Transcript RPC and RMI - Wayne State University

Communication in Distributed
Systems
Cheng-Zhong Xu
Taxonomy
Communication
Point-to-point
Two-way comm
Socket prog
Multicast group comm
One-way comm
RPC/RMI
Message-oriented
© C. Xu, 1998-2011
Outline
 TCP/UDP
socket programming
 RPC/RMI programming
– RPC: Remote Procedure Call
– RMI: Remote Method Invocation
 Distribute
Objects
– Local object vs remote object
 Message-oriented
comm
© C. Xu, 1998-2011
Limitations of Socket Programming

Limited support for data structure in comm
– In Java TCP/UDP programming, data in transmission is defined as a
stream of bytes
– client and server are coupled in development: they need to know how
the data are interpreted at other side

Limited support for heterogeneity
– Network protocols: Internet protocol masks the differences between
networks, but
– computer hardware: e.g. data types such as integers can be represented
differently
– operating systems, e.g. the API to IP differs from one OS to another
– programming languages, data structures (arrays, records) can be represented
differently
– implementations by different developers,
» they need agreed standards so as to be able to inter-work
© C. Xu, 1998-2011
Architectural heterogeneity: an example
Representations of integer 5 and String “JILL” on different
machines:
•
•
•
Original message on the Pentium (little endian)
The message after receipt on the SPARC (big endian)
The message after being inverted. The little numbers in boxes indicate the
address of each byte
•
Integers are reversed by byte ordering, but strings are not!!
© C. Xu, 1998-2011
Remote Procedure Call (RPC)
Remote Method Invocation (RMI)
 Middleware
provides a simple programming
abstraction and masks heterogeneity of client and
server systems
 Similar interface as in procedure call or method
invocation between client and service
– client sends command and arguments
– server computes
– Server sends back reply data
 Implementation
of the procedures or methods are
on remote servers
© C. Xu, 1998-2011
Conventional Procedure Call
For example,
count = read(fd, buf, nbytes);
a)
b)
Parameter passing in a local procedure call: the stack before the
call to read
The stack while the called procedure is active
© C. Xu, 1998-2011
Remote Procedure Call (RPC)

Build a mechanism, which allows clients
– call procedures over a network as if the code was stored and
executed locally


The mechanism automates the writing of data-handling
and formatting code
Advantages: fewer errors, faster to code and evolve interfaces
© C. Xu, 1998-2011
RPC Structure: Client/Server Proxies (stub, skeleton)
client
program
server
program
call return
return
client
stub
call
server
skeleton
network
1.
2.
3.
4.
5.
6.
Client procedure calls client stub in normal way
Client stub builds message, calls local OS
Client's OS sends message to remote OS
Remote OS gives message to server stub/skel
Server stub unpacks parameters, calls server
Server does work, returns result to the stub
7.
8.
9.
10.
Server stub packs it in message, calls local OS
Server's OS sends message to client's OS
Client's OS gives message to client stub
Stub unpacks result, returns to client
© C. Xu, 1998-2011
Passing Parameters

Steps involved in doing remote computation through RPC
2-8
© C. Xu, 1998-2011
Outline
 TCP/UDP
socket programming
 RPC/RMI programming
– RPC: Remote Procedure Call
– RMI: Remote Method Invocation
 Distribute
Objects
– Local object vs remote object
 Message-oriented
comm
© C. Xu, 1998-2011
Distributed Objects

An object is an individual unit of run-time data storage used as
the basic building block of programs.
– Each object is capable of receiving msgs, processing data, and sending
msgs to other objects; methods together form the object’s interface
with the outside world or other objects.
– A class is the blueprint from which individual objects are created.


