Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Building reliable distributed
systems
J2EE and JBoss Clustering
Giorgia Lodi
[email protected]
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Outline
•  J2EE specification
•  An introduction to JBoss application server
–  JBoss 5.x architecture
–  How to install and run it
•  The JBoss Clustering Service
–  The clustering framework
–  Automatic discovery of clustered nodes
•  How JBoss uses JGroups for group communication
–  Distributed cluster-wide hot deployment
–  HA-JNDI
–  Fail-over and load balancing
•  At RMI
•  At HTTP
–  Clustering Enterprise Java Beans
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Java 2 Enterprise Edition
•  J2EE technology provides a component-based
approach to the design, development, assembly
and deployment of enterprise applications
•  J2EE offers a multi-tiered distributed application
model
–  The ability to reuse components
–  Integrated eXtensible Markup Language (XML)-based
data interchange
–  A unified security model
–  Flexible transaction control
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Distributed Multi-tiered Applications
•  J2EE application logic is divided into components
according to function
–  Application components
•  can be installed on different machines depending on the tier in the
multi-tiered J2EE environment
•  Four tiers
–  Client-tier
–  Web-tier
–  Business tier
–  Enterprise information system (EIS)-tier
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Distributed Multi-tiered Applications
Client tier!
Application Client!
Web tier!
Business tier!
EIS tier!
Browser!
Facelet/JSP/
Servlet!
Enterprise Beans!
DB!
Client
machine!
J2EE Server
machine!
Enterprise Beans!
DB!
Database
Server
machine!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE containers
•  Runtime environments
–  They host application components
•  Application components
–  use their protocols and methods for interacting with other components
and platform services
–  never interact directly with other J2EE components
•  Provide the required middleware services to the
application components they host
–  A federated view of the underlying J2EE APIs
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Types of containers
•  Different containers depending on the kind of
application component they host
–  Application client container
–  Applet container
•  manages the execution of applets. Consists of a Web browser and
Java Plug-in running on the client together
–  Web container
•  manages the execution of JSP page and servlet components for
J2EE applications
–  Enterprise JavaBeans (EJB) container
•  manages the execution of enterprise beans for J2EE applications
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE components
•  A J2EE component is a functional software unit
•  Five types of components:
–  Application clients
–  Web clients
–  Java Servlet
–  JavaServer Pages (JSP)
–  JavaServer Faces (JSF)
–  Enterprise Java Beans (EJBs)
•  J2EE components written in Java and compiled in the
same way as any program in Java
•  J2EE components are assembled into a J2EE
application and deployed to production
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Application clients
•  Run on a client machine
•  Exhibit a graphical user interface (GUI)
–  Swing or Abstract Window Toolkit (AWT) APIs
–  command-line interface
•  They may directly access enterprise beans running
on the business tier
•  They may communication with a servlet running on
the Web tier by opening a HTTP connection
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Web Clients and Applets
•  Web pages (HTML,XML…) generated by Web
Components
•  Web browser
–  it renders the pages received from the server
•  Sometimes called thin clients
Applets
•  Small client applications written in Java
–  execute in the JVM installed in the Web browser
•  Necessary
–  Java Plug-in and security policy file in order to successfully
execute the applet in Web browsers
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Web Components: Servlets
•  Java objects that dynamically process requests and
construct responses
–  Serve as a HTTP-aware middle layer that resides on the
server side
•  They know how to communicate with both the clients, in HTTP, and
EJBs with Remote Method Invocation (RMI)
–  Typically take a HTTP request as input, parse its data,
perform some logic, and then construct a response to the
client
–  They are not scripts; the HTML part is embedded inside the
Java code
•  response.write(“<html> <body> Hello Guys!! </
body></html>”);
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Web Components: Servlets
•  The javax.servlet and the
javax.servlet.http provide interfaces and
classes for writing servlets
•  When implementing a generic service
–  use the GenericServlet class provided with the Java
Servlet API
•  When handling HTTP-specific services
–  use the HttpServlet class
•  provides methods such as doGet and doPost
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Servlet life cycle
•  Controlled by the Web container
•  When a request is mapped to a servlet, the container
performs the following steps
–  If an instance of the servlet does not exist
•  Loads the servlet class
•  Instantiates an instance of the servlet class
•  Initializes the servlet instance by calling the init method
–  Invokes the service method, passing a request and
response object
•  Service method can be the service method of the
GenericServlet or a doMethod (e.g., doGet, doPost) of the
HTTPServlet
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Servlet Example
public class MyServlet extends HTTPServlet {
………
public void init(ServletConfig config)
throws ServletException {
super.init(config);
System.out.println(“Servlet initialized”)
}
public void destroy() {
super.destroy();
System.out.println(“Servlet destroyed”);
}
public void doGet(HttpServletRequest req, HttpServletResponse
resp) throws ServletException, IOException {
…………
}
}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Web Components: Java Server Pages (JSP)
•  Text-based documents
–  allow developers to create Web contents with both static and
dynamic components
