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Presentation 4:Principles of
Object-Oriented Middleware
Ingeniørhøjskolen i ÅrhusSlide 2 af 33
Outline
• Students are assumed to be knowledgeable about Computer Networks – including the OSI model and the TCP, UDP protocols. If not … please read ; )
• Types of Middleware – Transaction-Oriented Middleware– Message-Oriented Middleware– Remote Procedure Calls
• Object-Oriented Middleware• Developing with Object-Oriented Middleware
Computer Networks- an ultra short introduction
Ingeniørhøjskolen i ÅrhusSlide 4 af 33
Physical
Application
Presentation
Session
Transport
Network
Data link
ISO/OSI Reference Model
MiddlewareSpecific
MiddlewareSpecific
Ingeniørhøjskolen i ÅrhusSlide 5 af 33
Physical
Application
Presentation
Session
Transport
Network
Data link
Transport Layer
• Level 4 of ISO/OSI reference model.
• Concerned with the transport of information through a network.
• Two facets in UNIX/Windows networks: – TCP and– UDP
• Actually TCP & UDP do not follow the OSI model!– rather as an abstract model
Ingeniørhøjskolen i ÅrhusSlide 6 af 33
Transmission Control Protocol (TCP)
• Provides bi-directional stream of bytes between two distributed components.
• Reliable but slow protocol.• Buffering at both sides de-couples computation
speeds.• Best at fairly unreliable Networks (as the Internet)• Very pratical
– As the WWW uses the TCP/IP protocol family– And most Networking Operating Systems as well
Ingeniørhøjskolen i ÅrhusSlide 7 af 33
Application
Presentation
Session
Transport
Application
Presentation
Session
TransportInput Stream
Output Stream
Requests
Results
Client Server
TCP for Request Implementation
Ingeniørhøjskolen i ÅrhusSlide 8 af 33
User Datagram Protocol (UDP)
• Enables a component to pass a message containing a sequence of bytes to another component.
• Other component is identified within message.• Unreliable but very fast protocol.• Restricted message length.• Queuing at receiver• Best at highly reliable networks (as a LAN)
Ingeniørhøjskolen i ÅrhusSlide 9 af 33Result Datagrams
Application
Presentation
Session
Transport
Application
Presentation
Session
Transport
Request Datagrams
Client Server
UDP for Request Implementation
Types of Middleware- and why to use …
Ingeniørhøjskolen i ÅrhusSlide 11 af 33
Rolling your own distributed object
• We may build directly on the TCP or UDP protocols to build a distributed system
• In Java: quite easy – thanks to the java.net package – which contains a complete network API incl. socket support for TCP/UDP
• BUT: if we want to ensure OO – more complex
• Lets take a look at how this may be done
Ingeniørhøjskolen i ÅrhusSlide 12 af 33
The Basic Class to OO Class
public class HelloWorldBasic { public static void main(String[] args) { System.out.println("HelloWorld!"); }}
HelloWorld – canit be more simple?
public class HelloWorldOO { public HelloWorldOO() { } public void sayHelloWorld() { return “HelloWorld"; } public static void main(String[] args) { HelloWorldOO helloWorldOO = new HelloWorldOO(); System.out.println(helloWorldOO.sayHelloWorld()); }}
To OO:
Ingeniørhøjskolen i ÅrhusSlide 13 af 33
First Issue: Making the Object Distributed- Method Call vs. Object Request
CalledCalledCalledCalled
StubStub
StubStubStubStub
CallerCaller
CalledCalledCalledCalled
CallerCallerCallerCaller
Transport Layer (e.g. TCP or UDP)Transport Layer (e.g. TCP or UDP)Transport Layer (e.g. TCP or UDP)Transport Layer (e.g. TCP or UDP)
Ingeniørhøjskolen i ÅrhusSlide 14 af 33
Server Class – The Skeleton (Stub)
…public class HelloWorldServerThread extends Thread { protected DatagramSocket socket = null; public HelloWorldServerThread() throws IOException { super(); socket = new DatagramSocket(4450); }
public void run() { while (true) { …
if (request.startsWith("sayHelloWorld")) { new String(packet.getData()); // figure out response String response = null; //Call the HelloWorldOO method & place it in response HelloWorldOO helloWorldOO = new HelloWorldOO(); response = helloWorldOO.sayHelloWorld(); buf = response.getBytes(); … socket.send(packet);…
CalledCalledCalledCalled
StubStub
Ingeniørhøjskolen i ÅrhusSlide 15 af 33
The Client Stub - HelloWorldProxypublic class HelloWorldProxy { public HelloWorldProxy() { } public String sayHelloWorld() {… // get a datagram socket socket = new DatagramSocket(); // send the request byte[] buf = new byte[256]; buf = methodCalled.getBytes(); InetAddress address = InetAddress.getByName("localhost"); DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4450); socket.send(packet); // get a response packet = new DatagramPacket(buf, buf.length); socket.receive(packet); response = new String(packet.getData()); } catch (Exception e) { e.printStackTrace(); } socket.close(); return response;…
StubStubStubStub
CallerCaller
Ingeniørhøjskolen i ÅrhusSlide 16 af 33
Achieving Access Transparency
package chapter1;
public class HelloWorldClient {
public static void main(String[] args) {
System.out.println("Recieved from Server: " +
new HelloWorldProxy().sayHelloWorld());
}
}
… and thus we have achieved access transparencyusing rather simple constucts (the Proxy pattern).
We could have achieved an even higher degree of transparency by using an interface definition – and thus a contract between Client & Server.
