CSC 322 Operating Systems Concepts Lecture - 11: b y Ahmed Mumtaz Mustehsan
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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 concepts and architecture
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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
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.
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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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
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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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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Downsizing and Client/Server
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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Downsizing and Client/Server
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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Preserving Your Mainframe
Applications Investment Through
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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Preserving Your Mainframe
Applications Investment Through
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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