Self-stabilization in NEST Mikhail Nesterenko (based on presentation by Anish Arora, Ohio State University)
Goals Scalable dependability via new notions of stabilization • e. g. weak, protective, bounded stabilization Stabilization at all levels of NEST system stack • e. g. , at application level, via component-frameworks and automated synthesis • e. g. , at middleware level, via stabilizing monitoring
Stabilization Notions: Original Concept legitimate states from where safety and liveness are satisfied • Closure: illegitimate states reached possibly due to faults Set of legitimate states is closed under system execution • Convergence: Starting from any system state, every system computation eventually reaches a legitimate state
Weak Stabilization • • Closure Weak Convergence: Starting from any system state, some system computation eventually reaches a legitimate state
Protective Stabilization • • • Closure Convergence (strong or weak) Protection: No transition is unsafe ( )
Bounded Stabilization Fault-span states, convergence time is bounded • • Closure Bounded Convergence: § Set of fault-span states is closed under system execution § Starting from any fault-span state, every system computation reaches a legitimate state in bounded time
Stabilization in NEST System Stack n Nonstabilizing synthesis Stabilizing application Stabilization synthesis framework component framework AP n Timed AP Implementing stabilizing apps APC n Stabilizing system/app monitoring
Project: Stabilizing Monitoring Service Model: • apps/daemons/nodes periodically send a refresh to service • period is chosen within some interval [LF. . HF] Service ensures in stabilizing manner: • apps/daemons/nodes are up • monitoring service of a node is up
Layered Architecture Layer 0: Hardware watchdog § implements a hardware self-rebooting mechanism Layer 1: Basic monitoring § ensures that registered app/daemons are up Layer 2: Remote and Advanced monitoring § ensures other nodes and distributed process groups are up § generation of suspicions for dependent apps/daemons § adaptation of refresh periods & registered apps/daemons
Project: Implementing Stabilizing Applications Input: a (weakly-) stabilizing protocol consisting of processes communicating via messages in Abstract Protocol (AP) notation Output: a weakly-stabilizing implementation using UNIX processes and UDP communication
Approach Input • Abstract AP timeouts • Zero message delay • Action/fault atomicity • Action fairness preserves all safety and liveness properties • Real Timed AP preserves some properties, including weakstabilization APC Output timeouts • Non-zero message delay • Action/fault atomicity • Action fairness • Real timeouts • Non-zero message delay • Event/weak fault atomicity • Weak action fairness
Project: Stabilization Synthesis Framework n Nonstabilizing AP n synthesis procedure Nonstabilizing APC Stabilizing APC dependability component framework
Approach • Exponential-time synthesis procedure, with adequate polynomial-time heuristic § sufficient for synthesis of byzantine agreement • Dependability component framework enables reuse of application-independent aspects of stabilization § application-dependent parameter used to instantiate this framework, e. g. network type, communication patterns
Sample Component Frameworks • Reactive link-predicate stabilization component § Retransmission based § Use of ACK/NACKs • Proactive link-predicate stabilization component § Forward error correction based § Sending parity packets in advance • Group-of-nodes state-predicate stabilization component
Deliverables and Milestones • Stabilizing Monitoring Framework: - Aug’ 02: Implementation of basic node monitoring - Aug’ 03: Implementation of advanced node/group monitoring - Apr’ 04: Demo of monitoring service use by NEST application • Implementing Stabilizing Applications: - Aug’ 02: AP-to-APC transformer implementation - Apr’ 03: Demo of stabilizing transformer-based NEST application - Aug’ 04: Transformer for stabilization of sequential processes • Stabilizing Synthesis Framework: - Aug’ 02: Demo of tool for synthesis of stabilizing AP protocols - Apr’ 03: BNF & semantics of APC dependability component composition language - Aug’ 03: Application-independent code for reactive & proactive component frameworks - Apr’ 04: Demo of stabilizing framework-based NEST application
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