1. CS551 Object Oriented Middleware (I) (Chap. 3 of EDO)
Yugi Lee
STB #555
(816)
CS551 - Lecture 9

2. Outline Computer Networks Types of Middleware
TCP
UDP
Types of Middleware
Transaction-Oriented Middleware
Message-Oriented Middleware
Remote Procedure Calls
Object-Oriented Middleware
Developing with Object-Oriented Middleware
CS551 - Lecture 9

3. Computer Networks
The communication between processes on different machines
At the high levels:
a connection to another process on another machine to simply send each message as a high level logical unit
Simple strings or arrays of bytes (binary data).
At the underlying layers:
Split message into smaller units for physical transfer called PDU's (protocol data units).
A wrapper with Address of sender/receiver, Error detection codes, Protocol control information
CS551 - Lecture 9

4. Transport Layer
Concerned with the transport of information through a network.
Two facets in UNIX/Windows networks:
TCP
UDP
ISO/OSI Reference Model
Application
Presentation
Session
Transport
Network
Data link
Physical
CS551 - Lecture 9

5. Protocols An agreement between entities on the transmission of data.
Different layers of network software have different concerns.
At lower levels: determining the amount of data to be sent, the size of packets and the speed of transmission.
Connection Control /Data transfer
Flow control/Error control
Synchronization/Addressing
At higher levels: cover the syntax and sequencing of logical messages.
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6. Transmission Control Protocol (TCP)
Provides bi-directional stream of bytes between two distributed components by establish a virtual connection
Steps: Connection establishment, Data transfer, Connection termination.
Reliable: send logical messages without having to explicitly deal with these other issues (sequencing and re-transmission of lost packets).
Slow: as more time is required to set up and terminate the link and buffering at both sides de-couples computation speeds.
UNIX rsh, rcp and rlogin are based on TCP.
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7. TCP for Request Implementation
Client
Server
Application
Application
Presentation
Presentation
Session
Session
Requests
Transport
Input Stream
Transport
Output Stream
Results
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8. Connection-oriented Communication
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9. User Datagram Protocol (UDP)
Enables a component to pass a message containing a sequence of bytes (packet) to another component:
A program may send information to another at an indeterminate time with no explicit prior co-ordination
the packet may get lost or packets may arrive in a different order to that in which they were sent.
reorganize the packets and make requests for new packets when problems occur.
dynamically routed; split into a number of PDUs that take different routes to their destination depending on network congestion.
Unreliable but very fast protocol: restricted message length, queuing at receiver, e.g. UNIX rwho command.
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10. UDP for Request Implementation
Client
Server
Application
Application
Presentation
Presentation
Session
Session
Request Datagrams
Transport
Transport
Result Datagrams
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11. Datagram Communication
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12. Direct Use of Network Protocols implies
Manual mapping of complex request parameters to byte streams
Manual resolution of data heterogeneity
Manual identification of components
Manual implementation of component activation
No guarantees for type safety
Manual synchronization of interaction between distributed components
No quality of service guarantees
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13. Middleware Layered between Application and OS/Network
Makes distribution transparent
Resolves heterogeneity of
Hardware
Operating Systems
Networks
Programming Languages
Provides development and run-time environment for distributed systems.
CS551 - Lecture 9

