CMPT Dr. Alexandra Fedorova Distributed Systems

2 CMPT 401 © A. Fedorova What is a Distributed System? Coulouris, et al: –communicate and coordinate their actions only by passing messages

3 CMPT 401 © A. Fedorova What is a Distributed System? Andrew Tanenbaum: –A collection of independent computers that appear to the users as a single coherent system –autonomous computers connected by a network –software specifically designed to provide an integrated computing facility

4 CMPT 401 © A. Fedorova What is a Distributed System? Leslie Lamport: –“You know you have a distributed system when the crash of a computer you’ve never heard of stops you from getting any work done.”

5 CMPT 401 © A. Fedorova What is a Distributed System? A broader definition: –A collection of processors executing independent instruction streams that communicate and synchronize their actions –Communication may be done via messages or shared memory –Includes multi-process and multithreaded programs running on a monolithic multiprocessor hardware

6 CMPT 401 © A. Fedorova Distributed System: the Internet ©Pearson Education 2001

7 CMPT 401 © A. Fedorova Distributed System: an Intranet ©Pearson Education 2001

8 CMPT 401 © A. Fedorova Distributed System: Mobile Devices ©Pearson Education 2001

9 CMPT 401 © A. Fedorova Other Examples of Distributed Systems Distributed Multimedia Systems –Teleconferencing –Distance learning Cellular phone systems IP Telephony Flight management system in an aircraft Automotive control systems (50+ embedded processors in a Mercedes S-class) Distributed file systems (NFS, Samba) P2P file sharing The World Wide Web

10 CMPT 401 © A. Fedorova Reasons for Distributing Systems I The need to share data across remote geographies –Online Encyclopedia Britannica is accessed by users all over the world –Computer users in different geographies send messages to each other Replication of processing power –Independent processors working on the same task –Distributed systems consisting of collections of microcomputers may have processing power of a large supercomputer Use of heterogeneous components –Compute-intensive sub-tasks of a problem are run on powerful computers –Less resource-demanding sub-tasks run on less powerful computers –More efficient use of resources

11 CMPT 401 © A. Fedorova Reasons for Distributing Systems II Cost of hardware and management –A collection of cheap computers may be less expensive than one large supercomputer –Small simple computers may be easier to manage than one large one Administrative/functional issues –Payroll database is separate from registrar’s database –Each is managed according to the needs of the organization –Each is equipped with hardware that answers the needs of an organization Resilience to failures –If one component fails, others can proceed with work on the task Scalability –The system can be extended by adding more components (i.e., WWW)

12 CMPT 401 © A. Fedorova Properties of Distributed Systems Heterogeneity –Systems consist of heterogeneous hardware and software components Concurrency –Multiple programs run together Shared data –Data is accessed simultaneously by multiple entities No global clock –Each component has a local notion of time Interdependencies –Independent components depend on each other

13 CMPT 401 © A. Fedorova Challenges of DS: Heterogeneity Different network infrastructures (Ethernet, – wireless) Hardware and software (e.g., operating systems, processors): how can an Intel/Windows system understand messages sent by an Macintosh OS X system? Programming languages – how can a Java program and a C program communicate?

14 CMPT 401 © A. Fedorova Challenges of DS: Security Shared data must be protected –Privacy – avoid unintentional disclosure of private data –Security – data is not revealed to unauthorized parties –Integrity – protect data and system state from corruption Denial of service attacks – put significant load on the system, prevent users from accessing it

15 CMPT 401 © A. Fedorova Challenges of DS: Synchronization Concurrent cooperating tasks need to synchronize –When accessing shared data –When performing a common task Synchronization must be done correctly to prevent data corruption: –Example: two account owners; one deposits the money, the other one withdraws; they act concurrently –How to ensure that the bank account is in “correct” state after these actions? Synchronization implies communication Communication can take a long time Excessive synchronization can limit effectiveness and scalability of a distributed system

16 CMPT 401 © A. Fedorova Challenges of DS: Absence of Global Clock Cooperating task need to agree on the order of events Each task has its own notion of time Clocks cannot be perfectly synchronized How to determine which even occurred first? Example: Bank account, starting balance = $100 –Client at bank machine A makes a deposit of $100 –Client at bank machine B makes a withdrawal of $150 –Which event happened first? –Should the bank charge the overdraft fee?

17 CMPT 401 © A. Fedorova Challenges of DS: Partial Failures Detection of failures – may be impossible –Has a component crashed? Or is it just slow? –Is the network down? Or is it just slow? –If it’s slow – how long should we wait? Handling of failures –Retransmission –Tolerance for failures –Roll back partially completed task Redundancy against failures –Duplicate network routes –Replicated databases

18 CMPT 401 © A. Fedorova Challenges of DS: Scalability Does the system remain effective as if grows? As you add more components: –More synchronization –More communication –> the system runs slowly. Avoiding performance bottlenecks: –Everyone is waiting for a single shared resource –In a centrally coordinated system, everyone waits for the coordinator

19 CMPT 401 © A. Fedorova Challenges of DS: Transparency I Concealing the heterogeneous and distributed nature of the system so that it appears to the user like one system Transparency categories –Access: access local and remote resources using identical operations (NFS or Samba-mounted file systems) –Location: access without knowledge of location of a resource (URL’s, ) –Concurrency: allow several processes to operate concurrently using shared resources in a consistent fashion (two users simultaneously accessing the bank account)

20 CMPT 401 © A. Fedorova Challenges of DS: Transparency II Transparency categories (continued) –Replication: use replicated resource as if there were just one instance –Failure: allow programs to complete their task despite failures –Mobility: allow resources to move around –Performance: adaption of the system to varying load situations without the user noticing it –Scaling: allow system and applications to expand without need to change structure of applications or algorithms

21 CMPT 401 © A. Fedorova Course Objective Comprehensive introduction to distributed systems Theoretical aspects: models and architectures of distributed systems Understand challenges in building distributed systems Practical aspect: implement advanced distributed systems Read latest research papers addressing distributed systems

22 CMPT 401 © A. Fedorova Topics Studied I Architecture models of distributed systems (client-server, P2P, etc.) Operating system support: processes and threads, synchronization and mutual exclusion Inter-process communication Distributed objects and remote invocation Distributed file systems Time and Global Clocks Coordination and agreement

23 CMPT 401 © A. Fedorova Topics Studied II Transactions and concurrency control Replication Distributed multimedia systems Peer-to-Peer systems Mobile and ubiquitous computing Biologically inspired distributed systems

24 CMPT 401 © A. Fedorova Course Structure Reading a textbook –Reading is assigned for every class –To be done before the lecture –Midterm and final will test reading Programming assignments and a project –Challenging assignments –Require strong programming skills –First assignment is in C; the rest are either C or Java Reading research papers –Takes time –Submit summaries of assigned articles

25 CMPT 401 © A. Fedorova Programming Assignments Assignment #1 (already posted) – due January 28 –Solve a synchronization problem –Multithreaded programming in C, using pthreads library and mutexes Assignment #2 – due February 25 –Implement a distributed file system with a transactional interface –Use C, C++ or Java

26 CMPT 401 © A. Fedorova Grading One midterm exam: 20% Two homework assignments: 30% Project assignment: 20% Paper summaries: 10% Final exam: 20%

27 CMPT 401 © A. Fedorova Course Web Site Linked at Visit often Contains: –Syllabus –Assignments –Deadlines –Instructor office hours and office location