Redistribution in Distributed Ada

In this paper we will provide a model using Ada and the Distributed Annex for relocating concurrent objects in a distributed dataaow application. This model will be used to provide passive and active error tolerance as well as the basis for a load balancing superstructure. Care will be taken to demonstrate how errors are trapped and propagated across partitions containing multiple threads of execution. The following is intended as an extended abstract. If accepted, the nal paper will contain pictures, code segments, examples, and execution speciics running under a network of workstations operating Solaris using a GNATTGLADE implementation. 2 The Model We rst summarize a model presented in a prior paper 3. A dataaow graph is deened as a directed acyclic graph with data passing through the directed edges, or queues, and dynamics at the nodes. Each queue is connected to exactly two nodes: an upstream node attached to its tail and a downstream node attached to its head data ows downstream. Each node possesses a set of input queues entering it, and a set of output queues leaving it, either set possibly being empty. Each queue contains a threshold and a buuer size and receives its data in a rst-in-rst-out fashion from its upstream node. Queues are said to be linked to nodes at ports, thus there are both output ports and input ports. When a queue accumulates a threshold of data, it is said to have reached threshold. When every upstream queue of a node has reached threshold, the node is ready to re. Upon ring, a node reads and consumes data from each of its upstream queues, processes this data, and writes results to its downstream queues. All nodes can function concurrently, and conceptually begin processing as soon as they reach a re condition. To allow for maximal concurrency in a distributed system, we implement queues as protected types, nodes as tasks, and provide an additional protected object a mailbox so that queues may send asynchronously to nodes. gure Each node has three mutually exclusive accept states Figure 1: 1. waiting to be initialized 2. waiting for the next command from its mailbox processing 3. waiting to be nalized 1