Distributed OS - An Introduction

Distributed Systems:
Principles and Paradigms
By Andrew S. Tanenbaum
and Maarten van Steen

Outline
1. Introduction
2. Communication
3. Processes
4. Naming
5. Synchronization
6. Consistency and Replication
7. Fault Tolerance
8. Security
9. Distributed Object-based Systems
10. Distributed File Systems
11. Distributed Document-based systems
12. Distributed Coordination-based Systems

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

Definition of a Distributed System (2)
A distributed system organized as middleware
Note that the middleware layer extends over multiple machines
1.1

Transparency in a Distributed System
Different forms of transparency in a distributed system
Transparency 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
Hide that a resource may be shared by several
competitive users
Failure Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or on
disk

Scalability Problems
Examples of scalability limitations
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

Scaling Techniques
1. Hiding communication latencies
2. Distribution
3. Replication

Scaling Techniques (1)
1.4
The difference between letting:
a) a server or
b) a client check forms as they are being filled

Scaling Techniques (2)
1.5
An example of dividing the DNS name space into zones

Hardware Concepts
1.6
Different basic organizations and memories in distributed computer
systems

Multiprocessors (1)
A bus-based multiprocessor
1.7

Multiprocessors (2)
a) A crossbar switch
b) An omega switching network
1.8

Homogeneous Multicomputer
Systems
a) Grid
b) Hypercube
1-9

Software Concepts
An overview of
• DOS (Distributed Operating Systems)
• NOS (Network Operating Systems)
• Middleware
System Description Main Goal
DOS
Tightly-coupled OS for multi-processors and
homogeneous multicomputers
Hide and manage
hardware
resources
NOS
Loosely-coupled OS 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

Uniprocessor Operating Systems
Separating applications from OS 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

(2)
Alternatives for blocking and buffering in message passing
1.15

(3)
Relation between blocking, buffering, and reliable
communications
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

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

Distributed Shared Memory Systems
(2)
False sharing of a page between two independent processes
1.18

Network Operating System (1)
General structure of a network operating system
1-19

Two clients and a server in a network operating system
1-20

Different clients may mount the servers in different places
1.21

Positioning Middleware
General structure of a distributed system as middleware
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
1.23

Comparison between Systems
A comparison between multiprocessor OS,
multicomputer OS, network OS, and middleware
based distributed systems
Item
Distributed 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 Files Model 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

A sample server

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

Distributed OS - An Introduction

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