•  static content expressed in any text-based format (HTML, XML …)
•  dynamic content constructed by JSP elements
•  Provide a more natural approach to creating static
contents
•  HTML pages with special embedded JSP tags
–  tags can contain Java code
•  NO Java classes
–  files with .jsp extension (they are not compiled like common
Java programs)
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Java Server Pages (JSP)
•  A JSP engine parses the .jsp and creates a Java servlet
source file (translation)
–  The source file is then compiled into a class file (compilation)
•  Both the translation and compilation phases can yield
errors
–  If an error occurs while the page is being translated (e.g., if the
translator encounters a malformed JSP element)
•  The server returns a ParseException and the servlet class source
file is empty or incomplete
–  If an error occurs while the JSP page is being compiled (e.g.,
due to a syntax error in a scriptlet tag)
•  The server returns a JasperException
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JSP Example
<html>
<head>
<title> My JSP Example </title>
</head>
<body>
JSP Hello word
<br><br>
<%! public String writeThisTest() {
String testToWrite=“Hello Guys!!”;
return testToWrite;
}
%>
<br>
<%= writeThisTest() %>
</body>
</html>
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JavaServer Faces
•  Based on Facelets
–  Facelets is a powerful but lightweight page declaration
language that is used to build JavaServer Faces views
–  Facelets views are usually created as XHTML pages
•  In a typical JavaServer Faces application, each
page of the application connects to a backing bean
–  A type of managed bean
•  Annotation @ManagedBean
•  A constructor and get and set methods
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Business Components
•  The Enterprise Java Beans (EJBs) of the J2EE
specifications
•  Handle the business code
–  i.e. the logic that meets the needs of a business domain
•  Retrieve data from J2EE clients, process it and send it
to the EIS tier (and vice versa)
•  Three kinds of EJBs (in old J2EE specifications)
–  Session Beans
–  Entity Beans
–  Message Driven Beans
•  New J2EE specification
–  Entity Beans have been deprecated
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE applications
•  J2EE applications packaged in Enterprise Archive
(EAR) files
–  EAR files are standard Java Archive (JAR) files with .ear
extension
•  The EAR file contains J2EE modules
–  J2EE modules (four types)
•  one or more J2EE components for same container type
•  a component Deployment Descriptor (DD) for that type
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE modules
•  EJB module
–  contains .class files for EJBs and an EJB DD
–  EJB module packaged as JAR file (.jar extension)
•  Web module
–  contains JSP files, .class files for servlet, HTML files and a Web DD
–  Web module packaged as JAR file with .war extension
•  Resource adapter module
–  contains Java interfaces, classes and libraries and the resource adapter
DD
–  Resource adapter module packaged as JAR file (.rar extension)
•  Application client module
–  contains .class files and an application client DD
–  Application client module packaged as JAR file (.jar extension)
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Deployment Descriptors (DDs)
•  A XML document
–  describes a component’s deployment settings (e.g.
transactional information, database mapping, security roles)
•  Used to inform the container about how to manage
the bean and its life cycle
•  Can be changed without modifying the bean source
code
–  it contains declarative information
–  at run time the J2EE server reads the DD and acts upon
the component accordingly
•  Different DDs depending on the J2EE modules
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
DD example
<?xml version=“1.0” encoding=“UTF-8”?>!
<ejb-jar>!
<description>Entity Bean Example </description>!
<display-name>Entity Beans</display-name>!
<enterprise-beans>!
<!-- Entity Beans -->!
<entity>!
<display-name>Container Managed Persistence Player</displayname>!
<ejb-name>CMPPlayer</ejb-name>!
<home>entity.ejb.PlayerHome</home>!
<remote>entity.ejb.Player</remote>!
<ejb-class>entity.ejb.CMPPlayerBean</ejb-class>!
<prim-key-class>java.lang.String</prim-key-class>!
<primkey-field>playerId</primkey-field>!
<cmp-field><field-name>name</field-name></cmp-field>!
…!
</entity> …!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE middleware services
•  Robust suite of middleware services
–  the services rely on the Java 2 Standard Edition (J2SE)
Technologies included in J2EE
•  Enterprise Java Beans (EJBs)
–  Defines how server-side components are written
•  Java Servlets and Java Server Pages (JSPs) Java
ServerFaces (JSF)
–  Defines and manages servlets JSPs Facelets
•  Java Remote Method Invocation (RMI) and RMI-IIOP
–  RMI allows for inter-process communication
–  RMI-IIOP is a portable extension of RMI that uses the InternetInter-ORB Protocol (IIOP) for CORBA integration
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE middleware services
•  HTTP
–  HTTP client-side API defined in the package java.net
–  HTTP server-side API defined by servlets and JSPs interfaces
–  HTTPs: HTTP over SSL
•  Java DataBase Connectivity (JDBC)
–  It is a relational database bridge that allows developers to invoke SQL commands
from Java methods
–  Generally used by EJBs but even servlet or JSP page can use it to access the
database directly
–  Composed by two parts: an application-level interface used by applications for
accessing database and a service provider interface to attach JDBC drivers to the
J2EE platform
•  Java Message Service (JMS)
–  It allows application components to create, send, receive and read messages
–  It enables distributed communication that is loosely coupled, reliable, and
asynchronous
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE middleware services
•  Java Naming and Directory Interface (JNDI)
–  provides methods for associating attributes with objects,
searching for objects using attributes
–  independent of any specific implementations
•  JNDI used to access multiple naming and directory services (e.g.
