chapter 1 introduction 1 introduction chapter 1
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Chapter 1 Introduction 1
Introduction
Chapter 1
Chapter 1 Introduction 2
Examples of Distributed Systems DNS
o Hierarchical distributed database WWW
o Origin servers and web cacheso Distributed database
Cray T3Eo 2048 tightly coupled homogeneous
processorso Distributed/parallel computing
Condor/RESo Loosely coupled heterogeneous workstationso Parallel/distributed computing
Chapter 1 Introduction 3
Other Distributed Systems Email Electronic banking Airline reservation system Peer-to-peer networks Etc., etc., etc.
Chapter 1 Introduction 4
Computer Revolution Processing power
o 50 years ago, $100M for 1 instr/seco 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 5
Computer Revolution High speed networks
o 30 years ago, networks were unknowno Today, Gigabit networks and the
Internet Before networks, centralized
systems Today, distributed systems
o Computers in many locations work as one
Chapter 1 Introduction 6
What is a Distributed System?
According to your textbooko “A collection of independent computers
that appears to its users as a single coherent system”
Two parts to definitiono Hardware machines are autonomouso Software machines appear as one
system• Implies that communication hidden from user• Implies that organization hidden from user
Chapter 1 Introduction 7
What is a Distributed System?
According to dict.die.neto 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 8
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 9
Role of Middleware
Distributed system as middleware Middleware extends over multiple
machines
Chapter 1 Introduction 10
Goals For a distributed system to be
worthwhile authors believe it shouldo Easily connect users to resourceso Hide fact that resources are
distributedo Be openo Be scalable
First 2 of these about transparency Transparent, open, scalable
Chapter 1 Introduction 11
TransparencyTransparency Description
AccessHide 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
PersistenceHide 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 12
Degree of Transparency Cannot hide physical limitations
o Time it takes to send packet May be a tradeoff between
transparency and performanceo What to do if Web request times out?o Keeping replicated data current
Chapter 1 Introduction 13
Openness Open == standards-based Provides
o Interoperabilityo Portability
Ideally, flexible, i.e., extensible But many useful systems follow
the “American standard”o Do whatever you want
Chapter 1 Introduction 14
Scalability
Concept Example
Centralized services A single server for all users
Centralized data A single on-line telephone book
Centralized algorithmsDoing routing based on complete information
Scalability issues/limitations
Chapter 1 Introduction 15
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 16
Scalability Decentralized algorithms
o No machine has complete system state
o Decisions based on local infoo Failure of one machine does not kill
entire algorithmo No assumption of global clock
Examples?
Chapter 1 Introduction 17
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 18
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 exampleo Replication allows for load
balancing Replication creates consistency
issues
Chapter 1 Introduction 19
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 20
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 21
Hardware Issues For our purposes, 2 kinds of
machines Multiprocessor
o Different processors share same memory
Multicomputero Each processor has it’s own memory
Each of these could use either bus or switched architecture
Chapter 1 Introduction 22
Hardware Issues
multiprocessor multicomputer
Chapter 1 Introduction 23
Multiprocessors
A bus-based multiprocessor Cache coherence is an issue
Chapter 1 Introduction 24
Multiprocessors
a) A crossbar switchb) Omega switching network
Chapter 1 Introduction 25
Homogeneous Multicomputer
Grid Hypercube
Chapter 1 Introduction 26
Software Concepts
DOS Distributed Operating Systems NOS Network Operating Systems Middleware self-explanatory
System Description Main Goal
DOSTightly-coupled operating system for multi-processors and homogeneous multicomputers
Hide and manage hardware resources
NOSLoosely-coupled operating system for heterogeneous multicomputers (LAN and WAN)
Offer local services to remote clients
MiddlewareAdditional layer atop of NOS implementing general-purpose services
Provide distribution transparency
Chapter 1 Introduction 27
Uniprocessor OSs
Separate apps from OS code via microkernel
Chapter 1 Introduction 28
Multiprocessor OSs
How to protect count from concurrent access?
monitor Counter {
private:
int count = 0;
public:
int value() { return count;}
void incr () { count = count + 1;}
void decr() { count = count – 1;}
}
Chapter 1 Introduction 29
Multiprocessor OSs
Protect count from concurrent accesso Using blocking
monitor Counter {
private:
int count = 0;
int blocked_procs = 0;
condition unblocked;
public:
int value () { return count;}
void incr () {
if (blocked_procs == 0)
count = count + 1;
else
signal (unblocked);
}
void decr() {
if (count ==0) {
blocked_procs = blocked_procs + 1;
wait (unblocked);
blocked_procs = blocked_procs – 1;
}
else
count = count – 1;
}
}
Chapter 1 Introduction 30
Multicomputer OSs
Multicomputer OS
Chapter 1 Introduction 31
Multicomputer OSs
???
Chapter 1 Introduction 32
Multicomputer OSs
Huh?
Synchronization point Send bufferReliable 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
Chapter 1 Introduction 33
Programming Issues Programming multicomputers
much harder than multiprocessors Why?
o Message passingo Buffering, blocking, reliable comm.,
etc. One option is to emulate shared
memory on multicomputero Large “virtual” address space
Chapter 1 Introduction 34
Distributed Shared Memory
a) Pages of address space distributed among 4 machines
b) After CPU 1 references pg 10
c) If page 10 read only and replication used
Chapter 1 Introduction 35
Distributed Shared Memory
False sharing of page between two processeso Two independent processors share same page
Chapter 1 Introduction 36
Network OS
Network OSo Each processor has its own OS
Chapter 1 Introduction 37
Network OS
Clients and server in a network OS Global shared file system
Chapter 1 Introduction 38
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 39
Positioning Middleware
A distributed system as middlewareo Individual node managed by local OSo Middleware hides heterogeneity of underlying systems
Chapter 1 Introduction 40
Middleware and Openness
Open middleware-based system Middleware layer should
o Use the same protocolso Provide same interfaces to apps
Chapter 1 Introduction 41
Comparison of Systems
Middleware rocks!
Item
Distributed OSNetwork OS
Middleware-based OSMultiproc. 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 managementGlobal, central
Global, distributed
Per node Per node
Scalability No Moderately Yes Varies
Openness Closed Closed Open Open
Chapter 1 Introduction 42
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 43
Client Server Model Read this section
Chapter 1 Introduction 44
Clients and Servers
Interaction between client and server
Chapter 1 Introduction 45
Example Client and Server
header.h o Used by
client o And by
server
Chapter 1 Introduction 46
Example Client and Server
A sample server
Chapter 1 Introduction 47
Example Client and Server
Client using server to copy a file
Chapter 1 Introduction 48
Processing Level
Internet search engine as 3 layers
Chapter 1 Introduction 49
Multitiered Architectures
Alternative client-server organizations
Chapter 1 Introduction 50
Multitiered Architectures
A server acting as client
Chapter 1 Introduction 51
Modern Architectures
Horizontal distribution of Web service
Chapter 1 Introduction 52
Summary Distributed system
o Autonomous computers that operate together as a single coherent system
Potential advantageso Can integrate systemso Scales well, if properly designed
Potential disadvantageso Complexityo Degraded performanceo Security
Chapter 1 Introduction 53
Summary Different types of dist systems Distributed OS
o For tightly coupled systemo Can’t integrate different systems
Network OSo For heterogeneous systemo No single system view
Chapter 1 Introduction 54
Summary Middleware systems based on
o Remote procedure callso Distributed objects, files, documentso Vertical organizationo Horizontal organization