1. INE4481 DISTRIBUTED DATABASES & CLIENT-SERVER ARCHITECTURES
Chapter 3.
Network architectures: Client/server & middleware

2. CONTENT Networks architectures Client/Server The middleware RPC model
Gateways 
Transparency 
Client server requirements 
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3. CONTENT (1) Networks architectures Yesterday Today
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4. Network architectures
Historically you had:
Centralized architecture :
Slave terminals, Mono-technology, Mono OS, proprietaries systems ,…
Non-relational DB: IDS2, DB2,..
Strutural Programing Languages: COBOL, …
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5. Networks architectures (2)
Then came distributed architecture:
Networks, more powerful PC, open OS.
Standards and API Interfaces,
relational DB
Query Languages 4GL: SQL, …
Development tools.
Transport (communications) tools.
Client/Server.
Rules
Applications
DBMS OS
Company network DB
Request
Response
Server
Clients
Windows
OS/2
Unix
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6. CONTENT (2) Networks architectures Client/Server Client/Server Model
Request ?
Response?
Examples?
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7. Client/Server Model Client: Server: Request: Response:
Process that request a server process to compute an operation, by sending a message containing details on the operation to compute. After a request is sent, the client waits the server response.
Server:
Process that receives client’s request, compute the demanded operation and send back a response to the requester.
Request:
Message transmitted from a client to a server, and containing a description of the tasks to be executed on behalf of (or for ) this client.
Response:
Message transmitted back by a server to a client after the requested operation has been executed, and containing the return status of the operation.
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8. Client/Server Model (2)
The most popular models are:
Peer to peer
Filter (gateway, firewall, proxy, …)
Client Server
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9. CONTENT (2) Networks architectures Client/Server Client/Server Model
Client/Server architectures
Fat vs thin
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10. Client/Server architectures
The model:
Many clients, one server:
Application
Hardware
Op. System
Client
process
Hardware
System
Server
process
Client
Master
Slave
Client
Server
One client, One server:
One client, many servers:
Client
Server
Client
Server
Server
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11. Fat vs thin
Clients and servers may be ``fat'' or ``thin'' (no normal size?)
Fat clients use up space on each client they run on, may have complex installation requirements, etc (e.g. Netscape 3.0)
Thin clients have reduced functionality, but are easier to manage(e.g cell phone browser and applications)
The network computer model favours thin clients.
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12. Client or server oriented C/S
Fat Client:
Stores data and application on localhost, and server stores updating files, …
Client makes a lot of processing tasks: e.g DB server, file server…
Server are often lightened.
Fat Server:
The maximum processing load is on the server: e.g groupware, transactions, objects, … servers.
Easier to manage, since features may be added on server side without changing and side-effects on clients.
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13. Client or server oriented C/S (2)
Thin Client:
Client with very little functionnalities: X-Terminals, workstations without hard-disk, Network Appliances, Network Computer, Networked PC,…
Maximum load on server side
Data and
applications
Applet, data
Local process,
Caching, …
Thin Client
Server
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14. CONTENT (2) Client/Server communication Client/Server messages
Networks architectures
Client/Server
Client/Server Model
Client/Server architectures
Fat vs thin
Client/Server communication
Client/Server messages
C/S messages exchange
C/S communication mode
Blocking tasks
Non blocking tasks
Communication examples 
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15. Client/Server communication
Service Primitives:
SendRequest()
ReceiveResponse()
ReceiveRequest()
SendResponse()
Request
Response
SendRequest()
SendResponse()
ReceiveRequest()
ReceiveResponse()
Session
Session
Transport
Transport
Network
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16. Client/Server messages
Three types of messages in general: REQ, RESP, and ACK.
Others types could exist.
AYA (Are You Alive):
Used by the client to avoid retransmission when the server is late in its answer.
IAA (I Am Alive):
Used by the server to delay client waiting (e.g the server is busy in a long task)
The client could make a timeout and stop refreshing requests after many tryings.
