Publication
It is noticeable that our society is increasingly relying on computer systems. Nowadays, computer networks can be found at places where it would have been unthinkable a few decades ago, supporting in some cases critical applications on which human lives may depend. Although this growing reliance on networked systems is generally perceived as technological progress, one should bear in mind that such systems are constantly growing in size and complexity, to such an extent that assuring their correct operation is sometimes a challenging task. Hence, dependability of distributed systems has become a crucial issue, and is responsible for an important body of research over the last years. No matter how much effort we put on ensuring our distributed system's correctness, we will be unable to prevent crashes. Therefore, designing distributed systems to tolerate rather than prevent such crashes is a reasonable approach. This is the purpose of fault-tolerance. Among all techniques that provide fault tolerance, replication is the only one that allows the system to mask process crashes. The intuition behind replication is simple: instead of having one instance of a service, we run several of them. If one of the replicas crashes, the rest can take over so that the crash does not prevent the system from delivering the expected service. A replicated service needs to keep all its replicas consistent, and group communication protocols provide abstractions to preserve such consistency. Group communication toolkits have been present since the late 80s. At the beginning, they were monolithic and later on they became modular. Modular group communication toolkits are composed of a set of off-the-shelf protocol modules that can be tailored to the application's needs. Composing protocols requires to set up basic rules that define how modules are composed and interact. Sometimes, these rules are devised exclusively for a particular protocol suite, but it is more sensible to agree on a carefully chosen set of rules and reuse them: this is the essence of protocol composition frameworks. There is a great diversity of protocol composition frameworks at present, and none is commonly considered the best. Furthermore, any attempt to defend a framework as being the best finds strong opposition with plenty of arguments pointing out its drawbacks. Given the complexity of current group communication toolkits and their configurability requirements, we believe that research on modular group communication and protocol composition frameworks must go hand-in-hand. The main goal of this thesis is to advance the state of the art in these two fields jointly and demonstrate how protocols can benefit from frameworks, as well as frameworks can benefit from protocols. The thesis is structured in three parts. Part I focuses on issues related to protocol composition frameworks. Part II is devoted to modular group communication. Finally, Part III presents our modular group communication prototype
Anastasia Ailamaki, Eleni Zapridou, Panagiotis Sioulas