Csc concepts

Dept of CSE, SKCET

CLIENT SERVER COMPUTING –

CONCEPTS

II ME CSE

2015-16 ODD SEMESTER (III SEMESTER)

Dept of CSE, SKCET

TOPICS

DBMS concepts and architecture

Single system image, Client Server architecture

Mainframe-centric client server computing

Downsizing and client server computing

Preserving mainframe applications investment

through porting

Client server development tools

Advantages of client server computing.

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DBMS system architecture

Centralized and Client-Server Systems

Server System Architectures

Parallel Systems

Distributed Systems

Network Types

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Centralized Systems

Run on a single computer system and do not interact with

other computer systems.

General-purpose computer system: one to a few CPUs

and a number of device controllers that are connected

through a common bus that provides access to shared

memory.

Single-user system (e.g., personal computer or

workstation): desk-top unit, single user, usually has only

one CPU and one or two hard disks; the OS may

support only one user.

Multi-user system: more disks, more memory, multiple

CPUs, and a multi-user OS. Serve a large number of

users who are connected to the system vie terminals.

Often called server systems.

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A Centralized Computer System

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Client-Server Systems

Server systems satisfy requests generated at m client

systems, whose general structure is shown below:

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Client-Server Systems (Cont.) Database functionality can be divided into:

Back-end: manages access structures, query evaluation and optimization, concurrency control and recovery.

Front-end: consists of tools such as forms, report-writers, and graphical user interface facilities.

The interface between the front-end and the back-end is through SQL or through an application program interface.

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Client-Server Systems (Cont.)

Advantages of replacing mainframes with networks

of workstations or personal computers connected to

back-end server machines:

better functionality for the cost

flexibility in locating resources and expanding facilities

better user interfaces

easier maintenance

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Server System Architecture

Server systems can be broadly categorized into

two kinds:

transaction servers which are widely used in

relational database systems, and

data servers, used in object-oriented database

systems

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Transaction Servers

Also called query server systems or SQL serversystems Clients send requests to the server

Transactions are executed at the server

Results are shipped back to the client.

Requests are specified in SQL, and communicated to the server through a remote procedure call (RPC) mechanism.

Transactional RPC allows many RPC calls to form a transaction.

Open Database Connectivity (ODBC) is a C language application program interface standard from Microsoft for connecting to a server, sending SQL requests, and receiving results.

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Transaction Server Process

Structure

A typical transaction server consists of multiple processes accessing data in shared memory.

Server processes

These receive user queries (transactions), execute them and send results back

Processes may be multithreaded, allowing a single process to execute several user queries concurrently

Typically multiple multithreaded server processes

Lock manager process

More on this later

Database writer process

Output modified buffer blocks to disks continually

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Transaction Server Processes

(Cont.) Log writer process

Server processes simply add log records to log record

buffer

Log writer process outputs log records to stable

storage.

Checkpoint process

Performs periodic checkpoints

Process monitor process

Monitors other processes, and takes recovery actions

if any of the other processes fail

E.g. aborting any transactions being executed by a server

process and restarting it

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Transaction System Processes

(Cont.)

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Transaction Server Processes

(Cont.) Shared memory contains shared data

Buffer pool

Lock table

Log buffer

Cached query plans (reused if same query submitted again)

All database processes can access shared memory

To ensure that no two processes are accessing the same data structure at the same time, databases systems implement mutual exclusion using either

Operating system semaphores

Atomic instructions such as test-and-set

To avoid overhead of interprocess communication for lock request/grant, each database process operates directly on the lock table

instead of sending requests to lock manager process

Lock manager process still used for deadlock detection

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Data Servers

Used in high-speed LANs, in cases where

The clients are comparable in processing power to the server

The tasks to be executed are compute intensive.

Data are shipped to clients where processing is performed, and then shipped results back to the server.

This architecture requires full back-end functionality at the clients.

Used in many object-oriented database systems

Issues:

Page-Shipping versus Item-Shipping

Locking

Data Caching

Lock Caching

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Data Servers (Cont.)

Page-shipping versus item-shipping Smaller unit of shipping more messages Worth prefetching related items along with requested item Page shipping can be thought of as a form of prefetching

Locking Overhead of requesting and getting locks from server is high

due to message delays Can grant locks on requested and prefetched items; with

page shipping, transaction is granted lock on whole page. Locks on a prefetched item can be P{called back} by the

server, and returned by client transaction if the prefetched item has not been used.