StubStubStubStub
CallerCaller
Ingeniørhøjskolen i ÅrhusSlide 17 af 33
Direct Use of Network Protocols implies
• Manual mapping of complex request parameters to byte streams or datagrams – is complex & error prone
• Manual resolution of data heterogeneity– encoding differs on NT & UNIX (what is the format of an Integer?)
• Manual identification of components– References must be established manually (host add., port nr. etc.)
• Manual implementation of component activation• No guarantees for type safety• Manual synchronization of interaction between distributed
components– Developers need to handle TCP/UDP output stream/responses
• No quality of service guarantees– Transactions, Security, Concurrency, Scalability, Reliability
Ingeniørhøjskolen i ÅrhusSlide 18 af 33
= Reason for Using 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
• “Gets you of the hook” – concerning the nasty stuff in network programming
Ingeniørhøjskolen i ÅrhusSlide 19 af 33
Forms of Middleware
• Transaction-Oriented– Suited for Database Applications– IBM CICS– BEA Tuxedo– Encina
• Message-Oriented– Suited for Asynchronous (EDI,Batch)– IBM MQSeries– DEC Message Queue– NCR TopEnd– (SOAP)
• RPC Systems– Suited for Access transparency etc– ANSA– Sun ONC– OSF/DCE– (SOAP)
• Object-Oriented– OMG/CORBA– DCOM– Java/RMI– (SOAP)
• First look at RPCs to understand origin of object-oriented middleware
Ingeniørhøjskolen i ÅrhusSlide 20 af 33
Remote Procedure Calls
• 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• RPC is the “legacy” of oo middleware
Ingeniørhøjskolen i ÅrhusSlide 21 af 33
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.
Ingeniørhøjskolen i ÅrhusSlide 22 af 33
InterfaceDefinition
Facilitating Type Safety
ServerClient
RequestRequest
ReplyReply
Client & Server has a contractClient & Server has a contract
Ingeniørhøjskolen i ÅrhusSlide 23 af 33
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;
} = 105040;
Ingeniørhøjskolen i ÅrhusSlide 24 af 33
Flashback to HelloWorld Example
• How to map complex types?– e.g. returning a struct (in C, C++) or an object (in C++,
Java, C#, etc.)?– How would you do it?– Use 5 minutes to discuss this with your neighbor
• What are the issues?• Which technology might help us?• How would you do this in Java?
Ingeniørhøjskolen i ÅrhusSlide 25 af 33
RPC Marshalling and Unmarshalling
• Marshalling: Disassemble data structures into transmittable form
• Unmarshalling: Reassemble the complex data structure
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.day,&dob.month, &dob.year,&name_len,name);};
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.day,&dob.month, &dob.year,&name_len,name);};
Ingeniørhøjskolen i ÅrhusSlide 26 af 33
RPC Stubs
• Creating code for marshalling and unmarshalling is tedious and error-prone.
• Code can be generated fully automatically from interface definition.
• Code is embedded (hidden away – de-coupled) 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.• NOT object-oriented, no support for exceptions or
advanced error handloing
Ingeniørhøjskolen i ÅrhusSlide 27 af 33
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
Object-Oriented Middleware
Ingeniørhøjskolen i ÅrhusSlide 29 af 33
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
Ingeniørhøjskolen i ÅrhusSlide 30 af 33
IDL Example
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);};
Ingeniørhøjskolen i ÅrhusSlide 31 af 33
Presentation Layer Implementation
• Ensuring compatible encoding across platforms• 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
Ingeniørhøjskolen i ÅrhusSlide 32 af 33
ObjectReferences Hosts Processes Objects
Session Layer Implementation- Mapping of Object references to hosts-Activation & deactivion of objects-Invocation of requested operation-Synchronization of client & server (state)
- Mapping of Object references to hosts-Activation & deactivion of objects-Invocation of requested operation-Synchronization of client & server (state)
Developing with Object-Oriented Middleware
Ingeniørhøjskolen i ÅrhusSlide 34 af 33
InterfaceDefinition
Design
Server StubGeneration
Client StubGeneration
ServerCoding
ClientCoding
ServerRegistration
Development Steps
SOAP: WSDLSOAP: WSDLJava2WSDLJava2WSDL
WSDL2JAVAWSDL2JAVA
AXISSOAPAXISSOAP
Ingeniørhøjskolen i ÅrhusSlide 35 af 33
Facilitating Access Transparency
• Client stubs have the same operations as server objects– If implemented …
• 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– Some Middleware products does this implicitly
Ingeniørhøjskolen i ÅrhusSlide 36 af 33
Facilitating Location Transparency
• Based on Object identity• … and this is 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?
– Need of a registry – an object adapter– Administrator uses Middleware tool for registering
• COM uses Windows Registry• Java RMI daemon• CORBA object Adapter• SOAP UDDI
– RPC’s – daemon “portmap”• Client contacts one portmap daemon or broadcasts
– Middleware resolves this (in some cases)• SOAP UDDI
Ingeniørhøjskolen i ÅrhusSlide 37 af 33
Team.idlTeam.idl
included ingeneratesreads
IDL-Compiler
Teamcl.hh
Teamcl.cc Teamsv.cc
Teamsv.hh
Stub Generation
Team.wsdlTeam.wsdlTeam.javaTeam.java
Team.midlTeam.midl
Ingeniørhøjskolen i ÅrhusSlide 38 af 33
C++ Compiler, Linker
Server
Client.ccClient.cc Server.ccServer.cc
C++ Compiler, LinkerC++ Compiler, Linker
Client
Team.idlTeam.idl
included ingeneratesreads
IDL-Compiler
Teamcl.hh
Teamcl.cc Teamsv.cc
Teamsv.hh
Client and Server Implementation