LDAP, DNS, NIS…)
•  Java Transaction API (JTA)
–  provides methods for managing transactions (e.g. begins,
rollbacks, commits)
•  JavaMail
–  Used to send e-mail notifications
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE middleware services
•  Java API for XML Processing (JAXP)
–  supports the processing of XML documents using either
Document Object Model (DOM) or Simple API for XML
Parsing (SAX)
–  enables applications to parse and transform XML
documents
•  Java Authentication and Authorization Service
(JAAS)
–  provides a way for J2EE application to authenticate and
authorize a specific user or group of users to run it
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE middleware services
•  Web Service management
–  provides support for both web service clients and web service
endpoints
–  includes
•  Java API for XML-RPC (JAX-RPC): supports for web services calls
using the SOAP/HTTP protocol
•  SOAP with Attachments API for Java (SAAJ): provides support for
manipulating low-level SOAP messages
•  Web Services
–  defines the deployment of web services clients and web service endpoints
–  Defines the implementation of web service endpoints using enterprise
beans
•  Java API for XML Registries (JAXR): provides client access XML
registry servers (UDDI)
–  NEW: Restful web services
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE middleware services
•  J2EE Connector Architecture (JCA)
–  used by tools vendors and system integrators to create
resource adapters
•  resource adapters are software components that allow J2EE
components to access and interact with enterprise information systems,
plugged into any J2EE product
•  resource adapters are useful to manage details of middleware
integration to existing systems
•  Java IDL
–  sun Microsystem’s Java-based implementation of CORBA
–  allows for integration with other languages (J2EE fully
compatible with CORBA)
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE overall architecture
…!
Web Container!
EJB Container!
J2SE!
Java MAIL!
JNDI!
JTA!
JCA!
Application client
container!
JDBC!
J2SE!
JMX!
J2EE Server!
Java MAIL!
JNDI!
JNDI!
J2SE!
JTA!
JCA!
Web Srvcs!
JMS!
SAAJ!
JAXR!
JAXRPC!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE application example
Web tier [Servlets/
JSPs/JSF]
H
T
T
P
customer
H
T
T
P
S
browse
select
secure
purchase
J2EE Server
Business tier
[EJBs]
catalog
J
D
Book B
Shopping order C
cart
Java Mail
Mail server
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Types of EJBs
•  In the past, 3 types of EJBs
–  Session Beans
•  Associated to a client
•  Represent business interactions
–  Entity Beans
•  Persistent
•  Represent business objects
–  Message Driven Beans
•  Act as an entry points for asynchronous messages
•  No interfaces defined, only bean class
•  Today
–  Only Session Beans
–  Message Driven Bean
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Session Beans
•  Useful for describing interactions with clients
•  They do not represent shared data in the database but they can access
shared data
•  In the past two types of Session Beans:
–  Stateless Session Beans
•  They do not maintain state across method invocations
•  Examples: credit card verifier
–  Stateful Session Beans
•  Dedicated to one client for the life of the bean instance
•  Act as client agents: retain state on behalf of the client
•  Example: checking account balance
•  Today
–  Singleton Session Beans
•  Instantiated once per application and exist for the lifecycle of the application
•  Offer similar functionality to stateless session beans
–  However only one singleton session bean per application, as opposed to a pool of stateless session
beans
•  Maintain their state between client invocations but are not required to maintain their state across
server crashes or shutdowns.
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
When to use Session Beans
•  Stateful session beans are appropriate if
–  A specific interaction between the bean and a client
–  The bean needs to hold information about the client across method
invocations
–  The bean mediates between the client and the other components of the
application, presenting a simplified view to the client
•  Stateless session beans are appropriate if
–  You want to improve performance
–  The bean’s state has no data for a specific client
–  In a single method invocation, the bean performs a generic task for all
clients
–  The bean implements a web service
•  Singleton session beans are appropriate if
–  State needs to be shared across the application
–  A single enterprise bean needs to be accessed by multiple threads
concurrently
–  The bean implements a web service
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Entity Beans
•  Represent business objects in a persistent storage
–  e.g., accounts, orders, customers
•  They model data
–  object oriented view of a database
•  Have a primary key
–  Each entity bean may be retrieved in much the same manner
you search records with a primary key in a database
•  Can be shared by multiple clients
–  As they are persistent and reachable by any client
•  they suffer from all the same types of problems associated with
concurrent access of a shared resource
•  Deprecated
–  New Persistence API in J2EE 6 specifications
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Message Driven Beans (MDBs)
•  Can consume and process messages
–  designed to consume messages from topics or queues
•  Act as a JMS message listener
–  they implement only one method onMessage()
•  Process only JMS messages (EJB 2.0)
•  A message-driven bean’s instances retain no data or conversational
state for a specific client
•  All instances of a message-driven bean are equivalent
–  this allows the EJB container to assign a message to any message-driven
bean instance
•  The container can pool these instances to allow streams of messages to be processed
concurrently.
When to use them
•  To receive messages asynchronously
•  When consuming JMS messages
–  impossible to write session or entity beans that respond to JMS messages
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Persistence
•  No persistence for Stateless Session Beans and
Message Driven Beans
•  Stateful Session Beans can be passivated
–  the state is defined by Java Serialization
•  Entity Beans are persistent
–  In the past, two types of persistence
•  Bean Managed Persistence (BMP)
•  Container Managed Persistence (CMP)
•  EJBQL query language
–  Today, Java Persistent API
•  Java Persistence Query Language (JPQL)
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Entity Relationships
•  Support
–  One-to-one
•  Each entity instance is related to a single instance of another entity
–  One-to-many
•  An entity instance can be related to multiple instances of the other entities
–  Many-to-one
•  Multiple instances of an entity can be related to a single instance of the other entity
–  Many-to-many
•  The entity instances can be related to multiple instances of each other
•  There exist specific Java Annotation that can be
used to specify a relationship
–  Es: @OneToMany
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Transactions
•  Container Managed Transactions (CMT)
–  Managed automatically by the EJB Container
–  No use of API
–  In DD the beans and the methods involved in the transaction
are specified
–  Supported by all bean types
–  The container
•  maps the transactions to DB transactions
•  begins a transaction immediately before an EJB’s method starts
•  commits the transactions just before the method exits
–  Each method is associated with a single transaction. Nested or multiple
transactions are not allowed within a method
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Transactions
•  Bean Managed Transactions (BMT)
–  Managed by the bean developer
–  Supported by the Session Beans and the Message
Driven Beans only
–  Developers must decide whether to use JDBC or JTA
transactions
N.B: Use them only when necessary!!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Summary of EJB container
responsibilities
•  Design of EJBs as single-threaded components (no worry about thread
synchronization with concurrent client accesses)
–  EJB container instantiates multiple instances of components to serve concurrent
client requests
•  Advantages: no need to worry about deadlock inside application code
•  EJB container uses management methods to alert beans when middleware
events take place
Client code!
