Scalable Termination Detection for Distributed Actor Systems

Automatic garbage collection (GC) prevents certain kinds of bugs and reduces programming overhead. GC techniques for sequential programs are based on reachability analysis. However, testing reachability from a root set is inadequate for determining whether an actor is garbage because an unreachable actor may send a message to a reachable actor. Instead, it is sufficient to check termination (sometimes also called quiescence): an actor is terminated if it is not currently processing a message and cannot receive a message in the future. Moreover, many actor frameworks provide all actors with access to file I/O or external storage; without inspecting an actor's internal code, it is necessary to check that the actor has terminated to ensure that it may be garbage collected in these frameworks. Previous algorithms to detect actor garbage require coordination mechanisms such as causal message delivery or nonlocal monitoring of actors for mutation. Such coordination mechanisms adversely affect concurrency and are therefore expensive in distributed systems. We present a low-overhead reference listing technique (called DRL) for termination detection in actor systems. DRL is based on asynchronous local snapshots and message-passing between actors. This enables a decentralized implementation and transient network partition tolerance. The paper provides a formal description of DRL, shows that all actors identified as garbage have indeed terminated (safety), and that all terminated actors--under certain reasonable assumptions--will eventually be identified (liveness).Comment: 23 pages, 7 figures. To appear in the proceedings of CONCUR 2020. Version 2: Fixed TeX error that omitted predicates in the third line of the Send rule: Actor $A$ must have active refobs $x$ and $y_1 \dots y_n

Journalistic Practice and the Cultural Valuation of New Media: Topicality, Objectivity, Network

Around the turn of the twenty-first century, American journalism is undergoing an existential crisis provoked by the emergence of digital and networked communication. As the economic model of producing journalism is undergoing significant changes, this study argues that the crisis of journalism is primarily a cultural crisis of valuation. Because the practices that traditionally defined the exclusivity of journalism as a form of public communication have been transposed to the online and digital environment through social media and blogs, such practices no longer value journalism in the same terms like in the age of mass media. The key to understanding the cultural crisis of journalism in the present, this study argues, is to revise the traditional narrative and its associated terminologies of the institutionalization of journalism. Journalism is thus defined as a structure of public communication, which needs to be enacted by producers and audiences alike to become socially meaningful. The consequence of seeing journalism as a structure sustained through social practices is that it allows to see the relation between audiences and their journalistic media as constitutive for the social function of new media in journalism. Through the analytically central dimension of practice, the study presents key moments in the history of modern journalism, where the meaning of new media was negotiated. These moments include the emergence of topical news media oriented toward a mass market (the penny press in the 1830s) and the definition of a schema of objectivity which valued journalistic practice in professional and scientific terms around the turn of the twentieth century in analogy to photographic media. In each phase, material, cognitive and social practices helped to define the value of a given new medium for journalism. Through the schemas of topicality and objectivity, journalistic practice institutionalized a privileged structure of public communication. The legacy of defining these schemas is then regarded as the central reason for the cultural crisis of journalistic practice in the present, as practices have been transposed and re-valued to sustain either forms of alternative journalism (as peer-production) or forms of self-communication in network media like blogs. Neither the form nor the technology of the blog alone can explain this differential social relevance but only the different ways in which social practices integrated and value new media. The study synthesizes an interdisciplinary array of concepts from cultural studies, sociology and journalism studies on subjects such as public communication, interaction, news production and cultural innovation. The theoretical framework of practice theories is then applied to an extensive body of primary and secondary source material, in order to retrace the cultural valuation of new media in a historically-comparative perspective. The study offers a theoretical and empirical contribution to the analysis of cultural innovation, which can be adopted to other cultural forms and media

Investigating and Writing Achitectural History: Subjects, Methodologies and Frontiers.

The volume contains the abstracts and full texts of the 157 papers and position statements presented and discussed at the III EAHN (European Architectural History) International Meeting, Torino 19-21 June 201

Proceedings of the 10th International Chemical and Biological Engineering Conference - CHEMPOR 2008

This volume contains full papers presented at the 10th International Chemical and Biological Engineering Conference - CHEMPOR 2008, held in Braga, Portugal, between September 4th and 6th, 2008.FC