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Introduction Chapter 1 Chapter 1 Introduction 1 Examples of Distributed Systems DNS WWW Cray T3E Condor/RES o Hierarchical distributed database o Origin servers and web caches o Distributed database o 2048 tightly coupled homogeneous processors o Distributed/parallel computing o Loosely coupled heterogeneous workstations o Parallel/distributed computing Chapter 1 Introduction 2 Other Distributed Systems Email Electronic banking Airline reservation system Peer-to-peer networks Etc., etc., etc. Chapter 1 Introduction 3 Computer Revolution Processing power o 50 years ago, $100M for 1 instr/sec o Today, $1K for 107 instructions/sec Price/perform. improvement of 1012 o If cars had followed same path as computers… o “…a Rolls Royce would now cost 1 dollar and get a billion miles per gallon” o And it would “explode once a year, killing everyone inside” Chapter 1 Introduction 4 Computer Revolution High speed networks o 30 years ago, networks were unknown o Today, Gigabit networks and the Internet Before networks, centralized systems Today, distributed systems o Computers in many locations work as one Chapter 1 Introduction 5 What is a Distributed System? According to your textbook o “A collection of independent computers that appears to its users as a single coherent system” Two parts to definition o Hardware machines are autonomous o Software machines appear as one system • Implies that communication hidden from user • Implies that organization hidden from user Chapter 1 Introduction 6 What is a Distributed System? According to dict.die.net o A collection of (probably heterogeneous) automata whose distribution is transparent to the user so that the system appears as one local machine o This is in contrast to a network, where the user is aware that there are several machines, and their location, storage replication, load balancing and functionality is not transparent Crucial point is transparency Chapter 1 Introduction 7 How to Implement a Dist. System? A distributed system is a collection of independent computers… …that acts like a single system How to accomplish this? Middleware o Make distributed system as transparent as possible Chapter 1 Introduction 8 Role of Middleware Distributed system as middleware Middleware extends over multiple machines Chapter 1 Introduction 9 Goals For a distributed system to be worthwhile authors believe it should o o o o Easily connect users to resources Hide fact that resources are distributed Be open Be scalable First 2 of these about transparency Transparent, open, scalable Chapter 1 Introduction 10 Transparency Transparency Description Access Hide different data representations, how resources 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 while in use Replication Hide that a resource is replicated Concurrency Hide that a resource may be shared by several users Failure Hide failure and recovery of a resource Persistence Hide whether a (software) resource is in memory or on disk Transparent system “acts” like one computer Various aspects of transparency listed above Chapter 1 Introduction 11 Degree of Transparency Cannot hide physical limitations o Time it takes to send packet May be a tradeoff between transparency and performance o What to do if Web request times out? o Keeping replicated data current Chapter 1 Introduction 12 Openness Open == standards-based Provides o Interoperability o Portability Ideally, flexible, i.e., extensible But many useful systems follow the “American standard” o Do whatever you want Chapter 1 Introduction 13 Scalability Concept Example Centralized services A single server for all users Centralized data A single on-line telephone book Centralized algorithms Doing routing based on complete information Scalability issues/limitations Chapter 1 Introduction 14 Scalability Authors believe centralized is bad o Centralized server is source of congestion, single point of failure o Centralized data leads to congestion, lots of traffic o Centralized algorithm must collect all info and process it (e.g., routing algs) Google? Napster? Chapter 1 Introduction 15 Scalability Decentralized algorithms o No machine has complete system state o Decisions based on local info o Failure of one machine does not kill entire algorithm o No assumption of global clock Examples? Chapter 1 Introduction 16 Geographic Scalability Big difference between LAN and WAN LANs have synchronous communication o Client can “block” until server responds On LAN, global time may be possible (to within a few milliseconds) WAN unreliable, point-to-point WAN has different admin domains o A security nightmare Chapter 1 Introduction 17 Scaling Techniques Scaling problems due to limited capacity of networks and servers Three possible solutions o Hide latencies do something useful while waiting (asynchronous comm.) o Distribution DNS, for example o Replication allows for load balancing Replication Chapter 1 Introduction creates consistency issues 18 Scaling Techniques Server or client check form as it’s filled out? Having client do more, as in (b), may reduce latency (but may cause security problems) Chapter 1 Introduction 19 Scaling Techniques DNS name space divided into zones Goto server in Z1 to find server Z2 and so on Like a binary search for correct server Chapter 1 Introduction 20 Hardware Issues For our purposes, 2 kinds of machines Multiprocessor o Different processors share same memory Multicomputer o Each processor has it’s own memory Each of these