What is a Distributed System? n From various textbooks: l “A distributed system is a collection of independent computers that appear to the users of the system as a single computer.” l “A distributed system consists of a collection of autonomous computers linked to a computer network and equipped with distributed system software.” l “A distributed system is a collection of processors that do not share memory or a clock.” l “Distributed systems is a term used to define a wide range of computer systems from a weakly-coupled system such as wide area networks, to very strongly coupled systems such as multiprocessor systems.”

n Is every system with >2 computers a distributed system?? l , ftp, telnet, world-wide-web l Network printer access, network file access, network file backup l We don’t usually consider these to be distributed systems… What is a Distributed System? (cont.) P1 P2 P3 P4P5 Network A distributed system is a set of physically separate processors connected by one or more communication links

Two Taxonomies for Classifying Computer Systems n Michael Flynn (1966) l SISD — single instruction, single data l SIMD — single instruction, multiple data l MISD — multiple instruction, single data l MIMD — multiple instruction, multiple data n More recent (Stallings, 1993)

Classification of MIMD Architectures Tightly coupled  parallel processing l Processors share clock and memory, run one OS, communicate frequently Loosely coupled  distributed computing l Each processor has its own memory, runs its own OS (?), communicates infrequently Tanenbaum (date?) MIMD parallel and distributed computers tightly coupled multiprocessors (shared memory) multicomputers (distributed / private memory) loosely coupled switchedbusswitchedbus Ultra- computer SequentHypercubeWorkstations on a LAN

Classification of Operating Systems n Multiprocessor Operating System l Tightly-coupled software (single OS) running on tightly-coupled hardware l A process can run on any processor n Single ready queue! l All memory is shared l File system similar to that on non-distributed systems n Network Operating System l Loosely-coupled hardware l Loosely-coupled software n Each computer runs its own OS n User knows which machine he/she is on l Goal: provide access other machines on the network, share resources l Typical utility programs: rlogin, rcp, telnet, ftp

Classification of Operating Systems (cont.) n “True” Distributed Operating System l Loosely-coupled hardware n No shared memory, but provides the “feel” of a single memory l Tightly-coupled software n One single OS, or at least the feel of one l Machines are somewhat, but not completely, autonomous P1P2P3 P4P5 Network M1M2M3 M4M5 Disk1 Disk5Printer4

Course’s Topics  Theoretical foundations Waves, Traversal Managing physical clocks in distributed systems Logical clocks: Lamport's and vector clocks Global state recording, Termination detection  OS concepts  Distributed mutual exclusion  Handling deadlocks  Resource management  Distributed file systems, examples: Sun's NFS, Andrew FS  Distributed shared memory  Load distribution, Process migration  Security Introduction to cryptography implementations: Kerberos, Secure Shell  Other Topics Clusters, High availability, Scalability, Fault tolerance