Distributed objects have a separation between interfaces and
objects that implement the interfaces; interfaces and the
objects are in different machines.
Allow programs to treat remote objects just like local objects
– remote references
– remote invocation


First good impl: DEC SRC network objects for Modula-3
Recent implementation: Java Remote Method Invocation
© C. Xu, 1998-2011
Organization of Distributed Objects


Separate proxy for each remote object that is referenced
proxy looks just like real object. but loaded at run-time
when remote object is to be bound.
– implements the same interface

proxy’s methods do RPC to real object
– handle arguments, return values, exceptions correctly

proxy code generated by RMI compiler
© C. Xu, 1998-2011
Remote Object References

Naming a remote object (“global address”)
–
–
–
–
–
machine (IP address) where actual object resides
port number, identifying the server that manages the object
(boot time)
unique object ID number (generated by server)
Object references can be passed between processes, as
parameters to method invocations
Before invocation, object ref. must first bind to its
object. Binding leads to a placement of corresponding
proxy
 Programs referenced a remote object by using stand-in
proxy object

© C. Xu, 1998-2011
Remote Invocation

when a proxy method is invoked:
– invocation message sent to remote process
» contains global address of object, method ID, arguments
– remote process invokes real object
– return message sent back to proxy object
» contains return value or exception
– proxy method call returns correct value, or throws
correct exception

Under the Hood
 each process keeps a table of all proxy objects
– maps global address to proxy object


use table to maintain one-to-one mapping from
proxy object to remote object
interactions with garbage collection
© C. Xu, 1998-2011
Building an RMI App
server
interface
client
code
client
stub
stub
generator
server
impl
server
skeleton
compiler/
linker
compiler/
linker
client
app
server
app
Server
code
© C. Xu, 1998-2011
RMI Binding (rmi registry)
 How
does a client find the service code
 Registry to keep track of distributed objects
– simply a name repository
– provides a limited central management point
for RMI
 Server
binds methods to names in the reg
 Client looks up names in the reg for
availability of an object
© C. Xu, 1998-2011
Interface Definition
 All
remote objects are referenced through
interfaces
– It extends the Remote interface as all RMI
interfaces must
– Methods must throw RemoteException
import java.rmi.*;
public interface myServer extends Remote {
public void doSomething( … ) throws RemoteException;
}
© C. Xu, 1998-2011
// myClient
// myServer object has registered itself as “xuObject”
import java.rmi.*;
import java.rmi.server.*;
public class myClient {
public static void main( String[] args ) {
System.setSecurityManager( new RMISecurityManager() );
try {
myServer ro= (myServer)Naming.lookup(“xuObject”);
ro.doSomething();
} catch (Exception e) {}
}
}
© C. Xu, 1998-2011
Notes
 All
RMI-based app need to import
– java.rmi and java.rmi.server packages
 Inside
the main method, the first thing is set
the Java security manager, which grant or
deny permissions on the operations
 create a try/catch block that performs the
remote method invocation
© C. Xu, 1998-2011
// myServer implementation
import java.rmi.*;
import java.rmi.server.*;
import java.rmi.registry.*;
public class myServerImpl extends UnicastRemoteObject
implements myServer {
public myServerImpl() throws RemoteException{ }
public void doSomething(… ) throws RemoteException{… …}
public static void main( String[] args) {
System.setSecurityManager( new RMISecurityManager());
try {
myServerImpl obj = new myServerImpl();
Naming.rebind(“xuObject”, obj);
System.out.println(“doSomething bound in registry”);
}catch (Exception e) {}
}
© C. Xu, 1998-2011
}
Notes
 myServerImpl
extends
– java.rmi.server.UnicastRemoteObject class, which
defines a non-replicated remote object whose
references are valid only while the myServer process is
alive (Transient Objects).
– It supports point-to-point active object reference via
TCP streams.
 Bind
vs rebind
 A remote implementation must have a zeroargument constructor
© C. Xu, 1998-2011
Deployment
 Generate
stub and skeleton
– rmic myServerImpl, which generates
» myServerImpl_Stub.class, myServerImpl_Skel.class
– Since JDK 5.0, stub classes can be generated ondemand at runtime
 rmiregistry
– starts the registry service in the background
 java
ServerImpl
– starts the server, which registers the remote
object with the registry
 java
Client
© C. Xu, 1998-2011
Passing Arguments
 Passing

primitive types (int, boolean, etc.)
Remote object vs. non-remote objects
– remote object is an instance of RemoteObject
– remote object refers to an object whose
method can be invoked from another JVM.
– Remote objects passed by reference, while nonremote objects passed by copy
A
difference between RPC and RMI
– passing objects,
– passed object may interact with receipt itself
© C. Xu, 1998-2011
Parameter Passing