Locks on the page can be deescalated to locks on items in the page when there are lock conflicts. Locks on unused items can then be returned to server.

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Data Servers (Cont.) Data Caching

Data can be cached at client even in between transactions

But check that data is up-to-date before it is used (cache

coherency)

Check can be done when requesting lock on data item

Lock Caching

Locks can be retained by client system even in between

transactions

Transactions can acquire cached locks locally, without

contacting server

Server calls back locks from clients when it receives

conflicting lock request. Client returns lock once no local

transaction is using it.

Similar to deescalation, but across transactions.

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Parallel Systems

Parallel database systems consist of multiple processors and multiple disks connected by a fast interconnection network.

A coarse-grain parallel machine consists of a small number of powerful processors

A massively parallel or fine grain parallel machine utilizes thousands of smaller processors.

Two main performance measures: throughput --- the number of tasks that can be

completed in a given time interval

response time --- the amount of time it takes to complete a single task from the time it is submitted

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Speed-Up and Scale-Up

Speedup: a fixed-sized problem executing on a small system is given to a system which is N-times larger.

Measured by:

speedup = small system elapsed time

large system elapsed time

Speedup is linear if equation equals N.

Scaleup: increase the size of both the problem and the system

N-times larger system used to perform N-times larger job

Measured by:

scaleup = small system small problem elapsed time

big system big problem elapsed time

Scale up is linear if equation equals 1.

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Speedup

Speedup

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Scaleup

Scaleup

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Batch and Transaction Scaleup

Batch scaleup:

A single large job; typical of most decision support

queries and scientific simulation.

Use an N-times larger computer on N-times larger

problem.

Transaction scaleup:

Numerous small queries submitted by independent

users to a shared database; typical transaction

processing and timesharing systems.

N-times as many users submitting requests (hence, N-

times as many requests) to an N-times larger

database, on an N-times larger computer.

Well-suited to parallel execution.

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Factors Limiting Speedup and

Scaleup

Speedup and scaleup are often sublinear due to:

Startup costs: Cost of starting up multiple

processes may dominate computation time, if the

degree of parallelism is high.

Interference: Processes accessing shared

resources (e.g.,system bus, disks, or locks) compete

with each other, thus spending time waiting on other

processes, rather than performing useful work.

Skew: Increasing the degree of parallelism

increases the variance in service times of parallely

executing tasks. Overall execution time determined

by slowest of parallely executing tasks.

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Interconnection Network

Architectures

Bus. System components send data on and receive data from a single communication bus; Does not scale well with increasing parallelism.

Mesh. Components are arranged as nodes in a grid, and each component is connected to all adjacent components Communication links grow with growing number of

components, and so scales better. But may require 2n hops to send message to a node (or n

with wraparound connections at edge of grid).

Hypercube. Components are numbered in binary; components are connected to one another if their binary representations differ in exactly one bit. n components are connected to log(n) other components

and can reach each other via at most log(n) links; reduces communication delays.

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Interconnection Architectures

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Parallel Database Architectures

Shared memory -- processors share a

common memory

Shared disk -- processors share a common

disk

Shared nothing -- processors share neither a

common memory nor common disk

Hierarchical -- hybrid of the above

architectures

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Parallel Database Architectures

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Shared Memory

Processors and disks have access to a common

memory, typically via a bus or through an

interconnection network.

Extremely efficient communication between

processors — data in shared memory can be

accessed by any processor without having to move it

using software.

Downside – architecture is not scalable beyond 32 or

64 processors since the bus or the interconnection

network becomes a bottleneck

Widely used for lower degrees of parallelism (4 to 8).

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Shared Disk

All processors can directly access all disks via an interconnection network, but the processors have private memories.

The memory bus is not a bottleneck

Architecture provides a degree of fault-tolerance — if a processor fails, the other processors can take over its tasks since the database is resident on disks that are accessible from all processors.

Examples: IBM Sysplex and DEC clusters (now part of Compaq) running Rdb (now Oracle Rdb) were early commercial users

Downside: bottleneck now occurs at interconnection to the disk subsystem.