Enterprise !
Bean!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Enterprise Java Beans (EJBs)
version 3.0
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
EJB 3.0 specification
•  The container does more work and the developers
do less work compared to previous version 2.1
•  Decreases the number of programming artifacts for
developers to provide
•  Reduces the complexity of the entity bean
programming model
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
EJB 3.0: advantages
•  Fewer number of classes and interfaces
–  In old specification two interfaces were required for EJBs
•  Home interface
•  Component interface
–  In the new specification
•  POJO/POJI-based components
–  Abstract bean classes replaced with Plain Old Java Objects (POJOs)
–  Component and Home interfaces replaced with Plain Old Java Interfaces
(POJIs)
»  Interfaces are optional
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
EJB 3.0: advantages
•  Deployment descriptors are optional
–  Use of Java metadata annotations
•  Simplification of APIs for accessing bean’s
environment
–  JNDI lookups are no longer necessary
•  New persistence APIs
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Annotations
•  Metadata
–  Additional definition that can be attached to an element
within the code in order to further characterize it
–  Can be applied to methods, variables, constructors,
package declarations, and so on
–  Begin with @ sign followed by the annotation name
•  Annotation name may be followed by annotation data
–  EJB 3.0 has defined annotations for use in EJB
development
•  The specification has also defined metadata to annotate deployment
information within the code
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Deployment Descriptor: optional
•  Deployment descriptors are redundant as developers
can use annotations in lieu of them
•  Deployment metadata can be included in the EJB
code
–  At deployment time, the metadata can be used to provide
the appropriate behaviour to the bean
•  Remember: they are optional
–  The bean developer can
•  Put all the deployment information in deployment descriptors
•  Distribute deployment information across the bean class and the
deployment descriptor
•  Use only annotations
•  Deployment descriptors override annotations
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Simplifying APIs access
•  Client applications for EJB 2.1 obtain a reference to
entity and session bean objects using the JNDI name
•  EJB 3.0 clients obtain them using annotations
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
From 2.1 to 3.0: interfaces
//Remote Interface EJB 2.1 BookCatalog.java
import java.rmi.RemoteException;
import javax.ejb.EJBObject;
public interface BookCatalog
extends EJBObject
{
public String getEdition(String title)
throws RemoteException;
}
//Home Interface EJB 2.1 BookCatalogHome.java
import java.rmi.RemoteException;
import javax.ejb.EJBObject;
import javax.ejb.CreateException;
public interface BookCatalogHome
extends EJBHome
{
public BookCatalog create()
throws RemoteException,
CreateException;
}
// EJB 3.0 BookCatalog.java
import javax.ejb.Remote
@Remote
public interface BookCatalogRemote {
public String getEdition
(String title);
}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
2.1 bean class
// BookCatalogBean.java
import javax.ejb.SessionBean;
import javax.ejb.SessionContext;
public class BookCatalogBean implements SessionBean {
private SessionContext ctx;
public String getEdition(String title){
if(title.equals("Java & XML"))
return new String("2nd edition");
if(title.equals("Java and XSLT"))
return new String("1st edition");
}
public void ejbCreate(){}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext ctx) {this.ctx=ctx;}}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
3.0 Bean Class
//BookCatalogBean.java EJB 3.0 Session Bean
import javax.ejb.Stateless
@Stateless
public class BookCatalogBean implements BookCatalogRemote {
public BookCatalogBean() {} //Constructor with no args
public String getEdition(String title){
if(title.equals("Java & XML"))
return new String("2nd edition");
if(title.equals("Java and XSLT"))
return new String("1st edition");
}}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Alternatively….
//BookCatalogBean.java EJB 3.0 Session Bean
import javax.ejb.Stateless
@Stateless
@Remote
public class BookCatalogBean {
public String getEdition(String title){
if(title.equals("Java & XML"))
return new String("2nd edition");
if(title.equals("Java and XSLT"))
return new String("1st edition");
}
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Dipartimento di Informatica e Sistemistica
2.1 deployment descriptor
<?xml version="1.0"?>
<!DOCTYPE ejb-jar PUBLIC
'-//Sun Microsystems, Inc.//DTD Enterprise JavaBeans 2.0//EN'
'http://java.sun.com/dtd/ejb-jar_2_0.dtd'>
<ejb-jar>
<enterprise-beans>
<session>
<ejb-name>BookCatalogEJB</ejb-name>
<jndi-name>ejb/BookCatalog</jndi-name>
<home>BookCatalogHome</home>
<remote>BookCatalog</remote>
<ejb-class>BookCatalogBean</ejb-class>
<session-type>Stateless</session-type>
<transaction-type>Container</transaction-type>
</session>
</enterprise-beans>
</ejb-jar>
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Dipartimento di Informatica e Sistemistica
3.0 deployment descriptor
•  I do not need it!
