Grid computing may be thought of as a set of distributed services that are
accessed securely by various remote clients. Example services include
secure remote command execution, file management, database access, and
access to queuing systems. To create, run, and access such diverse
services in a consistent way, the Open Grid Services Architecture (OGSA)
defines the framework one may follow when building and running new
services. Grid clients put services to work in useful ways. Computing web
portals are typical Grid clients that may connect to, and invoke, remote
services in a point-to-point fashion (via name or IP label). Grid
computing has an extensive literature, including several overviews and a
current snapshot of the state of the art.
We believe there is an important component missing that should link the
services and client environments: a software layer that manages the
messages and data traffic between the two. In current systems, this
traffic is point-to-point messages. This makes it difficult to build
collaboration services, provide reliability and redundancy for downed
services, work through firewalls, and address many other problems, such as
logging and backups. We propose that by inserting a message-relaying
framework between the clients and services, we may elegantly add
collaboration, resiliency, and redundancy to Grid computing. Many of the
services required by the VLab, such as job submission, file and metadata
management, are standard Grid tools that we will adopt. Our innovation
will be to place these services within a natural collaborative framework.
Our proposed architecture is based on the so-called publish/subscribe
paradigm. It effectively abstracts all connections to the middleware in
terms of topics. A simple analogy is a news service. One or more
publishers submit news items to newsgroups (identified by topics), while
subscribers choose which newsgroups they wish to read: they subscribe to
one or more topics. The user is not interested in where the news items are
actually stored or which machines execute the requests, except as relates
to efficiency. At the core, VLab will be driven by a collection of servers
whose sole purpose is to accept input messages from publishers and
redirect them to subscribers. Unless specifically disallowed, several
publishers will be able to submit to the same topic, while several
subscribers will be able to retrieve the contents of identical topics.
This strategy, at its core, supports the notion of collaboration.
We believe that the publish/subscribe approach supports all the stated
requirements of VLab. In such a system, messages have many encoding
(SOAP-RPC, audio-video streams, binary data files, etc.) and are
transmitted by any one of several protocols (TCP, UDP). The only
requirement is the presence of a topic in the message header. Resources
(storage, computational, portals, project managers, databases,
visualization servers, Grid services and infrastructure, audio-video
services, etc.) connect to the middleware. Because all resources
communicate only through topics, it is straightforward to add any number
of useful utilities such as logging and backup services. These would
respond to a broader range of topics than the more specialized resources.
The NaradaBrokering (NB) system, developed at the Community Grids
Laboratory, is a distributed publish/subscribe system. Instead of a single
message broker (i.e., server), NB uses a distributed brokering network
that can perform sophisticated and failsafe message routing. Advanced
features for security and reliable messaging are in development. Previous
work by Dr. Pierce and co-workers has shown that these systems support
traditional Grid and Web service systems.
