Distributed Systems Algorithms Review: Mutual Exclusion & Deadlock Detection

1. Outline • Distributed Mutual Exclusion • Token-based algorithms • Distributed Deadlock Detection • Review and overview • Agreement Protocols • Introduction • Lamport-Shostak-Pease algorithm • Dolev et al’s algorithm • Applications of agreement protocols COP5611

2. Announcements • There will be a quiz next Tuesday • It will be an open book quiz • It covers the things we have covered so far • It will be at the end of the class • You can turn in your homework #2 next Tuesday • However, no submission after that date will be accepted • Please do not run your programs on diablo COP5611

3. Distributed Deadlock Detection • In distributed systems, the system state can be represented by a wait-for graph (WFG) • In WFG, nodes are processes and there is a directed edge from node P1 to node P2 if P1 is blocked and is waiting for P2 to release some resource • The system is deadlocked if there is a directed cycle or knot in its WFG • The problem is how to maintain the WFG and detect cycle/knot in the graph COP5611

4. Distributed Deadlock Detection – cont. • Centralized detection algorithms • Distributed deadlock algorithms • Path-pushing • Edge-chasing • Diffusion computation • Global state detection • You need to know the basic ideas but not the details about those algorithms COP5611

5. Agreement Protocols • In distributed systems, sites are often required to reach mutual agreement • In distributed database systems, data managers must agree on whether to commit or to abort a transaction • Reaching an agreement requires the sites have knowledge about values at other sites • Agreement when the system is free from failures • Agreement when the system is prone to failure COP5611

6. The System Model • There are n sites in the distributed system and at most m can be faulty • Each site can communicate with all other sites through message passing • A receiver always knows the identity of the sender of a message • The communication is reliable COP5611

7. The System Model – cont. • Synchronous vs. asynchronous computations • In a synchronous computation, processes in the distributed system run in lock step manner • In each step, a process receives messages from other sites sent in the previous step, performs a computation, and sends messages to other processes COP5611

8. The System Model – cont. • Processor Failures • Crash fault • Omission fault • Malicious fault • Authenticated vs. non-authenticated messages COP5611

9. The System Model – cont. • Performance Analysis • Time to reach an agreement under a given protocol • Message traffic measured by the number of messages exchanged to reach an agreement • Storage overhead COP5611

10. Agreement Problems • There are three well known agreement problems • Byzantine agreement problem • Consensus problem • Interactive consistency problem COP5611

11. Byzantine Agreement Problem • A chosen process, called source process, broadcasts its initial value to all other processes • A solution should COP5611

12. The Consensus Problem • In this problem, every process broadcasts its initial value to all other processes, where the initial values of different processes can be different COP5611

13. The Interactive Consistency Problem • In this problem, every process broadcasts its initial value to all other processes, where the initial values of different processes can be different COP5611

14. Lamport-Shostak-Pease Algorithm COP5611

15. Lamport-Shostak-Pease Algorithm – cont. COP5611

16. Lamport-Shostak-Pease Algorithm – cont. • Performance analysis • Time • Message traffic • Storage overhead COP5611

17. Fault-Tolerant Clock Synchronization • The interactive convergence algorithm COP5611

18. Fault-Tolerant Clock Synchronization – cont. • The interactive consistency algorithm COP5611