Remote object vs Local Object
– Pass-by-value is ok for all distributed object references, but not efficient

The situation when passing an object by reference or by value
– E.g. Client on machine A calls a method of remote object in machine C,
which contains two parameters: O1 in machine A and O2 in machine B
© C. Xu, 1998-2011
Passing objects
 Object
Serialization
– To be passed between client/server, objects have
to be serialized
 For
example,
– CloudServer, which do jobs for clients in clouds
– Different clients may ask CloudServer do
different jobs
– Each job is specified in arguments passed from
a client to the server
© C. Xu, 1998-2011
//Task.java
import java.io.*;
public class Task implements Serializable{
public Object execute() { return null; }
}
//foo.java
public class foo extends Task{
int n;
public foo (int m) {n=m;}
public Object execute() {
return new Integer( n*n );
}
}
© C. Xu, 1998-2011
// CloudServer.java
import java.rmi.*;
import java.util.*;
public interface CloudServer extends Remote {
Object execute( Task o)
throws RemoteException
}
© C. Xu, 1998-2011
// CloudServerImpl.java
import java.rmi.*;
import java.io.*;
import java.rmi.server.*;
public class CloudServerImpl implements CloudServer {
public CloudServerImpl() throws RemoteException{}
public Object execute( Task t)
throws RemoteException {
return t.execute();
}
public static void main (String args[]) {
System.setSecurityManager( new RMISecurityManager());
try {
CloudServer csr = new CloudServerImpl();
Naming.rebind(“CloudServer”, csr);
} catch (IOException ioe) {}
© C. Xu, 1998-2011
}
// example.java
// java example registry-host
import java.rmi.*;
import java.io.*;
public class example{
public static void main( String[] args ) throws RemoteException {
System.setSecurityManager( … );
new example(args[0]);
}
public example( String host ) {
try {
CloudServer csr = (CloudServer)
Naming.lookup(“rmi://”+host+“/CloudServer”);
System.out.println(
csr.execute( new foo(10) );
catch (IOException ioe) {}
© C. Xu, 1998-2011
Running on different machines

Server site
–
–
–
–
–

CloudServer.class
CloudServerImpl.class
CloudServerImpl_Stub.class
CloudServerImpl_Skel.class
Task.class
 Client
–
–
–
–
–
site
CloudServer.class
example.class
CloudServerImpl_Stub.class
Task.class
foo.class
java CloudServerImpl
At client site
java -Djava.rmi.server.codebase=‘…’ example server
© C. Xu, 1998-2011
Remote Object References
 Asynchronous
Client/Server Comm
– Client calls CloudServer’s asynExecute(…)
– On the completion of the work, the
CloudServer notifies the client of the result
 Client
itself is declared as a Remote Object,
which reference is passed to CloudServer
 CloudServer calls workCompleted(…) of
the remote object.
© C. Xu, 1998-2011
// CloudServer.java
import java.rmi.*;
import java.util.*;
public interface CloudServer extends Remote {
Object execute( Task work )
throws RemoteException
void asynExecute( Task work,
WorkListener listener)
throws RemoteException
}
© C. Xu, 1998-2011
Public class CloudServerImpl
extends UnicastRemoteObject
implements CloudServer {
……
public void asynExecute(Task work,
WorkListener listener)
throws RemoteException {
Object result = work.execute();
listener.workCompleted( work, result );
}
}
© C. Xu, 1998-2011
WorkListener Remote Interface
// WorkListener.java
public interface WorkListener extends Remote {
public void workCompleted( Task request,
Object result )
throws RemoteException;
}
Example.java will be modified to implement the remote
interface. This turns example into a remote object
© C. Xu, 1998-2011
import java.rmi.*;
import java.io.*;
public class example extends UnicastRemoteObject
implements WorkListener {
public static void main( ) throws RemoteException {…}
public void workCompleted( Task t, Object result)
throws RemoteException {
System.out.println(“Async work result = “ + result);
}
public example ( String host ) throws RemoteException {
try {
CloudServer csr = (CloudServer)
Naming.lookup(“rmi://”+host+“/CloudServer”);
csr.asynExecute( new foo(10), this );
catch (IOException ioe) {}
}
© C. Xu, 1998-2011
Remarks
 Asynchronous
RPC: A client and server
interacting through two asynchronous RPCs
Asynchronous RMI ??
© C. Xu, 1998-2011
RPC vs RMI
 RMI
supports system-wide object references
 RMI can have object-specific stubs, not
necessarily have general-purpose client-side
and server-side stubs
 Pass objects as arguments (object migration)
 Migrated objects can interact with both original
and new hosts
© C. Xu, 1998-2011
Mobile Objects

