Distributed Systems Definition

Definition of a Distributed System A distributed system is a collection of independent computers that appears to its users as a single system.

A definition (Coulouris, et al) System of networked computers that communicate and coordinate their actions only by passing messages concurrent execution of programs components fail independently of one another

A definition (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.” inter-dependencies shared state

Distributed System

Operating System types Centralized Systems Process management Network System Share resources Remote access Telnet / FTP No direct control from machine to another Distributed system Global view of files system Global name Global time….

Distributed system properties Connect users and resources Transparency Scalability Openness

Transparency To hide the fact that machines are physically distributed and how much the distributed system appears as single system

Transparency in a Distributed System Description Access Hide differences in data representation and how a resource is accessed Location Hide where a resource is located Migration Hide that a resource may move to another location Relocation Hide that a resource may be moved to another location while in use Replication Hide that a resource may be shared by several competitive users Concurrency Failure Hide the failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk Different forms of transparency in a distributed system.

Types of transparency Location Transparency: users cannot tell where hardware and software resources such as CPUs, printers, files, data bases are located. Migration Transparency: resources must be free to move from one location to another without their names changed. E.g., /usr/lee, /central/usr/lee Replication Transparency: OS can make additional copies of files and resources without users noticing. Concurrency Transparency: The users are not aware of the existence of other users. Need to allow multiple users to concurrently access the same resource. Lock and unlock for mutual exclusion. Parallelism Transparency: Automatic use of parallelism without having to program explicitly. The holy grail for distributed and parallel system designers. Users do not always want complete transparency: a fancy printer 1000 miles away

Scalability It is to able to send anything to anyone anywhere System scalability to size means easily add more users & resources Scalability of geographically

Scalability A distributed system is scalable if it remains effective as the number of users and/or resources increase Challenges: Controlling resource costs Controlling performance loss Preventing resources from running out Avoiding performance bottlenecks

Scalability Problems No machine has complete information about the system state. Machines make decisions based only on local information. Failure of one machine does not ruin the algorithm. There is no implicit assumption that a global clock exists.

Openness / Flexibility The distributed system has a clear rules controls its services Open distributed system is flexible means easy to configure system for different developers

Openness When protocols are known to developers extensibility and maintainability becomes possible Openness allows re-implementation of different components of the system Important factors: Specification Documentation Published interfaces (often bypassing standards organizations)

Reliability Distributed system should be more reliable than single system. Example: 3 machines with .95 probability of being up. 1-.05**3 probability of being up. Availability: fraction of time the system is usable. Redundancy improves it. Need to maintain consistency Need to be secure Fault tolerance: need to mask failures, recover from errors.

Security Three components: Confidentiality (protection against disclosure to unauthorized individuals) Integrity (protection against alteration or corruption) Availability (protection against interference with the means of accessing the resources) The challenge: sending sensitive information in a network message in a secure manner efficiently

Security Scenario 1: Accessing exam information via a network file system Authentication: how do we know for sure that the user is a teacher who should have access to the data? Scenario 2: Sending a credit card number over the Internet Confidentiality: no other than the recipient should be able to read the data

Hardware concept of DS Distributed system may be multi-processor or multi-computers

Hardware Concepts Tightly Coupled versus Loosely Coupled Tightly coupled systems (multiprocessors) shared memory intermachine delay short, data rate high Loosely coupled systems (multicomputers) private memory intermachine delay long, data rate low

Multicomputers Bus-Based Multicomputers Switched Multicomputers easy to build communication volume much smaller relatively slow speed LAN (10-100 MIPS, compared to 300 MIPS and up for a backplane bus) Switched Multicomputers interconnection networks: E.g., grid, hypercube hypercube: n-dimensional cube 11/14/00

Multi-processor Multi-computers Hardware Concepts 1.6 Multi-processor Multi-computers

Bus-based Multiprocessors cache memory hit rate cache coherence write-through cache: propagate write immediately snoopy cache: monitor when its entry becomes obsolete 11/14/00

Multiprocessors 1.7

Switched Multiprocessors for connecting large number (over 64) of processors crossbar switch: n**2 switch points omega network: 2x2 switches for n CPUs and n memories, log n switching stages, each with n/2 switches, total (n log n)/2 switches building a large, tightly-coupled, shared memory multiprocessor is possible, but is difficult and expensive 11/14/00

Multiprocessors (2) 1.8

Multicomputer Systems Grid Hypercube 1-9

Software Concepts DOS (Distributed Operating Systems) Description Main Goal DOS Tightly-coupled operating system for multi-processors and multicomputers Tries to maintain global view of resources NOS Loosely-coupled operating system for multicomputers (LAN and WAN) Manages collection of machines with local services and OS Middleware Additional layer atop of NOS implementing general-purpose services Provide distribution transparency DOS (Distributed Operating Systems) NOS (Network Operating Systems) Middleware

Software Concepts Software more important for users Three types: Network Operating Systems (True) Distributed Systems Multiprocessor Time Sharing 11/14/00

Uniprocessor Operating Systems Separating applications from operating system code through a microkernel. 1.11

Network Operating Systems loosely-coupled software on loosely-coupled hardware A network of workstations connected by LAN each machine has a high degree of autonomy Files servers: client and server model Clients mount directories on file servers Best known network OS: Sun’s NFS (network file servers) for shared file systems

Multicomputer Operating Systems (1) General structure of a multicomputer operating system 1.14

Positioning Middleware General structure of a distributed system as middleware. 1-22

(True) Distributed Systems tightly-coupled software on loosely-coupled hardware provide a single-system image or a virtual uniprocessor a single, global interprocess communication mechanism, process management, file system; the same system call interface everywhere Ideal definition: “ A distributed system runs on a collection of computers that do not have shared memory, yet looks like a single computer to its users.”

Comparison between Systems Item Distributed OS Network OS Middleware-based OS Multiproc. Multicomp. Degree of transparency Very High High Low Same OS on all nodes Yes No Number of copies of OS 1 N Basis for communication Shared memory Messages Files Model specific Resource management Global, central Global, distributed Per node Scalability Moderately Varies Openness Closed Open A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware based distributed systems.

Distributed object architecture

A data mining system

Data mining system The logical model of the system is not one of service provision where there are distinguished data management services It allows the number of databases that are accessed to be increased without disrupting the system It allows new types of relationship to be mined by adding new integrator objects