1. Distributed Processing, Client/Server and Clusters
Chapter 16

2. Client/Server Computing
Client machines:
single-user PCs or workstations that provide a highly user-friendly interface to the end user
The server provides a set of shared user services to the clients
The server enables many clients to access shared resources (e.g. a large database).
The server typically runs on a cloud or a high-performance computer system.

3. Client/Server Applications
Client and server platforms/OS may be different
These lower-level differences are irrelevant as long as a client and server share the same communications protocols (ex: TCP/IP) and support the same applications
Actual functions performed by the application can be split up between client and server in a way that optimizes the use of resources
Optimize the ability of users to perform various tasks and to cooperate with one another using shared resources
Heavy emphasis on providing a user-friendly Graphical User Interface (GUI) on the client side (presentation services layer)

4. Generic Client/Server Architecture

5. Database Applications
One of the most common families of client/server applications
The server is a database server responsible for maintaining the database
Interaction between client and server is in the form of transactions
the client makes a database request and receives a database response
A variety of different client applications can use the same database server; all using the same interface/protocol

6. Client/Server Architecture for Database Applications
e.g. SQL: Structured Query Language

7. Client/Server Database Usage Example:
think of an application in which we need to compute the mean of the ages of a certain population and the search criteria returns 300K records
 heavy network traffic
To optimize performance: server must be equipped with application logic for performing data analysis (computation of mean).

8. Client/Server Interaction Yet another example:
think of an application in which we need to search for a person named Jo Smith, born before 1980.
If initial query results in 100K possible records, the server may just indicate that without sending the records.
Then search can be narrowed down and the # of possible records is reduced drastically.
After a few iterations we may receive the desired record.
Do not return the records

9. Classes of Client/Server Applications
Host-based (dumb terminal)
not true client/server computing
traditional mainframe environment.
Text-based interface to a mainframe.
(e.g. used only for login, issuing shell commands, viewing results, and logout)
Server-based (thin client)
server does all the processing
User(client) workstation provides a graphical user interface
(e.g. Business-specific applications:
Taking orders and entering them into a database.
Producing and printing receipts etc.)

10. Classes of Client/Server Applications
Fat client models
Takes advantage of desktop power and can serve large number of clients
by offloading application processing from servers to the clients.
Cooperative
application processing is performed in an optimized fashion
complex to set up and maintain but
greater user productivity gains and greater network efficiency
Client-based
Most common client/server model
all application processing done at the client
data validation and other database logic functions are done at the server
Some of the database logic functions are housed on the client side
(e.g. hotel search on expedia – narrow down search based on star/price/etc)

11. Three-Tier Client/Server Architecture
Application software distributed among three types of machines
User machine
thin client
Middle-tier server
Gateway
Converts protocols
Map from one type of database query to another
Merge/integrate results from different data sources
Assumes both roles:
server & client
Backend server
Legacy applications

12. File Cache Consistency
File caches hold recently accessed file records
Cache consistency problem:
Caches are consistent when they contain exact copies for remote data
Simple solution: File-locking prevents simultaneous access to a file
Complicated approach: allow multiple read but one write access; when there is an access for ‘write’, mark the file as non-cacheable for others.

13. Middleware
Lack of standards for client/server models makes it difficult to implement an integrated, multivendor, enterprise-wide client/server configuration
Middleware: Set of tools that provide a uniform means and style of access to system resources across different platforms. Goal: to enable an application or user at a client to access a variety of services on servers without being concerned about differences among them
Provides standard programming interfaces/protocols that sit between the application above and the communications software+OS below.
Capable to hide the complexities and disparities of different network protocols and OS
Enable programmers to build applications that look and feel the same with little effort
Enable programmers to use the same method to access data

14. Distributed Message Passing
Middleware products typically use one of the two underlying mechanisms for communication.
Message-passing or RPC (Remote procedure calls)

15. Message-passing schemes
Reliable
Guarantees delivery if possible - Not necessary to let the sending process know that the message was delivered
Unreliable
Send the message out into the communication network without reporting success or failure - Reduces complexity and overhead
Blocking
Send does not return control to the sending process until the message has been transmitted
OR does not return control until an acknowledgment is received
Receive does not return until a message has been placed in the allocated buffer
Nonblocking
Process is not suspended as a result of a Send or a Receive
Efficient and flexible
Difficult to debug

16. Clusters Clusters Compared to SMP
Alternative to symmetric multiprocessing (SMP)
Group of interconnected, whole computers working together as a unified computing resource
illusion is one machine
system can run on its own
Clusters Compared to SMP
SMP is easier to manage and configure
SMP takes up less space and draws less power
Clusters are better for incremental and absolute scalability
Add new systems in small increments
Can have dozens of machines each of which is a multiprocessor
Clusters are superior in terms of availability
Failure of one node does not mean loss of service
Clusters have superior price/performance

17. Beowulf and Linux Clusters
Mass market commodity components (No custom components)
A dedicated, private network (LAN or WAN or internetworked combination)
Easy replication from multiple vendors
Scalable I/O
A freely available software base

18. Issues on Clusters Failure management Load balancing
Highly available vs. fault-tolerant clusters
Highly available clusters offers a high probability that all resources will be in service
Fault-tolerant cluster ensures that all resources are always available (use of redundant disks/processors etc.)
Load balancing
When a new computer is added to the cluster, the load-balancing facility should automatically include this computer in scheduling applications
Parallelizing Computation
Parallelizing compiler
Parallelized application