istributed system

Principles and characteristics of
distributed systems and
environments

Definition of a distributed system
 Distributed system is a collection of independent
computers that appears to its users as a single coherent
system.
 Distributed computing is decentralised and parallel
computing, using two or more computers communicating
over a network to accomplish a common objective or
task. The types of hardware, programming languages,
operating systems and other resources may vary
drastically.
 Leslie Lamport: A distributed system is one I which the
failure of a computer you didn’t even know existed can
render your computer unusable.

Definition cont.
 2 aspects
Deals with hardware: machines are autonomous
Deals with software: the users think they are dealing
with a single system
 Important characteristics
Communication is hidden from users
Applications can interact with uniform and consistent
way
Relatively easy to expand or scale (DS will normally be
continuously available)
A distributed system should be functionally equivalent to
the systems of which it is composed.

Distributed system as Middleware service
If the system as a whole looks and acts like a classical
single-processor timesharing system, it qualifies as a
distributed system.

Goals
A DS should easily connect users to
resources; it should hide the fact that
resources are distributed across a
network; should be open and scalable.
4 goals:
Connecting users and resources
Transparency
Openness
Scalability

Connecting users and resources
Share resources (hw components, data)
Groupware
Security aspects

Transparency
Transparent ~ present itself to users and
applications as if it were only a single
computer system
Different forms of transparency in a distributed system
ANSA Reference Manual [1989] and ISO Reference Model
For Open Distributed Processing (RM-ODP) [1992/7]

Openness
 System that offers services according to standard rules
that describe the syntax and semantics
 Generally specified through interfaces, often described
in Interface Definition Language (IDL)
 Interoperability – characterizes two implementations of
system can co-exists and work together
 Portability – characterizes to what extent an application
developed for a distributed system A can be executed,
without modification, on distributed system B that
implements the same interface as A
 Small replaceable and adaptable parts - provide
definitions of not only the highest-level interfaces, those
used by applications, but also definitions for interfaces to
internal parts

Scalability
Measure at least 3 different dimensions
Size – easily add more users and resources to
system
Geographically scalable system – lie far apart
Administratively scalable – independent
administrative organizations
Scalability in one or more dimensions
often means loss of performance – should
not be significant.

Scalability cont.
 Performance
 Congestion of communication links
 Only decentralized algorithms should be used (no
machine has complete information; decisions based on
local information; failure of one machine does not ruin
the algorithm; no implicit assumption of global clock)
Example of scalability limitations

Scaling techniques
3 techniques for scaling
Hiding communication latencies - Asynchronous
communication
Distribution - domains
Replication – caching -> consistency problems !

Why Distributed Systems?
Geographically distributed environment
Speed up
Parallel vs. Distributed Systems (synchronous
SIMD or MIMD; asynchronous process)
Resource sharing
HW and SW (databases)
Fault tolerance
Detection, Recover
Logical distribution of functional capabilities: multiple processes, interprocess
communication, disjoint address space, collective goal

Issues in Distributed Systems
 Knowledge of process
 Identity, Identity of it’s immediate neighbors, communication
channel to neighbors
 Network topology
 Completely or sparsely connected, message routing, channel
uni/bi-directional
 Degree of synchronization
 Clock drift, propagation delay
 Failures
 Type of failure and duration of failure
 Scalabilty
 Time and space complexity (O(log N) ~ excellent, O(N) ~ pure)

Common Subproblems
Leader election
Mutual exclusion
Time synchronization
Global state
Replica management

RM-ODP
Reference Model – Open Distributed
Processing
ITU-T X.9xx standards, X.901 – Overview
Viewpoints (and their languages)
Enterprise
Information
Computational
Engineering
Technology

Hardware concepts
DS consist of multiple CPU – several
different ways of HW organization
Multiprocessors – have shared memory
Multicomputers – without shared memory

Hardware concepts cont.
Architecture of interconnection network
Bus
Switched
Distributed computer system (significant
for multicomputers)
Homogenous – single interconnection network
Heterogeneous – different networks (LAN
connected through FDDI, ATM)

Multiprocessors
Coherent
Overloaded bus –> cache memory
between CPU and bus (hit rate)
Bus suitable until 256 CPUs

Multiprocessors cont.
 Crossbar switch – crosspoint switch
n CPUs and n memories = n2 crosspoint switches
 Omega network
2x2 switches, each two inputs and two outputs
Fast switching, not cheap
 NUMA (NonUniform Memory Access)
Fast access to local memory, slow to other’s
Better average access times than omega networks

