introduction
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Distributed Systems CS 3850 Soufiane Noureddine Lectures MWF 14:00 – 14:50 (PE207D) Office Hours MW 11:00 – 12:00 (C520). Introduction. Chapter 1. Computer Networks. - PowerPoint PPT PresentationTRANSCRIPT
Distributed SystemsCS 3850
Soufiane Noureddine
LecturesMWF 14:00 – 14:50 (PE207D)
Office HoursMW 11:00 – 12:00 (C520)
Introduction
Chapter 1
Computer Networks
A collection of computers communicating through an
underlying network is called a computer network (in contrast to
a single-processor system)
Local Area Network: LAN10 to 1000 Mb/sec
Wide Area Network: WAN64 kbps to gigabits/sec
Definition of a Distributed System (1)
A distributed system is:
A collection of independent computers that appears to its
users as a single coherent system.
A. Tanenbaum
Definition of a Distributed System (2)
“You know you have one when the crash of a computer you never heard of stops you from getting
any work done.”
L. Lamport
Organization of a Distributed System
A distributed system organized as middleware.Note that the middleware layer extends over multiple machines.
1.1
Characteristics of a distributed System
Differences between computers are hidden
Communication between computers is hidden
Internal organization of the system is hidden
Single system image: interaction with the system is independent from location (and time)
Ease of extension
High availability
Examples of distributed Systems1. Network of workstations + Pool of processors:
- Single file system (uniform naming scheme)
- Processors are allocated dynamically when needed (load sharing)
- System acts like a single-processor system
2. Workflow systems
- Orders arrive dynamically (e.g. via laptops, cellular phones)
- System assigns orders to the corresponding departments and initiates the needed business processes and users are unaware of the internal flow of orders
- System acts like a centralized database
3. WWW
- No need to know where documents are stored (at least in theory)
- Accessing remote documents is like accessing local ones
Goals of distributed Systems1. Connecting users and resources:
consequence: communication and collaboration (e.g. joint editing)
issue: security
2. Openness
- Services should obey to standard rules specifying syntax and semantics
- Services are specified in general as interfaces in an interface definition language
- IDL includes rather syntax and no semantics
- Specification of interface should be completed and neutral (w.r.t. implementation)
- Openness promotes interoperability and portability
3. Transparency (see next slides)
4. Scalability (see next slides)
Transparency in a Distributed System
Different forms of transparency in a distributed system.
Degree of transparency: More transparency means less performance
In reality: Systems are transparent only to certain degree
Transparency Description
AccessHide 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
RelocationHide that a resource may be moved to another location while in use
Replication Hide that a resource is replicated
ConcurrencyHide that a resource may be shared by several competitive users
FailureHide the failure and recovery of a resource
PersistenceHide whether a (software) resource is in memory or on disk
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Company A
Company B
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Company A
Company B
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Company A
Company B
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Company A
Company B
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Applications
Operating System & Hardware
Clients Software Developers
e.g. Company
Network
(e.g. Telecom)
Network Provider
Applications
Operating System & Hardware
AEM: Availability Enhancing Middleware
Scalability Problems
Examples of scalability limitations.
Centralized solutions tend to be non-scalable
Decentralized solutions promote scalability
Concept Example
Centralized servicesA single server for all users (e.g. a single DNS server)
Centralized dataA single on-line telephone book (e.g. single table in a frequently used database)
Centralized algorithmsDoing routing based on complete information
A system is scalable when it is easy to extend without loss of performance.
Decentralized Algorithms
1. No machine has complete information about the system state
2. Machines make decisions based only on local information
3. Failures of one machine does not ruin the whole algorithm
4. No assumption on the existence of a global time
Scaling Techniques (1)
Ways to solve scalability problems:
a) Hide communication latencies (geographical scalability)
b) Use of distribution (in order to avoid bottlenecks)
c) Use of replication
Ad a)
Batch processing: Asynchronous communication instead of synchronous communication
Interactive applications: Reduce overall communication
Example: Move part of computation from server to client (e.g. Java applet)
Scaling Techniques (2)
1.4
The difference between letting:
a) a server or
b) a client check forms as they are being filled
Scaling Techniques (3)
1.5
An example of dividing the DNS name space into zones.