Shared-disk systems can scale to a somewhat larger number of processors, but communication between processors is slower.

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Shared Nothing

Node consists of a processor, memory, and one or more disks. Processors at one node communicate with another processor at another node using an interconnection network. A node functions as the server for the data on the disk or disks the node owns.

Examples: Teradata, Tandem, Oracle-n CUBE

Data accessed from local disks (and local memory accesses) do not pass through interconnection network, thereby minimizing the interference of resource sharing.

Shared-nothing multiprocessors can be scaled up to thousands of processors without interference.

Main drawback: cost of communication and non-local disk access; sending data involves software interaction at both ends.

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Hierarchical

Combines characteristics of shared-memory, shared-disk, and shared-nothing architectures.

Top level is a shared-nothing architecture – nodes connected by an interconnection network, and do not share disks or memory with each other.

Each node of the system could be a shared-memory system with a few processors.

Alternatively, each node could be a shared-disk system, and each of the systems sharing a set of disks could be a shared-memory system.

Reduce the complexity of programming such systems by distributed virtual-memory architectures

Also called non-uniform memory architecture (NUMA)

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Distributed Systems Data spread over multiple machines (also referred

to as sites or nodes).

Network interconnects the machines

Data shared by users on multiple machines

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Distributed Databases Homogeneous distributed databases

Same software/schema on all sites, data may be partitioned among sites

Goal: provide a view of a single database, hiding details of distribution

Heterogeneous distributed databases

Different software/schema on different sites

Goal: integrate existing databases to provide useful functionality

Differentiate between local and global transactions

A local transaction accesses data in the single site at which the transaction was initiated.

A global transaction either accesses data in a site different from the one at which the transaction was initiated or accesses data in several different sites.

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Trade-offs in Distributed Systems

Sharing data – users at one site able to access the data

residing at some other sites.

Autonomy – each site is able to retain a degree of control

over data stored locally.

Higher system availability through redundancy — data

can be replicated at remote sites, and system can

function even if a site fails.

Disadvantage: added complexity required to ensure

proper coordination among sites.

Software development cost.

Greater potential for bugs.

Increased processing overhead.

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Implementation Issues

Atomicity needed even for transactions that update data

at multiple sites

The two-phase commit protocol (2PC) is used to ensure

atomicity

Basic idea: each site executes transaction until just before

commit, and the leaves final decision to a coordinator

Each site must follow decision of coordinator, even if there

is a failure while waiting for coordinators decision

2PC is not always appropriate: other transaction models

based on persistent messaging, and workflows, are also

used

Distributed concurrency control (and deadlock detection)

required

Data items may be replicated to improve data availability

Details of above in Chapter 22

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Network Types

Local-area networks (LANs) – composed of

processors that are distributed over small

geographical areas, such as a single building or a

few adjacent buildings.

Wide-area networks (WANs) – composed of

processors distributed over a large geographical

area.

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Networks Types (Cont.)

WANs with continuous connection (e.g. the Internet)

are needed for implementing distributed database

systems

Groupware applications such as Lotus notes can

work on WANs with discontinuous connection:

Data is replicated.

Updates are propagated to replicas periodically.

Copies of data may be updated independently.

Non-serializable executions can thus result. Resolution

is application dependent.

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TOPICS


Single system image, Client Server

architecture




through porting



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Single system image

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Single system image

Definition - What does Single System Image

(SSI) mean?

A single system image (SSI) is a distributed

computing

method in which the system hides the

distributed

nature of the available resources from the

users.

The computer cluster, therefore, appears to be

a single

computer to users. This property can be

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Single system image

Techopedia explains Single System Image

(SSI)

The features of a single system image include:

Single User Interface: Users interact with the

cluster through a single GUI.

Single Process Space: Every user process

holds a unique cluster-wide process ID. A

process on a node creates a child process on

the same or a completely different node.

Communication between processes residing

on different nodes is also possible.

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Transparen

t access

Personal

Workstation

Processing service

Print service

Communication service

Other Users

Other Services

Storage Retrieval

Service

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Single system image Single Entry Point: Users connect to multiple

nodes in the cluster through a virtual host, which

acts as single entry point. The connection

request moves to different hosts to balance the

entire load.

