Cs architecture

Computer Science Program, The University of Texas, Dallas

Client-Server Architecture
Clients and Servers

Client/Server with File Servers
Client/Server with Database Servers
Client/Server Communication
Client/Server with Transaction Processing
Client/Server Groupware
Web Client/Server

Lawrence Chung

Paradigm Shift: Past, Present and Future

W Centralized processing
A host computer (often a mainframe) handls all processing,
including input, output, data storage and retrieval

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W Distributed processing
A number of computers (minis, workstations, PCs, ...) handle all processing,
They are distributed physically and connected thru a communications
network

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W Cooperative processing
A number of computers (minis, workstations, PCs, ...) handle all processing,
They are distributed physically and connected thru a communications
network
Processing thru sharing of resources, transparently to the users

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Lawrence Chung

Clients and Servers
v Basic Definition
3 Client: requests for services
3 Server: provides services
2 Service: any resource
(e.g., data, type defn, file, control, object, CPU time, display device, etc.)

v Typical Properties
u A service request is about what is needed, and it often made abstractly
It is up to the server to determine how to get the job done
- the notion of module
(cf. MILs, ADTs --- Larch, Z)

u The ideal client/server software is independent of hardware or OS platform
u The location of clients and servers are usually transparent to the user
u A client may become a server; a server may become a client
u A client/server system can be scaled
horizontally, i.e., by adding/removing client workstations
with only a slight performance impact
vertically, i.e., by migrating to a larger and faster server machines
or multiservers

Lawrence Chung

Client/Server with File Servers

Client Client Client
Application Application Application

Centralized

File Server

3 The client passes requests to the file server (software) for file records
3 Clients can reside in the same machine or separate machines (typically PCs)
3 Requests can be either local or over a network
3 Indispensable for documents, images, drawings, and other large data objects


Distributed Network

File Server File Server

Lawrence Chung

Client/Server with Database Servers


insert/update/delete
Network
tuple(s)

Database
Server

3 At present the majority of existing client/server-based software is to be found in the area of
databases, and it is here that the greatest challenge to any corporation currently lies.
[Richard Finkelstein, President, Performance Computing]

3 Events (violation of integrity constraints, temporal conditions, errors) trigger event handlers
- implicit invocation, blackboards, events

3 A DBMS also offers features for recovery and concurrency control
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Lawrence Chung

Sockets - RPCs - MOMs and ORBs
Sockets
3 introduced in 1981 as the Unix BSD 4.2 generic interface
3 provide Unix-to-Unix communications over networks
3 The Windows socket API (Winsock): based on the Unix BSD 4.2 sockets interface
Net_id.Host_id
Internet Address Port Address
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q
3 Principal transport service calls
d ` X Y ` X T U V p ` Y

S T T U V
Socket (addr format, type: connection-oriented/connectionless, protocol:TCP/IP/...)
W X Y ` X a S T V W W a X b ` Y U X c d ` e W U f T g h V d ` X Y ` i T U V p ` Y

q d ` X Y
Bind(...) ` X r f ` f ` s U d T Y U d W a t W a V U b W a t V U a a ` V Y W U a d ` r f ` T Y T u v g U V p W a t w a U a x v g U V p W a t y

€ ` b U
Listen(...) e ` X V U a a ` V Y W U a d ` r f ` T Y s d U b Y ` r f ` f ` U d ‚ X W Y s U d U a ` u v g U V p W a t w a U a x y

ƒ a W Y W X
Accept(...) Y ` X V U a a ` V Y W U a ‚ W Y X d ` b U Y ` T U V p ` Y

„ ` d b
Connect(local_socket, remote_socket)
W a X Y ` Y ` V U a a ` V Y W U a U a X T U V p ` Y

… ` a i
Shutdown(local_socket, remote_socket) X b ` T T X t ` Y d f X t U e ` a T U V p ` Y

€ ` V `
Send(local_socket, remote_socket, buffer, bytes)
W e ` X b ` T T X t ` Y d f X t U e ` a T U V p ` Y

q ` V
Recv(local_socket, remote_socket, buffer, bytes)
p X T ` Y U s T U V p ` Y T Y U T ` ` W s X a h V X a v ` d ` X i U d ‚ d W Y Y ` a u v g U V p W a t y

Select(...)
Lawrence Chung

¤ RPCs (Remote Procedure Calls)
3 a transparent mechanism to give the client procedure the illusion that
it is making a direct call on the distant server procedure
3 stubs:
local procedures (e.g., read(file_id, buffer, count)) hiding details of network comm.

3 10 steps to execute a RPC
Client prog/proc Server prog/proc

10. result 5. parameter 6. server proc
unmarshalling unmarshalling returns
1. stub call (unpack the msg (unpack the msg
into result) into parameters);
server proc call

Client stub Server stub

2. parameter 4. pass the msg 7. result
marshalling 9. pass the msg marshalling
(collect pack (pack the result
the parameters into a msg);
into a msg); a sys call
a sys call
3. send the call msg
Transport entity Transport entity
8. send the return msg
Lawrence Chung

u MOM (Message-Oriented Middleware)
Every DAD (Distributed Application Development) needs a MOM

u v u
¤ RPC
¤ MOM

3 Many-to-many messaging via queues

Server
Client

Server
Client
post office-like
asynchronous
no fixed sequence Server
Client a single queue sharable

Lawrence Chung



transactions
(banking, Network
reservation,...)

