web-grid
The OODT grid services (product and profile services) use CORBA or
RMI as their underlying network transport. However, limitations
of CORBA and RMI make them inappropriate for large-scale
deployments. For one, both are procedural mechanisms, providing a
remote interface that resembles a method call. This makes
streaming of data from a service impossible, because there are
limitations to the sizes of data structures that can be passed
over a remote method call. Instead, repeated calls must be made
to retrieve each block of a product, making transfer speeds
horribly slow compared to HTTP or FTP. (Block-based retrieval of
profiles was never implemented, resulting in out of memory
conditions for large profile results, which is another problem.)
Second, both CORBA and RMI rely on a central name registry. The
registry makes an object independent of its network location,
enabling a client to call it by name (looking up its last known
location in the registry). However, this requires that server
objects be able to make outbound network calls to the registry
(through any outbound firewall), and that the registry accept
those registrations (through any inbound firewall). This required
administrative action at institutions hosting server objects and
at the institution hosting the registry. Often, these firewall
exceptions would change without notice as system adminstrators
changed at each location (apparently firewall exceptions are
poorly documented everywhere). Further, in the two major
deployments of OODT (PDS and EDRN), server objects have almost
never moved, nullifying any benefit of the registry. This
project, OODT Web Grid Services, avoids the prolems of CORBA and
RMI by using HTTP as the transport mechanism for products and
profiles. Further, it provides a password-protected mechanism to
add new sets of product and profile query handlers, enabling
seamless activation of additional capabilities.