A Distributed Publish/Subscribe System for RDF Data - Data Management in Cloud, Grid and P2P Systems - page 46

to multiple peers. Also, note that this algorithm cannot deal with the situation
where a subscription is created before an event but reaches a peer after. Correctly
managing this case requires falling back to CSMA, which we do.
To summarize, the different properties of the two algorithms are presented in
Table 1.

Tabl e 1. Comparison of the two pub/sub algorithms

Routed
Element

Happen-
Before

Matching Steps Duplicates

Individual
quadruples

Multiple, Chain-like
and Reconstruction

Yes, filtering
required

CSMA Enforced

Whole
Compound Event

Requires
CSMA

OSMA Single No

5 Experiments

The latest version of our system dubbed EventCloud is publicly available 1 as an
open source project under the AGPL license. From an implementation point of
view each peer embeds Jena TDB instances for data and subscription storage.
The experiments introduced hereafter have been performed on 29 nodes of the
Grid'5000 testbed. Each machine embeds a Xeon E5520 @ 2,26 GHz with 32 GB
RAM, a hard disk drive at 7200 RPM. The partition used for data storage is an
EXT3 partition mounted with options noatime and nobarrier for performance
reasons. Java 7 was used with JVM option -server . Each result is the average
execution on 6 runs where the first run is laid aside due to JVM warmup.
The workload we are using is made of x synthetic events and y subscriptions
that are generated to be distributed uniformly among the available peers. This
allows us to evaluate the performance of the algorithms when the number of
peers involved is the largest.
Subscriptions are generated to embed k quadruple patterns of the form
(? g, ? s 1 ,p 1 , ? o 1)

(? g, ? o k− 1 ,p k , ? o k ). Compound Events
are generated to evenly match subscriptions.
In the first experiment we evaluate the effect of increasing the network size.
For this purpose we place 1 peer per machine and vary the total number of peers
from 1 to 25. There is only one subscriber with a subscription made of k =5
patterns. One publisher publishes 3

∧ (? g, ? o 1 ,p 2 , ? o 2) ∧ ... ∧

10 3 CEs, each one containing 5 quadruples
for an approximate size of 670 Bytes. Figure 2(a) depicts the average subscriber

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1 http://eventcloud.inria.fr