written to make use of an API for which a DODS client library exists can be re-linked and can access data via the DODS HTTP-base data servers. Because the data servers and client libraries use a single protocol to provide access to the data regardless of the data's storage format, user programs written to read data using one API can actually access data through DODS data servers in several APIs.
The current design of the DODS data delivery mechanism as described here and in ``DODS--Data Delivery Architecture'' and ``DODS--Data Access Protocol'' is the result of our previous experience building a similar system using Sun Microsystem's Remote Procedure Call (RPC) technology. That system demonstrated the feasibility of re-linking user programs with re-implemented API libraries and using those re-linked programs to access remotely stored data. That design, however, did not use a single transmission protocol for all servers and libraries. Thus it was not able to provide interoperability between APIs. This design reflects our desire to achieve that goal.
The decision to abandon the RPC-based transfer technology in favor of HTTP-based communications rests on the growing prominence of that technology, the convenience that its supporting software provides and the likelihood that a variety of interfaces can be constructed to access the HTTP-based data servers. Because HTTP is receiving widespread use, there are many active efforts to refine its weak points. By adopting HTTP, DODS stands to benefit from these refinements. In addition, using text MIME documents to transmit two of the three responses from the data servers makes interfacing our data servers with many different programs, in addition to programs re-linked with DODS client libraries, possible. For example, it is possible to use popular WWW browsers to read information about the contents of DODS data servers.