14. Forms of Middleware Transaction-Oriented Message-Oriented RPC Systems
IBM CICS
BEA Tuxedo
Encina
Message-Oriented
IBM MQSeries
DEC Message Queue
NCR TopEnd
RPC Systems
ANSA
Sun ONC
OSF/DCE
Object-Oriented
OMG/CORBA
DCOM
Java/RMI
EJB
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15. Remote Procedure Calls
First look at RPCs to understand origin of object-oriented middleware
Enable procedure calls across host boundaries
Call interfaces are defined using an Interface Definition Language (IDL)
RPC compiler generates presentation and session layer implementation from IDL
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16. IDL Example (Unix RPCs)
const NL=64;
struct Player {
struct DoB {int day; int month; int year;}
string name<NL>; };
program PLAYERPROG {
version PLAYERVERSION {
void PRINT(Player)=0;
int STORE(Player)=1;
Player LOAD(int)=2;
}= 0;
} = ;
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17. Resolution of data heterogeneity Marshalling and Unmarshalling
ISO/OSI Presentation Layer
Resolution of data heterogeneity
Common data
representation
Transmission of
data declaration
Marshalling and Unmarshalling
static
dynamic
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18. Marshalling and Unmarshalling
char * marshal() {
char * msg;
msg=new char[4*(sizeof(int)+1) +
strlen(name)+1];
sprintf(msg,"%d %d %d %d %s",
dob.day,dob.month,dob.year,
strlen(name),name);
return(msg); };
void unmarshal(char * msg) {
int name_len;
sscanf(msg,"%d %d %d %d ",
&dob.day,&dob.month,
&dob.year,&name_len);
name = new char[name_len+1];
sscanf(msg,"%d %d %d %d %s",
&dob.year,&name_len,name);
Marshalling: Disassemble data structures into transmittable form
Unmarshalling: Reassemble the complex data structure.
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19. Transport Layer (e.g. TCP or UDP)
Method Call vs. Object Request
Caller
Called
Called
Stub
Caller
Stub
Transport Layer (e.g. TCP or UDP)
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20. Stubs
Creating code for marshalling and unmarshalling is tedious and error-prone.
Code can be generated fully automatically from interface definition.
Code is embedded in stubs for client and server.
Client stub represents server for client, Server stub represents client for server.
Stubs achieve type safety.
Stubs also perform synchronization.
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21. Synchronization
Goal: achieve similar synchronization to local method invocation
Achieved by stubs:
Client stub sends request and waits until server finishes
Server stub waits for requests and calls server when request arrives
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22. Type Safety How can we make sure that
servers are able to perform operations requested by clients?
actual parameters provided by clients match the expected parameters of the server?
results provided by the server match the expectations of client?
Middleware acts as mediator between client and server to ensure type safety.
Achieved by interface definition in an agreed language.
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23. Facilitating Type Safety
Interface
Definition
Request
Server
Client
Reply
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24. Session Layer Implements identification of RPC servers Application
activation of RPC servers
dispatch of operations
Application
Presentation
Session
Transport
Network
Data link
Physical
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25. Example: RPC Server Identification
print_person(char * host, Player * pers) {
CLIENT *clnt;
clnt = clnt_create(host, , 0, "udp");
if (clnt == (CLIENT *) NULL) exit(1);
if (print_0(pers, clnt)==NULL)
clnt_perror(clnt, "call failed");
clnt_destroy(clnt); }
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26. Interface Definition Language
Every object-oriented middleware has an interface definition language (IDL)
Beyond RPCs, object-oriented IDLs support object types as parameters, failure handling and inheritance
Object-oriented middleware provide IDL compilers that create client and server stubs to implement session and presentation layer
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27. Why do we use an IDL? PL PL PL PL PL PL PL IDL PL PL PL PL PL 1 2 1 26 3 6 3 PL PL PL PL 5 4 5 4
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28. Smalltalk C++ Ada-95 C Java Cobol
CORBA Programming Language Bindings
Smalltalk
C++
Ada-95
IDL
Common
Object
Model C
Java
Cobol
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29. IDL Example (OO Middleware)
interface Player : Object {
typedef struct _Date {
short day; short month; short year;
} Date;
attribute string name;
readonly attribute Date DoB; };
interface PlayerStore : Object {
exception IDNotFound{};
short save (in Player p);
Player load(in short id) raises(IDNotFound);
void print(in Player p);
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30. Presentation Layer Implementation
In addition to RPC presentation layer implementation, object-oriented middleware needs to
define a transport representation for object references
deal with exceptions
need to marshal inherited attributes
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31. Session Layer Implementation
Object
References
Hosts
Processes
Objects
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32. Development Steps Design Interface Definition Server Stub Generation
Client Stub
Server
Coding
Client
Server
Registration
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33. Facilitating Access Transparency
Client stubs have the same operations as server objects
Hence, clients can
make local call to client stub
or local call to server object
without changing the call.
Middleware can accelerate communication if objects are local by not using the stub.
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34. Facilitating Location Transparency
Object identity
Object references
Client requests operation from server object identified by object reference
No information about physical location of server necessary
How to obtain object references?
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35. Stub Generation included in generates reads Team.idl IDL-Compiler
The CORBA IDL is translated
into stubs and skeletons (server stubs):
IDL-compiler generates four files
(two for client stubs and two for
server stubs)
included in
generates
reads
Team.idl
IDL-Compiler
Teamcl.hh
Teamsv.hh
Teamcl.cc
Teamsv.cc
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36. Client and Server Implementation
Client.cc + Teamcl.hh => to check consistency between the declaration and use of client stub implementation. Client.cc +Teamcl.cc => an executable code (Client)
Similar to Server.
C++ Compiler, Linker
Server
Client.cc
Server.cc
Client
Team.idl
included in
generates
reads
IDL-Compiler
Teamcl.hh
Teamcl.cc
Teamsv.cc
Teamsv.hh
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37. <<interface>>
Type Safe Server Object Implement.
Inheritance is used in classes
Play_Dispatch and Player_Impl,
which are contained in the server stub
for object type Player.
An Interface-based implementation
with the relationships of interfaces
and classes involved on the server side
<<uses>>
<<uses>>
Player_Impl
<<interface>>
Player_Dispatch
Player_Impl
Player_Dispatch
<<implements>>
Player_Server
Player_Server
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38. Server Registration
Object adapters need to be able to locate and start servers
Server objects are registered in some form of implementation repository
Registration processes is middleware and product-specific
Object adapter performs implementation repository lookup prior to activation
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39. Key Points Middleware builds on the transport layer
There are several forms of middleware
Object-oriented middleware provides IDLs
Object-oriented middleware implements session and presentation layer
Presentation layer implementation in client/server stubs is derived from IDL
Session layer is implemented in object adapters
CS551 - Lecture 9