TP Monitor

Database
Server

3 Transaction changeAddress (p: String, newAddress: Address)
begin
oldAddress - retrieve address from Person where name = p
if oldAddress = newAddress then return error1
else update Person where p = name
with address - newAddress
end

3 Transaction increaseSalary (increment: Dollar)
begin
...
end
Lawrence Chung

3 Transactions are a way to make ACID operations a general commodity
[Transaction Processing Concepts and Techniques, Jim Gray and Andreas Reuter, 1993]

Ë Atomicity
t a transaction is an indivisible unit of work
t an all-or-nothing proposition
t all updates to a database, displays on the clients’ screens, message queues
t e.g., salary increase for all 1 million employees or none

Ë Consistency
t a transaction is an indivisible unit of work
t S - [T | abort] - S
t integrity constraints (e.g., mgr.salaray salary)

Ë Isolation
t a transaction’s behavior not affected by other transactions running concurrently
t e.g., reserve a seat
t serialization techniques
Ë Durability
t persistence
t a transaction’s effects are permanent after it commits.
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Lawrence Chung

v Two-Phase Commit Protocol
3 To synchronize updates on different machines so that they either all fail or all succeed
3 centralize the decision to commit but giving each participant the right of veto
3 like a Christian marriage
Commit Coordinator Subordinate Subordinate
(Root) Node Node

Prepare-to-commit Prepare-to-commit Prepare-to-commit
Phase 1

Record in Log Ready-to-commit Ready-to-commit

Commit Commit Commit
Phase 2

Complete Complete Complete

t If the subordinates have spawned pieces of the transaction on other nodes,
they must propagate the prepare-to-commit-command in the first phase
- a transaction tree
t If any of the participants returns a transaction failure,
the root node tells all its subordinates to perform a rollback
Lawrence Chung

3 Why?
support for business reengineering: bcfh - maximize profit
change the way people communicate with each other
helps manage (and track) the product thru its various phases
computer-supported cooperative work (CSCW)
collaborative/workgroup computing
return on investment: 16% to 1666% on a median investment of $100K
[a study of 65 Notes users in 1994]
3 Five foundation technologies t u v w x y z y z { | } u ~ z v v { y } € z ‚ u ƒ } | y z { z } „ € … z } ƒ { … t

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from electronic imaging (scanning, digitization, display, storage and retrieval)
to document (component types: text, image, graphics, faxes, mail, voice clips, BBs)
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thru microphones, video windows, electronic whiteboards,
controlled access to shared document
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electronic scheduling of meetings, sharing calendars and to do lists, triggering events
e Why not just DBMS?
DBMS: highly structured data
Groupware: highly unstructured data document database
e Why not just TP Monitors?
TP Monitors: transaction processes
Groupware: (currently) not transaction-oriented in the ACID sense
Lawrence Chung

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S a X g h T W T q f T Y U b ` d

Form DB e-mail fax spreadsheet
Processing query to accountg
conferencing
d Many office workers spend a high percentage of their time processing documents
d The workflow river carries the flow of work from port to port,
value being added along the way
d The workflow definition must take into consideration:
h Routes along which the object moves
h Rules about what information is routed and when
E.g., If the loan is over $100, 000, send it to the supervisor within the next hour
or else send it to the next hop
h Roles define job functions independently of the people who do it
E.g., the supervisor role can be handled by users Adam and Eve
Lawrence Chung

Web Client/Server
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1 Page
2 3 Object
4
Internet
6 TCP/IP HTML
5 Documents
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You select a target URL (Univeral Resource Locator - platform-independent naming scheme)
1
http ://www.utdallas.edu :80 :~ttsx19c/java/toys.html
Protocol Server Address Port Number Target Resource Path

2 Brower takes the URL, embeds it inside an HTTP request, and sends it to the server
h HTTP (HyperText Transfer Protocol), a stateless RPC, is used to retrieve URL-named resources
1) establishes a client/server connection, 2) transmits and receives parameters including a returned file,
and 3) breaks the client/server connection
STATELESS: a new connection for each user request - simple but inefficient
3 The HTTP server receives the reqest on the port, makes a socket connection,
4 The server finds the requested HTML file (Page Object) and concatenates it with status info
d HTML (HyperText Markup Language) describes the structure of a Web document,
provide font and graphics info, and define hyperlinks to other Web pages Internet resources)
5 The server ships back the file, and closes the connection
6 The Browser interprets the HTML command and displays the page content
and invokes a helper application (e.g., xview, audiotool)
Lawrence Chung

Web Client/Server
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Var.
1 2
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5
Internet HTTP 4
Srvr CGI
TCP/IP Prog
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You click on the form’s submit button
1
The Web browser collects the data within the form, assembles it into a string of name/value pairs,
specifies a POST method, the URL of the target program in the cgi-bin directory
2 The HTTP server receives the method invocation via a socket connection
3 The server parses the message to discover that it’s a POST for the cgi-bin program
- start a CGI interaction:
The HTTP server sets up the environment variable (e.g., content_length), in a blackboard
4 The HTTP server starts a CGI program
5 The CGI program reads the environment variables
6 The CGI program receives the message body via the input pipe
7 The CGI program does some work, typically by interacting with a DBMS, TP Monitor
8 The CGI program returns the results via the output pipe
9 The HTTP server returns the results to the Web brower
Lawrence Chung

Web Client/Server
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Java
Applet

Bytecode Verifier

Class Loader 4 Java Compiler

Java Just-in-Time
Run-time
Compiler
Interpreter

Applet
1 Bytecode
2
Internet
5 TCP/IP
3
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1 A Web browser requests a Java applet
The Browser initiates a separate TCP/IP session to download each applet within a Web page
2 The server retrieves the requested applet
3 The server sends the applet
4 The browser loads the applet into the client’s memory and executes it
A poor man’s compound document architecture
The applet’s region does NOT visually integrate with the rest of the page
5 The browser deletes the applet from memory when it exits the Web page
Lawrence Chung

Cs architecture

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