3. Distributed Systems –
Architecture Models
Part 2
Object Relationships
2
Broker Pattern:
Consequences
• Benefits
–
–
–
–
–
Location transparency
Changeability and extensibility of components
Portability of a Broker system
Interoperability between different Broker systems
Reusability
• Liabilities
– Restricted efficiency
– Lower fault tolerance (server fails, broker fails, ...)
– Testing and debugging
3
SUMMARY of Proxy Design Pattern
Problems
Context
• The configuration of
components in
distributed systems is
often subject to change as
requirements evolve
• Low-level message passing is fraught with accidental
complexity
• Remote components should look like local components
from an application perspective
• i.e., clients & servers should be oblivious to
communication issues
Solution
Apply the Proxy design pattern to
provide an OO surrogate through
which clients can access remote
objects
: Client
: Proxy
: Service
service
pre-processing:
marshaling
service
post-processing:
unmarshaling
AbstractService
Client
service
Proxy
service
1
1
Service
service
•A Service implements the object, which is not
accessible directly
•A Proxy represents the Service and
ensures the correct access to it
•Proxy offers same interface as Service
6
•Clients use the Proxy to access the Service
Client-server Architecture
Client-Server Model
7
Multi-Tier Architecture Model
Client-Server Model
a client, a server, and network
Client
Server
Network
Client machine
Server machine
9
The Client-Server Model
Client
Machine
Mobile
Client
Server
Desktop
Client
10
Client-server
• Client/server is a distributed computing model in
which client applications request services from server
processes. Clients and servers typically run on
different computers interconnected by a computer
network.
– Client application is a process or program that sends
messages to a server via the network. Those messages
request the server to perform a specific task, such as
looking up a customer record in a database or returning a
portion of a file on the server’s hard disk.
– Server process or program listens for client requests that
are transmitted via the network. Servers receive those
requests and perform actions such as database queries
and reading files.
2-Tier Architecture
 It is client-server architecture
 Direct communication
 Run faster(tight coupled)
2-Tier Architecture
2-Tier Architecture
 Two parts:
1) Client Application (Client Tier)
2) Database (Data Tier)
 On client application side the code is written for saving the data
in the SQL server database
3-Tier Architecture
 Web based application
 Three layers:
1) Client layer
2) Business layer
3) Data layer
Client Layer
Contains UI part of our application
 This layer is used for the design purpose where data is
presented to the user or input is taken from the user
Business layer
All business logic written like validation of data,
calculations, data insertion etc…
 This acts as a interface between Client layer and Data
Access Layer
 Make communication faster between client and data
layer
Data layer
 Contains methods to
connect with database and
to perform insert, update, delete, get data from database
3-Tier / Multi-Tier Architecture Model
Classical Layered Structure of Software Systems
Three-tier Architectures
• An important design consideration for large client/server
systems is whether a client talks directly to the server, or
whether an intermediary process is introduced in-between
the client and the server. The former is a two-tier
architecture, the latter is a three-tier architecture.
• In the three-tier architecture, process between Server and
client (intermediary) process is:
– separate the clients and servers.
– cache frequently accessed server data to ensure better performance
and scalability.
– Performance can be increased by having the intermediary process to
distribute client requests to several servers such that requests execute
in parallel.
– The intermediary can also act as a translation service by converting
requests and replies to and from a mainframe format, or as a security
service that grants server-access only to trusted clients.
The 3-Tier Architecture Model
• The 3-tier architecture consists of the following tiers
(layers):
– Front-end (client layer)
• Client software – provides the UI of the system
– Middle tier (business layer)
• Server software – provides the core system logic
• Implements the business processes / services
– Back-end (data layer)
• Manages the data of the system (database / cloud)
21
The 3-Tier Architecture Model
Data Tier
(Back-End)
Middle Tier
(Business Tier)
Client Tier (Front-End)
Client
Machine
Mobile
Client
Database
Business
Logic
Desktop
Client
22
Multitiered Architectures
(3 Tier Architecture)
Multi-Tier Architecture Model
24
MVC
(Model-View-Controller)
Model-View-Controller (MVC)
• Model-View-Controller (MVC) architecture
– Separates the business logic from application data
and presentation
• Model
– Keeps the application state (data)
• View
– Displays the data to the user (shows UI)
• Controller
– Handles the interaction with the user
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Peer-to-Peer System Architecture
27
Based on System Architecture :
Peer-to-Peer Model
• All the nodes in the network have equal privilege
• No special routing server required. The nodes
themselves take care of routing the data
• Peers – both suppliers and consumers
• As the number of nodes increases, the bandwidth
increases
• Reduces single point failures
• It is less secure
• Usually used for sharing of resources – files,
audio - visual media
28
A Distributed Application Based on
Peer Processes
29
Fail-over
Fail-over
31
Fail-over
32
Fail-back
33
Replication
35
Master-Slave Replication
36
Master-Slave Replication
37
Tree Replication
38
Master-Master Replication
39
Buddy Replication
40
Buddy Replication
41
Brief Early History of Computing
Intergalactic era
client/server
Ethernet era
client/server
First Wave
Second Wave
Database
servers
File
servers
1982
1986
Third Wave
Distributed
objects
1990
1994
1998
43
Future of Distributed Computing…
• An expanded peer-to-peer network with every
personal computer interconnected with every
other.
• Utilization of CPU cycles and hard disks during
idle time
• Improvements in security and performance
that void the need for a centralized control
server and a data center
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45
Crab Computing
46
DESIGN REQUIREMENTS FOR
DISTRIBUTED ARCHITECTURES
Distributed system (Lamport):
a system in which the failure of a
computer you didn't even know
existed can render your own computer
unusable
48
Distributed System Challenges
49
Design Requirements for Distributed
Architectures
50
Guidelines for Designing for
Performance
51
Some Pitfalls when Developing
Distributed Systems
52