could use either bus or switched architecture Chapter 1 Introduction 21 Hardware Issues multiprocessor Chapter 1 Introduction multicomputer 22 Multiprocessors A bus-based multiprocessor Cache coherence is an issue Chapter 1 Introduction 23 Multiprocessors a) b) A crossbar switch Omega switching network Chapter 1 Introduction 24 Homogeneous Multicomputer Grid Chapter 1 Introduction Hypercube 25 Software Concepts System Description Main Goal DOS Tightly-coupled operating system for multiprocessors and homogeneous multicomputers Hide and manage hardware resources NOS Loosely-coupled operating system for heterogeneous multicomputers (LAN and WAN) Offer local services to remote clients Middleware Additional layer atop of NOS implementing general-purpose services Provide distribution transparency DOS Distributed Operating Systems NOS Network Operating Systems Middleware self-explanatory Chapter 1 Introduction 26 Uniprocessor OSs Separate apps from OS code via microkernel Chapter 1 Introduction 27 Multiprocessor OSs monitor Counter { private: int count = 0; public: int value() { return count;} void incr () { count = count + 1;} void decr() { count = count – 1;} } How to protect count from concurrent access? Chapter 1 Introduction 28 Multiprocessor OSs monitor Counter { void decr() { private: if (count ==0) { int count = 0; blocked_procs = blocked_procs + 1; int blocked_procs = 0; wait (unblocked); condition unblocked; blocked_procs = blocked_procs – 1; public: } int value () { return count;} else void incr () { count = count – 1; if (blocked_procs == 0) } count = count + 1; else signal (unblocked); } } Protect count from concurrent access o Using blocking Chapter 1 Introduction 29 Multicomputer OSs Multicomputer OS Chapter 1 Introduction 30 Multicomputer OSs ??? Chapter 1 Introduction 31 Multicomputer OSs Synchronization point Send buffer Reliable comm. guaranteed? Block sender until buffer not full Yes Not necessary Block sender until message sent No Not necessary Block sender until message received No Necessary Block sender until message delivered No Necessary Huh? Chapter 1 Introduction 32 Programming Issues Programming multicomputers much harder than multiprocessors Why? o Message passing o Buffering, blocking, reliable comm., etc. One option is to emulate shared memory on multicomputer o Large “virtual” address space Chapter 1 Introduction 33 Distributed Shared Memory a) b) c) Pages of address space distributed among 4 machines After CPU 1 references pg 10 If page 10 read only and replication used Chapter 1 Introduction 34 Distributed Shared Memory False sharing of page between two processes o Two independent processors share same page Chapter 1 Introduction 35 Network OS Network OS o Each processor has its own OS Chapter 1 Introduction 36 Network OS Clients and server in a network OS Global shared file system Chapter 1 Introduction 37 Distributed System Distributed OS not a distributed system by our definition Network OS not a distributed system by our definition What we need is middleware… Chapter 1 Introduction 38 Positioning Middleware A distributed system as middleware o Individual node managed by local OS o Middleware hides heterogeneity of underlying systems Chapter 1 Introduction 39 Middleware and Openness Open middleware-based system Middleware layer should o Use the same protocols o Provide same interfaces to apps Chapter 1 Introduction 40 Comparison of Systems Item Distributed OS Network OS Middlewarebased OS Multiproc. Multicomp. Degree of transparency Very High High Low High Same OS on all nodes Yes Yes No No Number of copies of OS 1 N N N Basis for communication Shared memory Messages Files Model specific Resource management Global, central Global, distributed Per node Per node Scalability No Moderately Yes Varies Openness Closed Closed Open Open Middleware rocks! Chapter 1 Introduction 41 Middleware Services Main goal is access transparency o Hides low level message passing Naming o Like yellow pages or URL Persistence o For example, a distributed file system Distributed transactions o Read and writes are atomic Security Chapter 1 Introduction 42 Client Server Model Read this section Chapter 1 Introduction 43 Clients and Servers Interaction Chapter 1 Introduction between client and server 44 Example Client and Server header.h o Used by client o And by server Chapter 1 Introduction 45 Example Client and Server A sample server Chapter 1 Introduction 46 Example Client and Server Client using server to copy a file Chapter 1 Introduction 47 Processing Level Internet search engine as 3 layers Chapter 1 Introduction 48 Multitiered Architectures Alternative client-server organizations Chapter 1 Introduction 49 Multitiered Architectures A server acting as client Chapter 1 Introduction 50 Modern Architectures Horizontal distribution of Web service Chapter 1 Introduction 51 Summary Distributed system o Autonomous computers that operate together as a single coherent system Potential advantages o Can integrate systems o Scales well, if properly designed Potential disadvantages o Complexity o Degraded performance o Security Chapter 1 Introduction 52 Summary Different types of dist systems Distributed OS o For tightly coupled system o Can’t integrate different systems Network OS o For heterogeneous system o No single system view Chapter 1 Introduction 53 Summary Middleware systems based on o Remote procedure calls o Distributed objects, files, documents o Vertical organization o Horizontal organization Chapter 1 Introduction 54