A Flexible Approach to Staged Events

The Staged Event Driven Architecture (SEDA) can be seen as a milestone as regards integration of threads and events in a single model. By decomposing applications into sets of multi-threaded stages connected by event queues, SEDA allows for the use of each concurrency model where most appropriate. Inside each SEDA stage, the number and scheduling policy of threads can be adjusted to enhance performance. SEDA lends itself to parallelization on multicores and is well-suited for many high-volume data stream processing systems and highly concurrent event processing systems. In this paper, we discuss the design of Leda, a platform that extends the staged model proposed by SEDA, decoupling application design from specific execution environments. In Leda, stages are defined purely by their role in application logic, with no concern for locality of execution, and are bound together through asynchronous communication channels, called connectors, to form a directed graph representing the flow of events inside the application. Decisions about the configuration of the application at execution time are delayed to later phases of the implementation process. Stages in the application graph can then be grouped to form clusters, and each cluster is mapped to an exclusive OS process, running on an arbitrary host. The paper briefly reviews a classification of hybrid concurrent models in order to establish SEDA´s and Leda´s role in this space. It then describes the design of Leda, which emphasizes decoupling at several levels, and discusses how its implementation in Lua helped to reinforce these design goals.