Message Oriented Middlewares

(1)

Message Oriented

Middlewares

Fabienne Boyer, [email protected]

Fabienne Boyer, Distributed Programming 2

Introduction

n 

RPC

n  Synchronous communications

n  Explicit identification of the receiver(s)

n  1-1 connexion

-> Strongly coupled components

n 

MOM

n  Asybchronous communications

n  Anonymous

n  1-N

-> Weakly coupled components

Fabienne Boyer, Basics of Distributed Programming 3

Application examples

n 

Event-based applications

n Network monitoring

n Workflow

n Administration

n 

EAI (Enterprise Application Integration)‏

n Heterogeneous information systems

n 

…

Monitoring application example

n 

Interrogation

n 

A client application monitors server nodes and application states

n 

Update a centralized database

n Complex configuration

n Dynamic (adding / removing server nodes as well as applications)

n 

Human administrators inspect the collected data

(2)

Application example

n 

Solution MOM

n 

Managed elements send messages

n When their state evolves

n Alerts, statistics

n 

One or more daemons receive messages

n Keep current system state in a database

n Raise alerts

MOM principles

n 

Two main communication patterns

n 

Message Queuing

n 

Publish/Subscribe

Message Queuing

n 

Communication via une file de message

n 

Asynchronisme

n 

Communication asynchrone

n 

Indépendance de l’émetteur et du récepteur (pas forcément

actif)

n 

Communication 1-1 et non anonyme

n 

Un seul récepteur identifié

Message Queuing

8

send recv

Client Serveur

Serveur

(3)

Publish-Subscribe

n 

Anonymous

n 

The sender emits a message

n Subject-based: (fsubject-based:failure,load,…)‏

n Content-based (content-based:*kernighan*)‏

n 

A receiver subscribe to a subject or content

n 

Communication 1-N

n 

Several receivers for a given message

Publish-Subscrive

consommateur

publish producteur

. .

.

  "

  "

subscribe

"

recv

MOM Properties

n 

Availability

n  Unavailability time less than few minutes per year

n 

Robustness

n  Message delivrance guarantee

n  Ordering

n  Atomicity (for a sequence of sending/receiving)

n 

Scalability

n  Tolerates load increases

n 

Security

n  Confidentiality

n  Integrity

n 

Persistence

n  Stopping the MOM è restart without state loss

MOM Design: centralized server

n 

Simplicity

n 

Low availability

n 

Low scalability

_Serveur

client

client client

client

(4)

MOM Design: distributed servers

n  Distributed servers

n Static or dynamic organization

n Message routing

n  Availability

n A server can be replaced by another

n Not transparent

n  Scalability

n Balance the load

n Strategies (by clients / by topic)

serveur serveur

serveur client

client client

client

serveur client

MOM Design: message bus

Machine A Système d’

exploitation Services du bus logiciel Processus_a

Machine B Système d’

exploitation Services du bus logiciel Processus_b

protocoles du bus logiciel

Interface Bus Interface Bus

MOM Example: AAA

n 

AAA Agent Platform

n 

A programming model

n 

Distributed agents communicating through events

n 

A runtime

n 

Based on a message bus

The AAA Platform

n 

Execution model

n  Reactive objects

n  React to a notification

n  Notifications sent through the message bus

n 

Agents

n  State

n  Reactions rules

Agent

Agent React SendTo

Message bus

(5)

AAA Properties

n 

Agents

n  Persistents (state saved on disk))

n  Atomic reactions (consistent state)

n 

Communications

n  Asynchrones (disconnected mode)

n  Robust (sent/received as soon as the network allows it )

n  Causally ordered

B

C A

AAA Programming Model

n 

Agents

n  Java objects that inherit from Agent

n  Identified by an agentId

public class HelloWorld extends Agent {

public HelloWorld(short to, String name) { super(to, name);

}

public void react(AgentId from, Notification not)‏

throws Exception { …

}

AAA Programming Model (2)

n 

Notifications

n  Serializable objects that inherits from Notification

n  Used at application level and at runtime level

public class HelloWorldNot extends Notification {

public String msg = "Hello world";

public HelloWorldNot(String msg) { this.msg = msg;

} }

AAA Programming Model (3)

n 

Notification sending

n  To an agent (message queue / file)‏

n  To a Role or RoleMultiple (pub/sub)‏

sendTo(AgentId ag, Notification n) ou sendTo(Role role, Notification n)‏

ag: receiver identification role: topic identification n: the notification

(6)

Agent identification

n 

Identification of the agent server that created the agent (from)‏

n 

Identification of the agent server that hosts the agent (to)‏

n 

Local stamp (from-stamp)‏

Creation engine Localisation engine Stamp

16 bits 16 bits 32 bits

Programming model (4)

n 

Event-reaction model

n  Event = notification sent by an agent to another one

n  Réaction method to process when receiving a notification

Agent

attribute1 : ...

attribute2 : ...

event1 (...)‏

event2 (...)‏

dest1 (...)‏

dest2 (...)‏

Agent example

Fabienne Boyer, Distributed Programming

public class HelloWorld extends Agent { public AgentId dest;

public HelloWorld(short to, String name) { super(to, name);

}

public void react(AgentId from, Notification not)throws Exception { if (n instanceof HelloWorldNot) { System.out.println(((HelloWorldNot)n).msg);

// send a new notification

sendTo(dest, new HelloWorldNotReceived());

} else

// core notification processing super.react(from,not);

} }

Agent creation and deployment

1.  Local instanciation

2.  Distributed deployment

short asId;

..

// Create agent ag

// ag should be deployed on AgentServer asId Ag ag = new Ag(asId);

// We can assign local attributes ag.x = 32;

// Effective deployment ag.deploy(),

..

(7)

Static distributed configuration

<?xml version="1.0"?>

<!DOCTYPE config SYSTEM "a3config.dtd">

<config name="test_conf1">

<host hostname="localhost">

<server id="0" name="s0" port="27300">

</server>

<server id="1" name="s1" port="27301">

</server>

</host>

</config>

Agent server creation (by command)

Server

Client Interface I

> java fr.dyade.aaa.agent.AgentServer num rac // num = numéro du serveur

// rac = racine de persistence

// exemple : java fr.dyade.aaa.agentServer 1 s1

Agent server creation (by program)

public class Launch {

public static void main (String args[]) { try {

AgentServer.init(args);

// Create agent ag to be deployed on AgentServer 0

Ag ag = new Ag((short)0);

ag.deploy(), // Send a notification

Channel.sendTo(ag.getId(), new not());

// Become an AgentServer AgentServer.start();

} ..

}

AAA Design

n 

Agent server

n  Notification transport‏

n  Persistent queues

n  Processing reactions

(8)

AAA Design

n 

Agent Server

n  JVM

n  Includes a bus and an agent factory

n  Allocates identifier and manages the agent persistence

n 

Local Bus

n  Manages local communications

n  Ensures atomicity and robustness

n  Interconnected with other bus

n  Composed of a Channel and an Engine

Agent server design

• 

Channel

• 

Localisation and transport of notifications

• 

Engine

• 

Execution server

Agent server design

Channel Agent

qin Engine

React SendTo

msg = qin.get();

agent = Agent.load(msg.to);

agent.react(msg.from, msg.not);

transaction.begin()‏

qin.pop();

channel.dispatch();

Agent.save();

transaction.commit()‏

Network Agent

qout n 

Agent server internals

n  Mono-threaded server

n  Any reaction processed as a transaction