cmpt 401 summer 2007 dr. alexandra fedorova distributed systems

CMPT 401 Summer 2007

Dr. Alexandra Fedorova

Distributed Systems

2CMPT 401 Summer 2007 © A. Fedorova

What is a Distributed System?

• Coulouris, et al:– communicate and coordinate their actions only by

passing messages



• 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



• 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.”


Distributed System: the Internet

©Pearson Education 2001


Distributed System: an Intranet



Distributed System: Mobile Devices



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


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


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)


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


Challenges of DS: Heterogeneity

• Different network infrastructures (Ethernet, 802.11 – 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?


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


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


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 $150– Client at bank machine B makes a withdrawal of $100– Which event happened first? – Should the bank charge the overdraft fee?


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


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 co-ordinator


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, e-mail)– Concurrency: allow several processes to operate concurrently

using shared resources in a consistent fashion (two users simultaneously accessing the bank account)


Challenges of DS: Transparency II

• Transparency categories (continued)– Replication: use replicated resource as if there was 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


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


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


Topics Studied II

• Transactions and concurrency control• Replication• Distributed multimedia systems• Peer-to-Peer systems• Mobile and ubiquitous computing• Biologically inspired distributed systems


Course Structure

• Reading a textbook– Reading is assigned for every class– To be done before the lecture– Midterms 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


Programming Assignments

• Assignment #1 (already posted) – due May 28– Solve a synchronization problem– Multithreaded programming in C, using pthreads library and

mutexes• Assignment #2 – due July 9

– Implement a distributed file system with a transactional interface– Use C, C++ or Java


Project Assignment

• Implement a distributed banking system, including banks and banking machines

• Specify the rules and protocols of the system• Required features:

– transactional interface, – distributed transactions with two-phase commit protocol,– replicated database (master- slave replication), – automated failover with master election, – synchronization using vector clocks.


Grading

• Two midterm exams: 30%• Two homework assignments: 20%• Project assignment: 20%• Paper summaries: 10%• Final exam: 20%


Course Web Site

• Linked off http://www.cs.sfu.ca/CC/index-by-course.html

• Visit often• Contains:

– Syllabus– Assignments– Deadlines– Instructor office hours and office location

cmpt 401 summer 2007 dr. alexandra fedorova distributed systems

Documents

task distributed systems

fedorova reasons

www slide

fedorova challenges

powerful computers

world wide web slide

collection of cheap

heterogeneous hardware