Objects be passed as arguments
– Single-hop vs multi-hop migration
– Dispatched by its host, or migrated according to its own itinerary

Mobile objects can be performed on hosts (cloud servers)
according to their own agenda, under a security sandbox
– access to local computing and data resources

Mobile objects can communicate back to their home hosts.
Weak mobility
Stop before migration, and restart on arrival
at a new server; running status not transferred
Strong mobility
Suspend before migration, and resume on arrival
at a new server; running status carried over
© C. Xu, 1998-2011
Naplet:
A mobile object middleware system

Structured Itinerary Mechanism
– Sequential/parallel/alternative visit of a group of servers




Naplet directory based location + Forward Pointer
Message-oriented communication
Persistent comm between mobile objects
Un/secure execution
– Multithreaded objects
– Secure access to local resource

Java based
Naplet 0.2 source code available
http://www.ece.eng.wayne.edu/~czxu/
software/naplet.html
© C. Xu, 1998-2011
Taxonomy
Communication
Point-to-point
Two-way comm
Socket prog
Multicast group comm
One-way comm
RPC/RMI
Message-oriented
© C. Xu, 1998-2011
Message-Oriented Persistent
Communication
© C. Xu, 1998-2011
Organization of Comm System
Examples:
Email --- Client/Outbound Mail Server/Inbound Server
MPI --- Message Passing Layer
© C. Xu, 1998-2011
Difference??
Another Example of Persistent Comm
 Persistent
communication of letters back in the
days of the Pony Express.
Although the means of transportation and the means by which letters
are sorted have changed, the mail principle remains the©same
C. Xu, 1998-2011
Persistence and Synchronicity
 Persistent
vs Transient Communication
– Persistent: submitted messages are stored by the comm.
system as long as it takes to deliver it the receiver
– Transient: messages are stored by the comm system as
long as the sending and receiving app. are executing.
 Asynchronous
vs synchronous
– Sync: a client is blocked until its message is stored in a
local buffer at the receiving host, or actually delivered
to the receiver
– Async: A sender continues immediately after it has
submitted its message for transmission. (The message
is stored in a local buffer at the sending host, or
otherwise at the first communication server.
© C. Xu, 1998-2011
a)
Persistent asynchronous communication
E.g. Email
b)
Persistent synchronous communication (??) (a
weaker form: when the sender is blocked until its
message is stored at receiver-side comm server )© C. Xu, 1998-2011
2-22.2
c)
d)
Transient asynchronous communication (e.g. UDP,
asynchronous RPC). Transmission fails if the receiver
is not executing at the time the msg arrives.
Receipt-based transient synchronous communication .
Sender is blocked until the msg is stored in a local
buffer at the receiving host. (Example?)
© C. Xu, 1998-2011
e)
f)
Delivery-based transient synchronous communication at
message delivery. (Sender is blocked until the msg is delivered
to the receiver)
Response-based transient synchronous communication (e.g.
RPC and RMI) (Sender is blocked until it receives a reply msg
from the receiver)
© C. Xu, 1998-2011
Different Forms of Communication
 Persistent
Asynchronous
 Persistent Synchronous
 Transient Asynchronous
 Transient Synchronous
– Receipt-based (weakest, e.g. async. RPC)
– Delivery-based
– Response-based (strongest, e.g. RPC)
 Need
for persistent comm. in middleware, largescale system
– Components of a large scale distributed system may
not always be immediately accessible
– Failure should be masked with a recovery procedure
© C. Xu, 1998-2011
– But, persistent comm. incurs long delays
Message-Oriented Transient Comm.


