Gridbus Workflow Enactment Engine - Grid Computing: Infrastructure, Service, and Applications - page 124

user's point of view, it is better to use a service that offers a higher perfor-
mance at a lower price. Therefore, a method is required to allow users to
i nd replicated services easily.
A directory service, called the grid market directory (GMD) [1], has been
developed to support service publication and discovery in a market-
oriented grid environment. GMD is an infrastructure that allows (1) the
creation of one or more registries for service providers (2) the service pro-
viders to register the resources/application services that they wish to pro-
vide and (3) users such as workl ow engines to discover resources/services
and their attributes (e.g., access price, location, and usage constraints) that
meet their QoS requirements.
Figure 5.7 illustrates service publishing and discovery in a grid envi-
ronment through GMD. Service providers i rst register with the GMD and
publish their static information such as location, service capability, and
access methods. A grid user such as the workl ow engine can query the
GMD to i nd a suitable service. After that, the user can also query and sub-
scribe to the service provider directly to obtain more dynamic information
such as service execution status.
A provider can provide specialist applications for others to access
remotely. Figure 5.8 shows an application service schema. An application
service provider may also provide hosted machine information such as
host name and host public key for remote secure access. Service providers
also need to indicate middleware through which grid users can access
the service.
The grid application model (GAM) is developed for application identi-
i cation. GAM is a set of specii cations and application programming
interfaces (APIs) for a grid application. GAM can be published by service
providers within the GMD, and the users can search GMD for services
conforming to a particular GAM. Applications with the same GAM name
provide the same function and API. In the case of the workl ow system,

Workflow engine Grid market directory

3. Publish

5. Subscribe

6. Notification

Repository

Service
information

Runtime data

1. Register

4. Create/update 2. Create/update

Service provider

FIGURE 5.7 Service discovery using GMD.