Design Issues of Distributed Systems
Designing issues of DS
·
Heterogeneity
·
Openness
·
Security
·
Synchronization
·
Absence of global clock
·
Partial failures
·
Scalability
·
transparency
Heterogeneity:
- The distributed system contains many different kinds of hardware
and software working together in cooperative fashion to solve problems.
- There may be many different representations of data in the system
this might include different representations for integers, byte streams,
floating point numbers and character sets.
- There may be many different instructions sets. An application
compiled for one instruction set cannot be easily run on a computer with
another instruction set unless an instruction set interpreter is provided
- Components in the distributed system have different capabilities
like faster clock cycles, larger memory capacity, bigger disk farms,
printers and other peripherals and different services
High Degree of node
heterogeneity:
- High-performance parallel systems (multiprocessors as well as
multicomputer)
- High-end PCs and workstations (servers)
- Simple network computers (offer users only network access)
- Mobile computers (palmtops, laptops)
- Multimedia workstations
High degree of network
heterogeneity:
- Local area gigabit networks
- Wireless connections
- Long-haul, high-latency connections
Observation: Ideally, a
distributed system must hide these differences
Openness:
- The openness of a computer system is the characteristic that
determines whether the system can be extended and reimplemented in various
ways
- The challenge to designers is to tackle the complexity of
distributed systems consisting of many components engineered by different
people
- Open systems are characterized by the fact that their key
interfaces are published\
- Open distributed systems are based on the provision of a uniform
communication mechanism and published interfaces for access to shared
resources
- Open distributed systems can be constructed from heterogeneous
hardware and software, possibly from different vendors
Security
- Shared data must be protected
o Privacy - avoid unintentional disclosure of private
data
o Security – data is not
revealed to unauthorized parties
o Integrity – protect data and
system state from corruption
- Denial of service attacks – put significant load on the system,
prevent users from accessing it
Security
in detail concerned in the following areas:
·
Authentication, Authorization/Access control: are the means to identify
the right user and user right
·
Critical Infrastructure Protection: CIP is the protection of
information systems for critical infrastructures including telecommunications,
energy, financial services, manufacturing, water, transportation, health care
and emergency services sectors
·
Distributed Trust and Policy Management: designed to address the
authorization needs for the next-generation distributed systems. A trust
management system is a term coined to refer to a unified framework for the
speciation of security policies, the representation of credentials, and the
evaluation and enforcement of policy compliances
·
Multicasting security and IPR Protection: defines the common
architecture for multicast security(MSEC) key management protocols to support a
variety of application, transport, network layer security protocol and the
intellectual property rights
·
Multimedia Security: is intended to provide an advanced multimedia
application course with its focus on security. two major areas of concern- to
ensure secure uses of multimedia data and to use multimedia data for security
applications
Ø Object security (OMG/CORBA
security, EJB Security, DCOM/COM Security)
Ø Privacy: is it a purely
political or moral issue?
·
Risk analysis, Assessment, Management: A security policy framework is
necessary to support the security infrastructure required for the secure
movement of sensitive information across and within national boundaries
Synchronization
·
Concurrent cooperating tasks need to synchronize
o When accessing shared data
o When performing a common
task
·
Synchronization must be done correctly to prevent data corruption:
o Example: two account owner;
one deposits the money, the other one withdraws; they act concurrently
o How to ensure the bank
account is in “correct” state after these actions?
·
Synchronization implies communication
·
Communication can take a long time
·
Excessive synchronization can limit effectiveness and scalability of
distribute system
Absence of Global Clock
- Cooperating task need to agree on the order of events
- Each task its own notion of time
- Clocks cannot be perfectly synchronized
- How to determine which even occurred first?
Example:
Bank account, starting
balance = $100
Client at bank machine A
makes a deposit of $150
Client at bank machine B
makes a withdrawal of $100
Which event happened first?
Should the bank charge the
overdraft fee?
Partial Failures
- Detection of failures - may be impossible
- Has a component crashed? Or is it just show?
- Is the network down? Or is it just slow?
- If it’s slow – how long should we wait?
- Handling of failures
- Re-transmission
- Tolerance for failures
- Roll back partially completed task
- Redundancy against failures
- Duplicate network routes
- Replicated databases
Scalability
- Does the system remain effective as of grows?
- As you add more components:
- More synchronization
- More communication à the system runs
slowly.
- Avoiding performance bottlenecks:
- Everyone is waiting for a single shared resource
- In a centrally coordinated system, everyone waits for the
co-coordinator
Transparency
Distributed systems
designers must hide the complexity of the systems as much as they can. Adding
abstraction layer is particularly useful in distributed systems.
Example: While users hit
search in google.com, they never notice that their query goes through a complex
process before Google shows them a result
- Concealing the heterogeneous and distributed nature of the system
so that it appears to the user like one system
Transparency categories
Access: access local and remote
resources using identical operations (NFS or Samba-mounted file systems)
Location: access without knowledge of
location of a resource (URL’s, e-mail)
Concurrency: allow several processes to
operate concurrently using shared resources in a consistent fashion (two users
simultaneously accessing the bank account)
Transparency
categories
Mobility: allow resources to move
around
Performance: adaption of the system to
varying load situations without the user noticing it
Scaling: allow system and
applications to expand without need to change structure of applications or
algorithms