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Dipartimento di Informatica e Sistemistica
Client accessing 2.1 session bean
import javax.naming.InitialContext;
public class BookCatalogClient {
public static void main(String[] argv){
try {
InitialContext ctx=new InitialContext();
BookCatalogHome catalogHome =
(BookCatalogHome) ctx.lookup(”ejb/BookCatalog");
BookCatalog catalog =
(BookCatalog) catalogLocalHome.create();
String title="Java and XML";
String edition = catalogLocal.getEdition(title);
System.out.println("Edition for Title: ” + title + " " + edition);
}catch(Exception e){}}}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Client accessing 3.0 session bean
public class BookCatalogClient {
@Inject BookCatalogBean;
BookCatalogBean catalogBean;
String title="Java and XML";
String edition=catalogBean.getEdition(edition);
System.out.println("Edition for Title: "
+ title + " " + edition);
}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Other types of injections
•  @EJB
–  Used to inject in particular EJB stubs
–  @EJB public BookCatalogBean catalogBean;
•  @Resource
–  Used to inject dependencies on resources in general
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
2.1 entity bean
import javax.ejb.*;
public abstract class BookBean implements EntityBean {
private String title;
private String author;
public
public
public
public
abstract
abstract
abstract
abstract
String getTitle(){return title;}
void setTitle(String title){this.title=title;}
void setAuthor(String author){this.author=author;}
String getAuthor(){return author;}}
public String ejbCreate(String title,String author)
throws javax.ejb.CreateException {
setTitle(title);
setAuthor(author);
return null;
}
//other ejb life cycle methods
}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Accessing entity instances in EJB 3.0
•  javax.persistence.EntityManager API is used
for creating, finding, updating and deleting entity
instances
•  You can inject an instance of EntityManager in a
session bean and use persist or find method to
create or query entity bean objects
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
3.0 Entity
import javax.persistence.*;
@Entity @Table(name=“BOOK”)
@NamedQuery(name="findByAuthor", queryString = "SELECT DISTINCT OBJECT(obj) FROM Book
obj WHERE obj.author = ?1")
public class BookBean implements java.io.Serializable {
private String title;
private String author;
public BookBean(){}
public BookBean(String title){this.title=title; }
@Id @Column(name=“title”)
(use @GeneratedValue if you want the container to manage the auto increment of
primary keys)
public String getTitle(){return title;}
public void setTitle(){this.title=title;}
@Column(name=“author”)
public void setAuthor(String author){this.author=author;}
public String getAuthor(){return author;}
}
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Accessing entity instances in EJB 3.0:
an example
…
@PersistenceContext
private EntityManager em;
private BookBean bb;
public BookBean findByPrimaryKey(String title) {
return ((BookBean) em.find(BookBean.class,title));
}
public BookBean finderAuthor (String auth) {
return (BookBean)
em.createNamedQuery(“findByAuthor”)
.setParameter(1, auth).getSingleResult();
}
public void createBook(String title, String author) {
if (bb == null) bb = new BookBean();
bb.setTitle(title);
bb.setAuthor(author);
em.persist(bb);
}…
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Persistence deployment descriptor
•  It is called persistence.xml
•  It is included in the META-INF directory
•  It looks like
<persistence>
<persistence-unit name="manager1">
<jta-data-source>java:/DefaultDS</jta-data-source>
<provider>org.hibernate.ejb.HibernatePersistence< /provider>
<properties>
<property name="hibernate.dialect”
value="org.hibernate.dialect.MySQLDialect"/>
</properties>
</persistence-unit>
</persistence>
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
J2EE application servers
•  Middleware platforms that implement the J2EE specifications
–  Open source
•  JBoss (JBoss group)
•  JOnAS (ObjectWeb consortium)
•  Geronimo (Apache Software Foundation)
–  …
–  Commercial
•  BEA Web Logic
•  IBM WebSphere
–  …
–  Other component-based technologies
•  Corba Component Model (CCM)
•  Microsoft .Net
•  The most popular open-source application server is JBoss
–  Many versions available
• 
• 
• 
• 
• 
3.x
4.x
5.x
6.0
7.0 Beta
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JBoss 4.x Application Server
•  Collection of middleware services for
communication, persistence, transactions,
security, and clustering
•  Services interoperate by means of a microkernel,
i.e., Java Management eXtension (JMX)
–  JMX provides Java developers with a common
software bus
•  Allows them to integrate components (e.g., modules,
containers)
–  Components are declared as Managed Beans
(MBeans) services
•  Implementation of manageable resources in JBoss
•  Represented by Java objects that expose interfaces
•  Consisting of methods to be used for invoking the MBeans
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JBoss 5.x Application Server
•  It uses the microcontainer
–  It is a refactoring of JBoss's JMX Microkernel to support direct
Plain Old Java Object (POJO) deployment and standalone use
outside the JBoss application server
–  It allows the application server to integrate enterprise services with
a Servlet/JSP and EJB containers, deployers and management
utilities in order to provide a standard Java EE environment
•  If you need additional services
–  simply deploy these on top of Java EE to provide the functionality you need
–  Since it is very lightweight and deals with POJOs it can be also
used to deploy services into a Java ME runtime environment
•  New possibilities for mobile applications to take advantage of enterprise
services without requiring a full J2EE application server
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Dipartimento di Informatica e Sistemistica
JBoss 5.x Application Server
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Dipartimento di Informatica e Sistemistica
JBoss Installation
•  You should have already installed java J2DK (latest
versions)
•  Set the JAVA_HOME to point where java is installed
•  Download the JBoss Application server
–  You can download the src code and build it
•  jboss-XXX-src.tar.gz
–  You can download the software already built (unzip the file)
•  jboss-XXX.zip
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
How to run and shutdown JBoss
Run
•  cd jboss-XXX/bin
–  ./run.sh (under Linux) or run.bat (under Windows)
•  You are running the default server
•  There exist three server confs:
–  Minimal
•  starts the core server container without any of the enterprise services
–  Default
•  all the basic services of the J2EE specifications
–  All
•  default conf + IIOP + clustering and load balancing services
•  New two configurations since JBoss AS 5
–  Standard
•  Deployment isolation properties, specific juddi implementation, …
–  Web
•  New experimental lightweight configuration created around JBoss Web
Shutdown
•  You can terminate the server by using ctrl-c
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Dipartimento di Informatica e Sistemistica
JBoss Clustering Service
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Dipartimento di Informatica e Sistemistica
Why Clustering?