Socket is a communication endpoint to which an appl can write data to
and from which incoming data can be read
Socket primitives for TCP/IP.
Primitive
Meaning
Socket
Create a new communication endpoint
Bind
Attach a local address to a socket
Listen
Announce willingness to accept connections
Accept
Block caller until a connection request arrives
Connect
Actively attempt to establish a connection
Send
Send some data over the connection
Receive
Receive some data over the connection
Close
Release the connection
© C. Xu, 1998-2011
Berkeley Sockets

Connection-oriented communication pattern using sockets.
© C. Xu, 1998-2011
MPI: Another Transient Comm

Basic message-passing primitives of MPI.
Primitive
Meaning
MPI_bsend
Append outgoing message to a local send buffer of MPI runtime
MPI_send
Send a message and wait until copied to local or remote buffer
MPI_ssend
Send a message and wait until receipt starts
MPI_sendrecv
Send a message and wait for reply
MPI_isend
Pass reference to outgoing message, and continue
MPI_issend
Pass reference to outgoing message, and wait until receipt starts
MPI_recv
Receive a message; block if there are none
MPI_irecv
Check if there is an incoming message, but do not block
© C. Xu, 1998-2011
Synchrony in MPI
MPI_bsend  Transient synchronous
 MPI_send (blocking send)

– Block the caller until the msg be copied to MPI runtime
system at the receiver’s side;  Receipt-based
transient comm
– or until the receiver has initiated a receive operation
delivery-based transient comm
MPI_ssend  delivery-based
 MPI_sendrecv  response-based

© C. Xu, 1998-2011
Message Queuing Model
MQ: a software-engineering component used for
interprocess comm. It utilizes a queue for
messaging—the passing of control or of content.
[Wikipedia]
 MQ provides a Persistent Asynchronous comm
protocol

– Sender and receiver do not need to connect to the msg
queue at the same time; msgs placed on the queue are
stored until the recipient retrieves them.
– Most msg queues have set limits on the size of data that
can be transmitted in a single msg. Those with no such
limits are often referred to as mailboxes.
© C. Xu, 1998-2011
MQ Model
2-26

Four combinations for loosely-coupled comm. using queues
(sender and receiver can execute completely independently of
each other)
© C. Xu, 1998-2011
MQ: Interface to a Queue
One-way Communication Primitives:
Primitive
Meaning
Put
Append a message to a specified queue (non-blocking)
Get
Block until the specified queue is nonempty, and remove the first message
Poll
Check a specified queue for messages, and remove the first. Never block.
Notify
Install a handler to be called when a message is put into the specified
queue.
• Addressing: System wide unique destination queue
• Active message: The handler to be executed on arrival at the dest.
• Mobile object: messages in principle contains any data. How
about objects?
© C. Xu, 1998-2011
Msg Oriented Middleware (MOM)
 Many
impl function internally in OS or appl
– In UNIX: msgctrl(), msgget(), msgsnd()
 Distributed
impl across different systems for msg
passing
– Enhanced fault resilient: msgs don’t get lost even in the
event of a system failure.
 Examples:
–
–
–
–
–
IBM’s WebSphere MQ (MQ series),
Oracle Advaced Queueing (AQ)
Microsoft’s MSMQ
Sun’s Java API: Java Message Service (JMS) since 2005
Amazon’s Simple Queue Service for building automated
© C. Xu, 1998-2011
workflow (aws.amazon.com/sqs)
MOM (Cont’)
Msgs are forwarded over comm servers and
eventually delivered to dest, even if it was down
when the msg was sent.
 Each app has its own queue and a queue can be read
only by its assoc. app.
 Multiple apps can share a single queue
 Guaranteed that a msg will eventually be inserted in
the recipient’s queue, but no guarantee about when,
or if the msg will actually be read.