Single I/O Space: This permits all nodes to

perform I/O operations on local or remote disk

devices.

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Single system image - benefits

The benefits of using a single system image

include:

It provides a similar syntax as that used in

other systems, reducing operating errors.

Users can work in their preferred interface,

which is then altered by the administrator to

manage the entire cluster as a single entity.

It reduces cost of ownership and simplifies

system management.

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Single system image - benefits

It provides a straightforward view of all

activities from a single node in the entire

cluster.

The end user is not concerned about where

the application runs.

It avoids using numerous skilled

administrators, because only one is needed to

centralize system management.

It promotes standard tool development.

It provides location-independent message

communication.

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Client Server ArchitectureClient/server model by Sybase

A client-user relies on the desktop workstation for all computing needs.

Whether the application runs totally on the desktop or uses services

provided by one or more servers—be they powerful PCs or mainframes—

is irrelevant.

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Client Server Architecture

Client: A client is a single-user workstation

that provides presentation services and the

appropriate computing, connectivity, and

database services and interfaces relevant to

the business need.

Server: A server is one or more multiuser

processors with shared memory providing

computing, connectivity, and database

services and interfaces relevant to the

business need.

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Application Model - Components

Graphical user interface (GUI)

stored data access

Business logic

Link services with other

applications

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Application Model

Effective client/server computing - fundamentally

platform-independent.

The application user - provides the business

functionality

Computing platform provides access to this

business functionality

Platform changes - transparent to user

Tools – provide transparent, portable application

development

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Existing environment.

All application logic resides

here

"dumb"

terminal

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Migration to C/S

In the client/server model

Low-cost processing power of the workstation -

replace host processing,

Application logic divided among platforms.

Distribution of function and data - transparent to

user and application developer.

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TOPICS


Single system image, Client Server architecture




through porting



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Mainframe-centric client server

computing

Uses presentation capabilities of the workstation

to front-end existing applications.

Character mode interface is remapped - Easel

and Mozart.

UI easy to use, information presented clearly.

Porting mainframe applications to workstation -

UniKix and IBM's CICS OS/2 and 6000.

Character mode applications – converted to GUI

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Easel i/f

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GUI advantages Workstation - GUI dynamic screen.

Screens with a variable format based on data values

Variable length – scrollable

Lists of fields - scrollable number of rows.

Larger virtual screen with no additional data communication between the client server.

Help text, valid value lists, and error messages

Easy navigation.

Additional information - display's colors, fonts, graphics icons, scrollable lists, pull-down lists, and option boxes

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GUI

GUI front end to an existing application is

frequently the first client/server-like application

implemented by organizations familiar with the

host mainframe and dumb-terminal approach

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Business Logic

Provision of edit and processing logic executing

in desktop workstation

Reduce the host transaction rate - sending up

only valid transactions.

network traffic reduced – performance improved

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EDI

Electronic data interchange (EDI) -example of this

front-end processing

Organizations communicate electronically with

suppliers or customers

EDI link work with the existing back-end host

system.

Messages are reformatted and responses

handled by EDI client

Application processing done by the application

server

Productivity enhanced by capturing information at

source - made available to authorized users

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Desktop application

integration.

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Desktop application

integration.

Multipart form for data capture.

Capturing information once to a server in a C/S

application, reusing

reduce errors,

lower data entry costs,

speed up the availability of information

Data available as soon as it is captured. No delay

while the forms are passed around the

organization.

Better than forms imaging technology - forms are

created and distributed internally in an

organization

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Downsizing and Client/Server

Computing

Rightsizing and downsizing - used with the client/servermodel.

Rightsizing and upsizing - addition of diverse or powerfulcomputing resources to enterprise computing environment.

The benefits of rightsizing reduction in cost and/or increased functionality, performance, flexibility in the enterprise applications

Significant cost savings - reduction in employee,hardware, software, and maintenance expenses.

Downsizing - flattening of the organizational hierarchy.

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Computing

Eliminating middle layers of management - empowerment to thefirst level of management with the decision-making authority.

The desktop-host integrated systems – quick decision making

Architects and developers work closely with business decisionmakers - new applications and systems are designed &integrated with effective business processes.

Poor return on technology investment - lack of understanding bydesigners on day-to-day business impact of their solutions.