•  It allows developers to run an application on several parallel
servers while providing a single view to application clients
•  It is crucial for
–  Fault tolerance
•  If one or more servers fails, the application is still accessible via the
surviving servers
–  Load Balancing
•  Load is distributed across different servers
–  Scalability
•  Performance can be improved by simply adding more nodes to the cluster
and performing load balancing
–  Highly availability
•  The clustering infrastructure supports the redundancy needed for high
availability
•  It is obtained through load balancing and fault tolerance as well
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Why JBoss Clustering?
•  Transparent
–  No stub re-compiling!
–  Client does not know about clustering
–  Cluster is maintained automatically
–  Modular/stack approach
•  Open source
–  No extra cost to activate clustering
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Overview of Clustering functionalities
R1
R3
Failover
Load
Balancing
R3
R2
R1
R3
R2
Public
network
Private
network
State Replication
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Running JBoss Clustering
•  Use JBoss’ “all” configuration
–  run.bat -c all
–  ./run.sh -c all
•  The “all” configuration contains everything needed
for clustering
–  It has all the libraries for clustering
•  E.g., JGroups.jar, etc.
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Modular/stack approach
JBossCache
HA-Remoting
Farming
Messaging
Hibernate
Distributed
State
jBpm
Entity
SFSB
HTTP Session
HA-JNDI
Distributed
RPC,
Membership
HASingleton
Distributed Replicant
DRM
Manager
JGroups
HAPartition
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering divided in 3 parts
•  Cluster communication (inside cluster)
–  Based on JGroups
–  Method calls across all cluster nodes
•  HA-JNDI, Farming, etc…
•  Client-cluster communication
–  Formerly HA-RMI
–  Load balancing and failover logic for fat clients
•  State replication (inside cluster)
–  JBoss Cache
•  HTTP session, SFSBs, DistributedState
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering divided in 3 parts
•  Cluster communication (inside cluster)
–  Based on JGroups
–  Method calls across all cluster nodes
•  HA-JNDI, Farming, etc…
•  Client-cluster communication
–  Formerly HA-RMI
–  Load balancing and failover logic for fat clients
•  State replication (inside cluster)
–  JBoss Cache
•  HTTP session, SFSBs, DistributedState
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Cluster definition
•  A JBoss cluster (or partition) consists of a set of
nodes
–  Nodes communicate with each other and work toward a
common goal
–  A node in JBoss is a JBoss server instance
–  More than one JBoss server instance can be deployed in
one single machine
•  Pay attention to the service ports: they must be changed!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JGroups Channels
•  Communication between nodes is handled by the JGroups group
communication library
•  Two main components
–  Channel
–  Protocol stack
•  A JGroups Channel
–  Tracks who is in the cluster
–  Enables reliable exchanging of messages between the cluster members
–  JGroups channels with the same configuration and name dynamically
discover each other and form a group
–  Nodes can be dynamically added to or removed from clusters at any time
•  by starting or stopping a Channel with a configuration and name that matches the
other cluster members
•  In essence, a JBoss cluster is a set of AS server instances each
of which is running an identically configured and named JGroups
Channel
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JGroups Channels
•  On a same network,
even for the same
service, we may
have different
clusters
–  In order to
differentiate them,
each cluster must
have an individual
name
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JGroups Configuration
•  It is built on top of a stack of
network communication protocols
–  Protocols provide transport,
discovery, reliability and failure
detection, and cluster membership
management services
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JGroups Configuration in old JBoss versions
•  cluster-service.xml file in /deploy folder
–  Describes configuration for the default cluster partition
•  JGroups configurations appear as a nested attribute in
cluster related MBean services
•  PartitionConfig attribute in the
ClusterPartition MBean is an XML string
–  Describes and configures the JGroups stack of protocols
–  All the JGroups configuration data is contained in the
<Config> element under the PartitionConfig attribute
–  Default configuration uses UDP with IP multicast
•  multicast identified by address and port number
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Dipartimento di Informatica e Sistemistica
JGroups Configuration in old JBoss versions
<mbean code="org.jboss.ha.framework.server.ClusterPartition" name="jboss:service=
{jboss.partition.name:DefaultPartition}">
...