Multiprocessors cont.
Crossbar switch Omega network

Homogenous Multicomputer Systems
 Relative easy acording multiprocessors
 CPU-to-CPU and CPU-to-memory
communication
 System Area Networks (SANs)
High performance interconnection network
 Bus-based – Fast Ethernet 25-100 nodes
 Switch-based – messages routed
Mesh, hypercube
Vary widely from Massively Parallel Processors
(MPPs) to Clusters of Workstations (COWs)

Heterogenous Multicomputer Systems
Most distributed systems build on it
Due to scale, inherent heterogeneity, and
most of all, lack of global system view,
sophisticated software is needed to build
apps for heterogeneous multicomputers.
 Provide transparency for apps.
Example
DAS http://www.cs.vu.nl/~bal/das.html
I-way project

Software concepts
 Determines what a DS actually looks like
 Resource manager for underlying hardware
 Operating systems for distributed computers
Tightly-coupled – distributed operating system
(multiprocessors, homogenous multicomputer systems)
Loosely-coupled – network operating system
(heterogeneous multicomputer systems)
Middleware – enhancements to services of network
operating systems – distribution transparency

Uniprocessor Operating Systems
Implement a virtual machine
Apps protected each other
OS should be in full control of how hw
resources are used and shared
Kernel mode
User mode
Architecture
Monolithic kernel
Microkernel

Multiprocessor Operating Systems
Support for multiple CPUs having access
to shared memory
IPC done through memory – protect
against simultaneous access
Synchronization primitives
Semaphore – atomic operations down and up
Monitor – private variable and public
procedures (operations)

Multicomputer Operating Systems
Different structure and complexity than
multiprocessor operating systems
Communication done by message
passing
Software implementation of shared
memory – not always
Message passing primitives may vary
widely in different OS

Multicomputer Operating Systems cont.

Multicomputer Operating Systems cont.
Reliability of message delivery ?

Distributed Shared Memory Systems
Page-based distributed shared memory
Address space divided into pages (4kB or 8kB)
When CPU references address not present
locally, trap occurs, OS fetches the page
Essential normal paging

Distributed Shared Memory Systems
cont.
(a) Pages in DSM
(b) CPU 1 references page 10
(c) Situation if page 10 is read
only and replication used

Network Operation Systems
Do not assume that underlying hardware
is homogeneous and that it should be
managed as if it were a single system
Facilities to allow users to make use of the
services available on specific machine
Service accessed commonly using
commands
rlogin machine
rcp machine1:file1 machine2:file2

Network Operation Systems cont.
Network operating systems are clearly more primitive than distributed
operating systems. The main distinction between the two types of operating
systems is that distributed operating systems make serious attempt to
realize full transparency, that is, provide a single-system view.

Middleware
DOS and NOS don’t qualify definition of
DS
Modern distributed systems are
constructed by means of an additional
layer called middleware.
Additional layer of software between
applications and the network operating
system

Middleware cont.
Goal is to hide heterogeneity of the underlying platforms from applications

Middleware cont.
Middleware models (paradigm)
Everything as file (simple) – Plan 9, UNIX
Distributed File Systems – reasonable scalable
which contributes popularity
Remote Procedure Calls – unaware of network
communication
Distributed Objects – unaware of network
communication
Distributed documents - WWW

Middleware cont.
 Middleware services
high-level Communication facilities - Access
transparency
Naming – allows entities to be shared and accessed
Persistence – through distributed file system or
database
Distributed transactions – atomic multiple read and
write operations; write operation succeeds or fails
Security – can’t rely on the underlying local operating
system, must be implemented in middleware, security
has turned out to be one of the hardest services to
implement

Middleware and Openness
 Modern distributed systems as middleware
 Applications are independent of operating
system
 Independence replaced by strong dependency
of specific middleware
 Need that middleware protocols and interfaces
are the same in different implementations

Client – Server Model
Server – process implementing a specific
service
Client – process that requests service
from server by sending request and
waiting for response

Client – Server Model cont.
Application layering
Distinction of 3 levels
The user-interface level
The processing level
The data level

Multitiered architectures
Three-tier architecture – physically

Modern architectures
 Vertical distribution – multitiered architectures
 Horizontal distribution – distribution split into
equivalent parts (clients or servers)
 Peer-to-peer distribution

istributed system

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