Clientserver Z1: nl.vu.cs.fluit
Server Z1 Client: address of Z2
Clientserver Z2: vu.cs.fluit
…
Ad b) Use of distribution
Scaling Techniques (4)
Ad c) Use of replication
Replication raises:
Availability: in general the primary goal
Performance:
By balancing the load among replicas
By locating replicas close to users/clients
Example: Caching (e.g. browser cache for used WWW pages)
Main issue in connection with replication: consistency
Hardware Concepts
1.6
Different basic organizations and memories in distributed computer
systems
Multiprocessors (1)
A bus-based multiprocessor.Issues:
Scalability
Cache consistency
1.7
Multiprocessors (2)
a) A crossbar switch: n2 switches!b) An omega switching network: less switches
1.8
Homogeneous Multicomputer Systems
a) Gridb) Hypercube
1-9
Software Concepts
An overview of • DOS (Distributed Operating Systems)• NOS (Network Operating Systems)• Middleware
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
Uniprocessor Operating Systems
Separating applications from operating system code through a microkernel.
1.11
Multiprocessor Operating Systems (1)
A monitor to protect an integer against concurrent access.
monitor Counter {
private:
int count = 0;
public:
int value() { return count;}
void incr () { count = count + 1;}
void decr() { count = count – 1;}
}
Multiprocessor Operating Systems (2)
A monitor to protect an integer against concurrent access, but blocking a process.
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;
}
}
Multicomputer Operating Systems (1)
General structure of a multicomputer operating system
1.14
Multicomputer Operating Systems (2)
Alternatives for blocking and buffering in message passing.
1.15
Multicomputer Operating Systems (3)
Relation between blocking, buffering, and reliable communications.
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
Distributed Shared Memory Systems (1)
a) Pages of address space distributed among four machines
b) Situation after CPU 1 references page 10
c) Situation if page 10 is read only and replication is used
Replicating all pages:
Coherence protocols:
a) strong (transparent)
b) weak (not transparent)
Distributed Shared Memory Systems (2)
False sharing of a page between two independent processes.
Problem with DSM: not as efficient as expected
1.18
Page size: small more communication overhead
large less communication, but false sharing may occur
Network Operating System - NOS (1)
General structure of a network operating system.
Main features: Independent operating systems
Services for accessing remote resources
1-19
Network Operating System (2)
Two clients and a server in a network operating system.
1-20
Network Operating System (3)
Different clients may mount the servers in different places.
1.21
NOS vs DOS (3)
Distributed operating system:
Fully transparent
For homogeneous systems computers are not independentMore secure, but less scalable and less open
Network operating system:
Not transparent
For heterogeneous system computers are independent
Easier to extend (e.g. adding a new node in the Internet)
Both do not really qualify as a distributed system!!!
Solution: Middleware atop of a NOS hiding heterogeneity and improving transparency
Positioning Middleware
General structure of a distributed system as middleware.
Middleware Services: rather a functionally complete set of services in order to hide heterogeneity, direct access to NOS is discouraged.
1-22
Middleware and Openness
In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications. implementations of the middleware should not use the NOS-provided protocols
1.23
Comparison between Systems
A comparison between multiprocessor operating systems, multicomputer operating systems, network operating systems, and middleware-based distributed systems.
ItemDistributed OS
Network 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 FilesModel
specific
Resource management
Global, central
Global, distributed
Per node Per node
Scalability No Moderately Yes Varies
Openness Closed Closed Open Open
Clients and Servers
General interaction between a client and a server.
1.25
An Example Client and Server (1)
The header.h file used by the client and server.
An Example Client and Server (2)
A sample server.
An Example Client and Server (3)
A client using the server to copy a file.
1-27 b
Processing Level
The general organization of an Internet search engine into three different layers
1-28
Multitiered Architectures (1)
Alternative client-server organizations (a) – (e).
1-29
Multitiered Architectures (2)
An example of a server acting as a client.
1-30
Modern Architectures
An example of horizontal distribution of a Web service.
1-31