<attribute name="PartitionConfig">
<Config>
<UDP mcast_addr="${jboss.partition.udpGroup:228.1.2.3}"
mcast_port="${jboss.hapartition.mcast_port:45566}”
tos="8”
… …
<PING timeout="2000"
down_thread="false" up_thread="false"
num_initial_members="3"/>
<MERGE2 max_interval="100000”
down_thread="false" up_thread="false"
min_interval="20000"/>
……
<FD timeout="10000" max_tries="5"
down_thread="false" up_thread="false" shun="true"/>
<VERIFY_SUSPECT timeout="1500" down_thread="false"
up_thread="false"/>
………
<pbcast.STATE_TRANSFER down_thread="false" up_thread="false"/>
… …
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Dipartimento di Informatica e Sistemistica
JGroups configurations in new JBoss version
•  Cluster directory in /deploy folder
–  Describes configuration for the default cluster partition
–  It includes jgroups-channelfactory.sar which in
turn embodies the xml configuration files of JGroups in
the META-INF directory
•  jgroups-channelfactory-stacks.xml
–  with the JGroups stack configurations
•  jgroups-channelfactory-jboss-beans.xml
–  Defines the bean JChannelFactory and uses the previous file for
the JGroups stack configuration
–  It includes the address used to determine the name node
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JGroups Channels
•  By default JBoss uses four separate JGroups
Channels
–  Two broad categories
•  The Channel used by the general purpose HAPartition service
•  Three Channels created by JBoss Cache for special purpose
caching and cluster wide state replication
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Dipartimento di Informatica e Sistemistica
HA Partition
•  HAPartition is a general purpose service used for a variety of
tasks in AS clustering
•  It is an abstraction built on top of a JGroups Channel
•  It provides support for making/receiving RPC invocations on/from
one or more cluster nodes
•  It supports a distributed registry of which clustering services are
running on which cluster members
•  It provides notifications to interested listeners when the cluster
membership changes or the clustered service registry changes
•  It is the core of many clustering services
– 
– 
– 
– 
– 
smart client-side clustered proxies
EJB 2 SFSB replication and entity cache management
Farming,
HA-JNDI,
…
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
HA Partition: example
<mbean code="org.jboss.ha.framework.server.ClusterPartition"
name="jboss:service=DefaultPartition">
<! -- Name of the partition being built -->
<attribute name="PartitionName">
${jboss.partition.name:DefaultPartition}</attribute>
<! -- The address used to determine the node name -->
<attribute name="NodeAddress">${jboss.bind.address}</attribute>
<! -- Determine if deadlock detection is enabled -->
<attribute name="DeadlockDetection">False</attribute>
<! -- Max time (in ms) to wait for state transfer to complete.
Increase for large states -->
<attribute name="StateTransferTimeout">30000</attribute>
<! -- The JGroups protocol configuration -->
<attribute name="PartitionConfig”>... ...
</attribute>
</mbean>
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
HA Partition
•  In order for nodes to form a cluster, they must
have
–  the exact same PartitionName and the
PartitionConfig elements
•  Changes to these elements on some nodes of the
cluster but not all nodes would cause the cluster
to split
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Types of deployments
•  JBoss’ staff recommends use of homogeneous
deployment
–  Each application component replicated in all clustered
nodes
•  Requests do not need to span on different nodes
•  Potentially, heterogeneous deployment is also
allowed
–  Application components can span on different nodes
•  Not recommended for lack of distributed transaction manager for
example in old JBoss versions
•  In new JBoss version a distributed transaction manager has been
introduced
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Homogeneous deployment
•  Realized using JBoss farming service
–  application copied into JBoss /farm directory
Node2
Node1
Copy .ear in /farm
ear
Node3
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Dipartimento di Informatica e Sistemistica
Clustering JNDI
•  The client obtains a HA-JNDI proxy object and
invokes JNDI lookup services on the remote
server through the proxy
•  On the server side, the HA-JNDI service
maintains a cluster-wide context tree
–  The tree is always available as long as there is one
node left in the cluster
–  Each node in the cluster also maintains its own local
JNDI context tree
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering JNDI
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering JNDI
•  A remote client does a lookup through HA-JNDI
–  If the binding is available in the cluster-wide JNDI tree,
return it
–  If the binding is not in the cluster-wide tree, delegate
the lookup query to the local JNDI service and return
the received answer if available
–  If not available, the HA-JNDI service asks all other
nodes in the cluster if their local JNDI services own
such a binding
–  If no local JNDI service owns such a binding, a
NameNotFoundException is finally raised
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering divided in 3 parts
•  Cluster communication (inside cluster)
–  Based on JGroups
–  Method calls across all cluster nodes
•  HA-JNDI, Farming, etc…
•  Client-cluster communication
–  Formerly HA-RMI
–  Load balancing and failover logic for fat clients
•  State replication (inside cluster)
–  JBoss Cache
•  HTTP session, SFSBs, DistributedState
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JBoss Clustering Configurations
•  Two types of clustering configurations
–  JBoss fat client clustering
–  JBoss thin client clustering
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Dipartimento di Informatica e Sistemistica
JBoss Fat Client Clustering
•  Client uses smart proxies
•  HA smart proxy has logic inside
–  Load Balancing policies, e.g., RoundRobin
–  Failover capabilities
•  HA smart proxy logic can be pluggable
–  User can define his/her own logic
•  Failover policy
•  Load balancing based on weight or load
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JBoss Thin Client Clustering
•  Front end load balancing mechanism
–  Hardware
•  JBoss group suggests F5 BIG-IP
–  Software
•  Apache + JBossWeb
–  Apache mod_jk/mod_proxy
–  mod_cluster
»  This latter fully integrated in JBoss AS6
–  Tomcat
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Fail-over and load balancing: RMI
•  RMI fail-over and load balancing (fat client clustering)
–  Clustering logic included into the client stubs (HA-RMI)
–  Stub contains the list of available clustered nodes + load balancing
policy
•  Random
•  Round Robin
•  First Available
–  If cluster topology changes, next client invocation server piggybacks
new list of target nodes
•  Works only on remote invocations
–  The list of target nodes is maintained automatically, using JGroups
–  The client stub
• 
• 
• 
• 
receives a reply from the invoked server
unpacks the list of target nodes from that reply
updates the current list of target nodes with the received one
terminates the client RMI
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Dipartimento di Informatica e Sistemistica
Fail-over and Load Balancing: RMI
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Dipartimento di Informatica e Sistemistica
HA RMI: Interceptors
•  When the stub’s interface is invoked, the invocation is
translated from a typed call to a de-typed call
•  The de-typed invocation is passed through a set of clientside interceptors
•  The load balancing and failover mechanisms are located
in the last interceptor of the chain
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Fail-over and load balancing: RMI
WEB Container
EJB Containers
JVM
JBoss
JVM
RMI fail-over and load balancing are possible!!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Fail-over and load balancing: RMI
WEB Container
EJB Containers
JBoss
JVM
RMI Fail-over and load balancing do not apply!!!