Downsizing - attempt to use cheaper workstation technologies toreplace existing mainframes and minicomputers.

Greater benefit - reengineering the business processes to usethe capabilities of the desktop environment.

Systems solutions are effective only - visible to actual user to add value to business process.

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Computing

Client/server technology - drive downsizing.

Makes the desktop the users' enterprise.

Moving from the machine-centered computing into the workgroup era- the desktopworkstation empowering the business user to regain ownership of information resource.

Client/server computing combines the best of the old with the new—the reliable multiuseraccess to shared data and resources with the intuitive, powerful desktop workstation.Object-oriented development concepts - SDE created for an organization from anarchitecturally selected set of tools.

The SDE provides more effective development and maintenance than companies have experienced with traditional host-based approaches.Client/server computing is open computing.

Mix and match is the rule.

Development tools and development environments - both openness and standards.

Mainframe applications rarely can be downsized—without modifications—to a workstationenvironment.

Modifications can be minor, wherein tools are used to port (or rehost) native COBOL compilers,functional file systems, and emulators for DB2, IMS DB/DC, and CICS

major, wherein the applications are rewritten using completely new tools. In porting, areavailable for workstations (PowerBuilder, Visual Basic, and Access, Forte and Dynasty. )

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Preserving Your Mainframe

Applications Investment Through

Porting

Downsize and still preserve investment in application code.

Micro Focus COBOL/2 Workbench by Micro Focus Company Inc., and XDB Systems Inc., bundles products from Innovative Solutions Inc., Stingray Software Company Inc., and XDB Systems Inc.,

ProxMVS product from Proximity Software, enable extensive unit and integration testing to be done on a PC LAN before moving the system to the mainframe for final system and performance testing. Used within a properly structured development environment, these products can dramatically reduce mainframe development costs.

Micro Focus COBOL/2 supports GUI development ,OS/2 Presentation Manager and Microsoft Windows 3.x. Dialog System, provides support for GUI and character mode applications that independent of COBOL applications.

Micro Focus Object Oriented (OO) - reusable components. Integration with Smalltalk/V PM applications .

IBM's CICS for OS/2, OS400, RS6000, and HP/UX -directly port applications using standard CICS call interfaces from the mainframe to the workstation

Applications can then run under OS/2, AIX, OS400, HP/UX, or MVS/VSE without modification.

Ceate applications for CICS MVS environment port to environments without modificationall of these platforms.

COBOL code generator, Computer Associates' (previously Pansophic) Telon PWS, provides systems development.

Object-oriented development - code and function reuse

Support prototyping and rapid application development.

Execute in the OS/2, UNIX AIX, OS400, IMS DB/DC, CICS DLI, DB2, IDMS, and Datacom DB environments.

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Porting

This combination— structured development environment with appropriate standards—provides the capability to build single-system image

These products + workstation=LAN an ideal development and maintenance environment for existing host processors

offloading the development three to six months.

Proper Systems development environment with a suite of tools with host capabilities + enhanced connectivity.

Appropriate standards and procedures to coordinate shared development.

The workstation environment will change.

Applications with common standards and procedures will be resilient enough to remain viable in the new environment.

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Porting

Movement toward client/server-based development, and the transfer of code between the host and client/server - better testing and fewer errors.

Testing facilities and usability of workstation - developer and tester more effective more accurate.

Business processes use database, communications, and application services

Pick the best servers.

Compromises around the existing technology, existing application products, training investments, product support, and a myriad other factors.

Success of full client/server applications – selecting an appropriate application and technical architecture for the

organization.

the tools are known.

Implement an SDE - define the standards to use tools effectively.

SDE - hardware, software, standards, standard procedures, interfaces, and training

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Client/Server Development

Tools

A successful builder - build according to standards.

Reusing the models for integration- cost and risk reduction.

Development using an SDE - reuse as much as possible to save costs, reduce risk, provide the users common "look and feel."

Select good set of tools lead to success.

Without comprehensive SDE, developers will not achieve success.

Object Technology based tools for client/server development –standards shared development,

reusable code,

interfaces to existing systems,

security,

error handling,

standard "look and feel.” Developers can build application systems closely tied to today's technology or use an SDE and develop applications that can evolve along with the technology platform.

Developers build application with current technology or use an SDE

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