TA (Try Again);
Used by the server when the REQ message can not be accepted (e.g mail box over flow, unknown destination address, …)
REQ
RESP
ACK S C
AYA
IAA
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17. C/S messages exchange
In an heterogenous environment, an adaquate data presentation is necessary:
Data translation: XDR (SUN) , ASN.1(CCITT),…
Marshalling parameters sent
Unmarshalling received parameters
Presentation
Session
Transport
Translation
Marshalling
Unmarshalling
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18. C/S communication mode
Synchronous mode :
Doesn’t use waiting queue
Messages are sent with no delay
Blocking mode (ex. RPC).
Asynchronous mode :
Uses waiting queue
Non blocking mode (ex.: FIFO, , …).
Uses Multithreading.
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19. Blocking tasks The process will stop untill the task ends: System call
Reading
Copy in the
buffer
Middleware
Resume
read()
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20. Non blocking tasks The process will not stop between two tasks:
System call
WOULD BLOCK
Resume
read()
loop
Waiting
for data
Copy in the
buffer
Activate SIGIO
System call
Resume
SIGIO Signal
signal()
handler()
read()
Waiting
for data
Copy in the
buffer
Events
Signals
The process will not stop between two tasks:
E.g: Multithreading
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21. A communication example
Procedure call (RPC or local) : Blocking tasks
ordinary procedure call
dynamic link library procedure call
remote procedure call
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22. CONTENT (2) Networks architectures Client/Server Client/Server Model
Client/Server architectures
Fat vs thin
Client/Server communication
Designing a C/S application
2 tiers Client/server
Components distribution  
3 tiers Client/server 
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23. Designing a C/S application
A C-S application must include the following components:
Presentation:
Textual interfaces or GUI, interactions, data retrieving, validation, …
Application Logic:
Process in relation with the client/server application’s data.
Data access :
Data store and query: DB, Web documents,…
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24. 2 tiers Client/server Gartner model for 2-tiers systems:
Presentation
Logic
Data
Distributed DB
Class 1
Remote Data
Class 2
Distributed
Transactions
Class 3
Remote
Presentations
Class 4
Class 5
Client
Server
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25. Components distribution
CGI, HTTPD,
SERVLETS
Distributed DB:
Presentation
Logic
Data
Class 1
Web:
HTML, JAVA, JSP
Class 3
File Service:
Class 2
This allows remote clients
acess to a shared file system
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26. Components distribution
Terminals emulation
X Window
Presentation
Logic
Data
Class 5
Envelop
Presentation
Presentation
X Wrapper
Logic
Presentation
telnetd
Data
Logic
Original
application
Data
Class 4
This allows a remote system
to act as a normal terminal
on a local system
Class 5
This decouples application
view of windowing and
display view of windowing
This acts as a wrapper around
a classical system, either to provide
an X Window interface or an
alternative character-based one
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27. 3 tiers Client/server Gartner Model for 3-tiers systems: Client
Middle-tier
Server
Presentation
Logic
Data
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28. CONTENT (3) Networks architectures Client/Server The middleware
The middleware : definitions
The middleware in networks
Types of middleware 
Middleware Model 
Middleware Components 
Middleware position 
Middleware services
Middleware functionnalities 
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29. The middleware: definitions
Set of software services built on top a transport protocol to enable and hide C/S requests/responses.
Makes the connexion between server (e.g DB ) and development tools on client device.
Middleware services are a set of distributed softwares that exist between the OS, the application and network services on a particular node.
Middleware examples:
CORBA, RMI, DCOM, .NET…
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30. The Middleware in networks
The network services include things like TCP/IP
The middleware layer is application-independent s/w using the network services
Examples of middleware are: DCE, RPC, Corba
Middleware may only perform one function (such as RPC or many (such as DCE)
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31. Types of middleware General purpose middleware:
Message-Oriented Middleware (MOM) which provides program-to-program data exchange, enabling the creation of distributed applications.
Communication protocols between shared directories, authentication services, time service, RPC, FIFO, ...
NOS(network OS) type distributed services: File services, printing services.
