Middleware Services for Dynamic Clustering of Application Servers (Ph.D. Thesis)

Nowadays, middleware are well-established technologies developed in order to make easier the implementation of distributed applications. Among these applications, this thesis focuses on the so-called enteprise applications. Usually, these applications exhibit stringent Quality of Service (QoS) requirements, which are to be met in order to enable them to carry out their tasks effectively. QoS has been widely defined in the literature; for the purposes of this thesis it is intended to be a set of non-functional application requirements that include availability, scalability, reliability and timeliness. In current industry practice, these requirements are usually specified within so-called Service Level Agreements (SLAs) that, in the context of this work, are contracts used to regulate the contractual relationships between application owners and middleware platform providers. Application owners own applications that can be deployed, run and maintained using component-based technologies termed application servers. These technologies support clustering of application server instances for scalability, load balancing and fault-tolerance purposes; however, current clustering mechanisms can meet only partially the above mentioned non-functional (i.e., the QoS) requirements of the applications they host, as these mechanisms are not designed to be QoS-aware. In this thesis, QoS-aware application servers (QaASs) are such component-based technologies in which their clustering mechanisms are capable of meeting QoS requirements that are specified within SLAs. Hence, the thesis proposes the design, implementation and experimental evaluation of an open source middleware architecture, constructed out of QoS-aware middleware services, that extends current application server technology so as to create QaASs. Specifically, the thesis focuses on three principal middleware services, termed Configuration Service (CS), Monitoring Service (MS) and Load Balancing Service (LBS). The CS, MS and LBS, according to the QoS specifications included into SLAs, are responsible for (i) configuring clusters of application servers in order to meet the SLA requirements; (ii) monitoring and adapting those clusters in case the QoS delivered by the resources of the clusters deviates from that required by the applications, and (iii) distributing the client requests among application server instances in the clusters so as to honor SLAs. Experimental evaluations of the QoS-aware middleware services described in this thesis show that these services can be used effectively to extend current application server technology so as to enable that technology to meet its SLAs. Part of the work described in this thesis has been developed within the context of the EU funded project TAPAS [TAP] and deployed for the production by the German Adesso AG company [ade], which participated to the TAPAS project. 1. Department of Computer Science, University of Bologna, Mura Anteo Zamboni 7, 40127 Bologna, Italy.