– Immediate-term storage capacity for messages

Messaging domain:
– Point-to-point vs publish/subscribe
© C. Xu, 1998-2011
General Architecture:

Source Queue, Destination Queue, or 3rd-party messaging service (Amazon)
Mapping of queue-level addressing to network-level addressing. (analogous
to DNS in email system)
Queue manager

Special manager working as routers: forward msg between queue managers


© C. Xu, 1998-2011
Queue Manager, Router, and Overlay Network
2-29
• Static vs dynamic routing with routers
• Multicasting with routers
© C. Xu, 1998-2011
Message Brokers (e.g. Amazon’s SQS



Integration of legend applications with new ones to form a single coherent
distributed information system
Message broker acts as an appl-level gateway to convert incoming msg to
a format that can be understood by the dest app.
The general organization of a message broker in a msg-queuing system.
© C. Xu, 1998-2011
Amazon Simple Queue Service
Amazon’s SQS offers a reliable, highly scalable,
hosted queue for storing messages as they travel
between computers. (aws.amazon.com/sqs)
 By using Amazon SQS, developers can simply move
data between distributed components of their
applications that perform different tasks, without
losing messages or requiring each component to be
always available.
 Make it easy to build an automated workflow
 Pricing: $0.01 for 10K SQS requests.

© C. Xu, 1998-2011
Basic Queue Requests








CreateQueue: Create queues for use with your AWS account.
ListQueues: List your existing queues.
DeleteQueue: Delete one of your queues.
SendMessage: Add any data entries to a specified queue.
ReceiveMessage: Return one or more messages from a specified queue.
DeleteMessage: Remove a previously received message from a
specified queue.
SetQueueAttributes: Control queue settings like the amount of time
that messages are locked after being read so they cannot be read again.
GetQueueAttributes: See information about a queue like the number of
messages in it.
© C. Xu, 1998-2011
Example: IBM WebSphere MQ




Launched in 1992, previously known as MQ series
De-facto standard for messaging across platforms
Queue Manager: handles storage, timing issues, triggering, and all
other functions not directly actual movement of data
QM comm with outside via local Binding, or remote Client connection
© C. Xu, 1998-2011
Message Channels


MC is a unidirectional, reliable connection between a
sending and a receiving queue manager
Each end is managed by a Message Channel Agent (MCA)
– check send queue, wrap messages into transport-level packets, send
to associated receiving MCA
– listening for incoming packets, unwrap them, store into queues
Attribute
Description
Transport type
Determines the transport protocol to be used
FIFO delivery
Indicates that messages are to be delivered in the order they are sent
Message length
Maximum length of a single message
Setup retry
count
Specifies maximum number of retries to start up the remote MCA
Delivery retries
Maximum times MCA will try to put received message into queue
© C. Xu, 1998-2011
Message Transfer




Address: queue manager + destination queue;
Each queue manager has a system-wide unique id; Local aliases
for queue manager names
Routing tables: (Dest, SendQueue)
Transfer along a channel can take place only if both MCAs are
up and runing
© C. Xu, 1998-2011
Message Queue Interface

Primitives available in an MQSeries MQI
Primitive
Description
MQopen
Open a (possibly remote) queue
MQclose
Close a queue
MQput
Put a message into an opened queue
MQget
Get a message from a (local) queue
MQSerie also provides facilities to signal applications
when msgs arrive. For more info, see
http://www.redbooks.ibm.com/abstracts/sg247128.html
© C. Xu, 1998-2011
Messaging Domain
 Point-to-Point
 Publish/Subscribe
– Clients address messages to a topic
© C. Xu, 1998-2011
Publish/Subscribe Messaging

Request/Reply (RPC, MRI) model has limitations in two
types of applications
– Update of state info that changes over time
» Send requests regularly for update of the new info. NOT efficient
– Action in response to event occurrence:
» Send event info to ALL nodes who are interested in

Publish/Subscribe messaging
– Any number of customers of info can receive messages that are
provided by one or more producers of the info
– Publisher: producer of the info
– Subscriber: consumer of the info
– Topic (or named logical channel) to be subscribed by consumers to
register their interests
– Publish/subscribe broker: track subscriptions to individual topics and
provide facilitates for a publisher to publish msgs on a given topic

Topic-based vs content-based vs hybrid systems
© C. Xu, 1998-2011
JMS Programming