E.g: DCE, Netware, Named Pipes, LAN Server, LAN Manager, Vines, TCP/IP, NetBios.
Middleware
Server
Client
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32. Types of middleware (2) Middleware Gateway Server Client
Specific types:
Databases Middleware: ODBC, IDAPI,DRDA, EDA/SQL,…
Transaction processing (TP) monitors which provide tools and an environment for developing and deploying distributed applications.
E.g. ATMI (Tuxedo), Transactional RPC (Ensina), …
Groupeware: MAPI, Lotus Notes, …
Object Request Brokers (ORBs) which enable the objects that comprise an application to be distributed and shared across heterogeneous networks. E.g. Corba ORB,...
Network management: SNMP, …
Middleware
Gateway
Server
Client
Middleware
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33. Middleware Model Distributed management GUI Logic OS Client
Transport
Network OS
Specific
Services
Middleware
Distributed
management
DBMS
Logic
OLPT OS
Server
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34. Middleware Components
Channels:
Communication services between components and applications:
DCE RPC (synchronous), ORB (synchronous), MOM (Message Oriented Middlware) (asynchronous).
Support for communication services
Corba SSL, directories (LDAP).
Platforms:
Application servers. Running on server side.
Offers communication channel
Makes dispatching (distributed OS), load balancing, transactions integrity, …
E.g: TP Monitors, OTM, Web based application servers, …
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35. Middleware position Examples:
Primitives services: terminals emulators, file transfer, ,…
Basic services: RPC
Integrated services : DCE, SE distributed by O/S.
Distributed objects: CORBA, COM/DCOM, OLE/ActiveX
World Wide Web
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36. Middleware services Specifiic service middleware
SQL/API TxRPC Mail ORB/COM
Distributed management
CMIP SNMP DME
Network OS
RPC NFS directories, Security, Message service
Transport
TCP/IP IPX/SPX NetBIOS SNA
Network, Data Link, Physical
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37. Middleware functionnalities
Connexion establishment procedure
Process requests
Return results
Close connexion
Initiation of processes at different computers
Directory services to allow clients to locate servers
Remote data access
Concurrency control to allow servers to handle multiple clients
Security integrity
Monitoring
Termination of processes both local and remote
Caching results
Caching requests
Session management
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38. Middleware examples
Primitive services such as terminal emulators, file transfer,
Basic services such as RPC
Integrated services such as DCE, Network O/S
Distributed object services such as CORBA, OLE/ActiveX
World Wide Web
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39. CONTENT (4) Networks architectures Client/Server The middleware
RPC model  
Gateways 
Transparency 
Client server requirements 
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40. RPC model Client RPC Server Server Stub Client Stub Procedure
Application
Procedure
Call
Procedure
return
Return
Procedure
Run
Procedure
Server Stub
Client Stub
Marshalling
Unmarshalling
Marshalling
Unmarshalling
SendRequest()
ReceiveResponse()
SendResponse()
ReceiveRequest()
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41. Gateways
A gateway is a generic term for an entity used to connect two or more networks
A repeater operates at the physical level copies the information from one subnet to another
A bridge operates at the data link layer level and copies frames between networks
A router operates at the network level and not only moves information between networks but also decides on the route
A proxy operates at the application level
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42. Transparency Networks Transparency: Servers Transparency :
Support of different type of networks: LAN, WAN, … TCP/IP, ISO, SNA, …
Servers Transparency :
Support of different type of servers: SGBD (Oracle, DB2, …), applications, …
Data Transparency:
Data formats translation: XDR, ASN.1, …
Localisation Transparency:
By using directory service.
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43. Client/Server points of failure
The client side of the application could crash
The client system may have h/w problems
The client's network card could fail
Network contention could cause timeouts
There may be network address conflicts
Network elements such as routers could fail
Transmission errors may lose messages
The client and server versions may be incompatable
The server's network card could fail
The server system may have h/w problems
The server s/w may crash
The server's database may become corrupted
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44. Client/Server requirements
Reliability
Performance
Responsiveness
Scalability
Capacity
Security
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