P/S Based Distributed Mediator

The mediator service can be built as an overlay network of distributed bro- kers that implement a content-based P/S system. In particular, we leverage CBPS so that each mediator broker receives information only regarding remote consumer components that are compatible (i. e. whose capabilities and compo- nent attributes match) with locally managed producer components. To do so, each broker has a forwarding table built according to the registered C-GMA registration and executes a routing strategy that satisfies the CBPS downstream replication and upstream evaluation principles introduced in Sect. 4.

In the proposed scheme (see Figure 3), producers drive the interaction (a consumer driven scenario is also possible). When a producer registers with a mediator broker, two actions are performed:

1 locally managed matching consumers are notified of the producer exis- tence,

2 the producer registration is broadcasted to the other brokers. This last step is necessary to ensure consistency across all brokers and correctly es- tablish the routes that C-GMA consumers' registration will follow in the CBPS overlay network. More specifically, whenever a mediator broker

162 Krajíček, Ceccanti, Křenek, Matýska, Ruda :z E l

O

o

Figure 3. The distributed mediator content-based replication strategy. Figure a) shows the

broadcasting of a producer C-GMA registration. In figure b), a C-GMA consumer registration is forwarded by each broker towards the matching producer. Finally, in figure c), the last mile broker performs the final matchmaking between component capabilities and data attributes and types and sends a matching proposal to interested C-GMA components.

receives a producer's registration from a neighbour, it updates its for­ warding table adding the received registration to the set of registrations associated with that specific neighbour.

Consumer C-GMA registrations are treated differently. Whenever a con­ sumer component registers itself, the local mediator broker starts a matchmak­ ing process comparing its registration with:

• locally managed producer registration, so that matching producers are immediately notified of the newly arrived consumer;

• producers registration appearing in the forwarding table. If a matching registration is found, the received consumer registration is forwarded towards the matching neighbours for further matchmaking by remote brokers.

The main advantage of the CBPS replication strategy is that it limits the spreading of consumer registration only where these are really needed for the matchmaking process, allowing for better scalability. The immediate conse­ quence is a gain in scalability and performance of the infrastructure, since the amount of administrative traffic introduced in the overlay is limited and the distributed matchmaking function is ran only when strictly necessary (i. e. on all the brokers appearing on the shortest path that connects the producer edge

Distributed Mediator for the C-GMA 163

broker with the consumer edge broker). In contrast, the naive replication ap- proach states that all C-GMA registrations are broadcasted to all the brokers so that the matchmaking process is executed on each broker even on registrations that will not match locally managed C-GMA components.

The C-GMA matchmaking performed by the brokers may happen at different levels. We assume here that only capabilities and attributes associated with the components are taken into consideration, leaving the check of compatibility between data attributes and components capabilities to the "last-mile" broker, i.e., the broker that actively notifies producers and consumers of the existing match (see Figure 3).

This approachhas several advantages. Firstly, we may configure the mediator network to provide a chain of C-GMA components that are compatible and then entitle those components to exchange a specific data type in a second time. Secondly, by limiting the matching at the component level, we reduce the complexity of the matching function implemented at each broker thus keeping the infrastructure lightweight and scalable.

6. Conclusions

The discussed C-GMA architecture offers a general approach to integrate different GMA implementations. The distributed mediator improves the scal- ability of the C-GMA matchmaking process by leveraging the CBPS commu- nication paradigm. We believe that the resulting architecture could provide highly scalable interoperability framework for various Grid monitoring tools. Performance evaluation as well as exploring other mechanisms for implement- ing distributed mediator service is subject of further research.

Moreover, the choice of propagating the producer descriptors through the whole network and matching the consumer registrations can be symmetrically replaced by propagating consumer descriptors and matching producer registra- tions. Assessment of effectiveness of these two approaches should be a subject of further evaluation.

A c k n o w l e d g e m e n t

The work described in this paper is the result of collaboration enabled through the EU Network of Excellence European Research Network on Foundations,

Software Infrastructures and Applications for large scale distributed, GRID and Peer-to-Peer Technologies, (CoreGRID, FP6-004265). Part of this work is

also supported by the MU Research Intent MSM0021622419.

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Appendix D