Available in Java System Message Queue V3.6
Interfaces:
– ConnectionFactory
» Administrated object used to create a connection to JMS provider
» Defined by administrator, rather than programmer
– Connection
» A conn represents a comm link between app and msg server
» Administrated object
– Destination: where msg is delivered and consumed
» Queue or topic
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MessageConsumer
MessageProducer
Message
Session: Single threaded context for sending and receiving messages
http://www.sun.com/software/products/message_queue/index.xml
© C. Xu, 1998-2011
JMS Programming Model
© C. Xu, 1998-2011
Producer (1/3)
injects resources for a connection factory, queue, and topic:
@Resource(mappedName="jms/ConnectionFactory")
private static ConnectionFactory connectionFactory;
@Resource(mappedName="jms/Queue")
private static Queue queue;
@Resource(mappedName="jms/Topic")
private static Topic topic;
Assigns either the queue or topic to a destination object, based on dest type:
Destination dest = null;
try {
if (destType.equals("queue")) {
dest = (Destination) queue;
} else {
dest = (Destination) topic;
}
© C. Xu, 1998-2011
} catch (Exception e) {}
Producer (2/3)
Creates a Connection and a Session:
Connection connection =
connectionFactory.createConnection();
Session session = connection.createSession(false,
Session.AUTO_ACKNOWLEDGE);
Creates a MessageProducer and a TextMessage:
MessageProducer producer = session.createProducer(dest);
TextMessage message = session.createTextMessage();
Sends one or more messages to the destination:
for (int i = 0; i < NUM_MSGS; i++) {
message.setText("This is message " + (i + 1));
System.out.println("Sending message: " + message.getText());
producer.send(message);
}
© C. Xu, 1998-2011
Producer (3/3)
Sends an empty control message to indicate the end of the message stream:
producer.send(session.createMessage());
Sending an empty message of no specified type is a convenient way to
indicate to the consumer that the final message has arrived.
Closes the connection in a finally block, automatically closing the
session and MessageProducer:
} finally {
if (connection != null) {
try {
connection.close();
} catch (JMSException e) {
}
}
}
© C. Xu, 1998-2011
Async Consumer
1. injects resources for a connection factory, queue, and topic.
2. Assigns either the queue or topic to a destination object, based on the specified dest type.
3. Creates a Connection and a Session.
4. Creates a MessageConsumer.
5. Creates an instance of the TextListener class and registers it as the message listener
for the MessageConsumer:
listener = new TextListener();
consumer.setMessageListener(listener);
6. Starts the connection, causing message delivery to begin.
7. Listens for the messages published to the destination, stopping when the user types the
character q or Q:
System.out.println("To end program, type Q or q, " + "then <return>");
inputStreamReader = new InputStreamReader(System.in);
while (!((answer == 'q') || (answer == 'Q'))) {
try {
answer = (char) inputStreamReader.read();
} catch (IOException e) { }
}
8. Closes the connection, which automatically closes the session and MessageConsumer.
© C. Xu, 1998-2011
Async Consumer (2/2)
When a message arrives, the onMessage method is called automatically.
The onMessage method converts the incoming message to a TextMessage and
displays its content. If the message is not a text message, it reports this fact:
public void onMessage(Message message) {
TextMessage msg = null;
try {
if (message instanceof TextMessage) {
msg = (TextMessage) message;
System.out.println("Reading message: " + msg.getText());
} else {
System.out.println("Message is not a " + "TextMessage");
}
} catch (JMSException e) } catch (Throwable t) { }
}
© C. Xu, 1998-2011
Email vs Message Queue

Email is a special case of Message Queue
– Email aims at providing direct support for end users
– Email makes direct use of the underlying transport services
(SMTP over TCP); routing is left out

General MQ can satisfy a different set of
requirements:
» guaranteed message delivery,
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message priorities,
logging facilities,
efficient multicasting,
load balancing,
fault tolerance, etc.
© C. Xu, 1998-2011
Comm in DS: In Summary
Communication
Point-to-point
Two-way comm
Socket prog
Multicast group comm
One-way comm
RPC/RMI
Message-oriented
© C. Xu, 1998-2011