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering divided in 3 parts
•  Cluster communication (inside cluster)
–  Based on JGroups
–  Method calls across all cluster nodes
•  HA-JNDI, Farming, etc…
•  Client-cluster communication
–  Formerly HA-RMI
–  Load balancing and failover logic for fat clients
•  State replication (inside cluster)
–  JBoss Cache
•  HTTP session, SFSBs, DistributedState
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
JBoss Cache
•  JBoss Cache is a fully featured distributed cache framework that
can be used standalone or in any application server environment
–  It caches frequently accessed Java objects in order to dramatically improve
the performance of applications
•  it is easy to remove data access bottlenecks - such as connecting to a database
•  In JBoss, it provides cache services for
– 
– 
– 
– 
HTTP sessions
EJB 3.0 session beans
EJB 3.0 entity beans
Each cache service creates its own JGroups Channel
•  JBoss Cache is a cluster-aware cache; that is, state is always kept
in sync with other servers in the cluster when there are concurrent
updates
–  Any state stored in JBoss Cache is resilient to server crashes or restarts
–  It can either invalidate or update its state
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Fail-over and Load Balancing: HTTP
•  Session state replication handled by JBoss
–  in “all” JBoss conf included files for session state replication
–  configure HTTP session replication with JBoss Cache
•  Load balancing handled by external software or hardware
components (thin client clustering)
–  typically use software components such as Apache+mod_jk
–  mod_cluster
•  Compared with the former mod_jk, mod_cluster accepts a dynamic
configuration of httpd workers
–  This can be done through an advertise mechanism where all httpd workers
communicate lifecycle events (like startup or shutdown) thus leveraging
dynamic configuration of nodes
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Dipartimento di Informatica e Sistemistica
Fail-over and Load Balancing: HTTP
WEB
Container
EJB
Containers
WEB
Container
EJB
Containers
HTTP State
replication
JBoss
JBoss
JVM
JVM
Apache mod_jk
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Alternatively….
•  If you use sticky sessions you do not need HTTP session replication
•  All requests coming from the same client are sent to the same node
•  In the example round robin policy is applied per-client
JBoss
Node 2
JBoss
Node 1
R1
R2
R2
JBoss
Node 3
R1
Apache mod_jk
R1
R1
R2
Client 1
R2
Client 2
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Dipartimento di Informatica e Sistemistica
Sticky Session: Pros and Cons
•  Pros
–  You may disable HTTP session replication that is an
expensive operation
•  All requests from the same clients are processed by the same server
•  Cons
–  If server that serves the requests fails, you loose your session
state
•  It is a tradeoff and depends on the type of application
you are deploying
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Hybrid solution: Sticky session and HTTP
state replication
•  You may enable both sticky session and HTTP session replication
–  In case of server crash the client session is replicated in the remaining
servers
•  Asynchronous vs synchronous HTTP replication
–  Synchronous: the application has to wait for the response until the state is
replicated to all nodes of the cluster
–  In case you use sticky session, it may be better enabling an asynchronous
HTTP replication that is more scalable
•  The application receives immediately the response and then the state is replicated to
the nodes of the cluster
•  You can also specify the granularity of the replication for optimization
purposes
•  Entire session
•  Modified attributes
–  More efficient than session granularity
•  Modified field
–  If size of the attribute is huge, replication of modified field is efficient
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
HTTP session replication: configuration
•  Session replication is enabled per web application in
WEB-INF/web.xml
–  Add <distributable/> to your webapp
•  JBoss specific configurations (asynchronous vs
synchronous replication) goes into WEB-INF/
jboss-web.xml
•  Session replication is configured in Tomcat
deploy/tc5-cluster-service.xml
...
<!-- Valid modes are LOCAL, REPL_ASYNC and REPL_SYNC ! -->
<attribute name="CacheMode">REPL_SYNC</attribute>
...
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Hybrid: Thin and Fat Client Clustering
WEB
Container
EJB
Containers
JBoss
Apache
mod_jk
JVM
WEB
Container
JVM
EJB
Containers
JBoss
JVM
JVM
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering EJBs
•  In general, you have to annotate your application in
order to instruct the EJB container that the bean is
clustered
•  Stateless Session Beans
–  As no state involved, calls can be load balanced on any node
•  Stateful Session Beans
–  It is necessary to manage the state
•  The states of all stateful session beans are replicated and
synchronized across the cluster
–  Each time the state of a bean changes
•  JBoss Cache provides the session state replication for EJB 3.0
stateful session beans
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Clustering EJBs
•  Entity Beans
–  2.x: do not use clustering
•  Introduces data synchronization problems
–  they do not have a distributed locking mechanism and a distributed cache
–  3.0
•  Entity bean clustering service deals with distributed caching and
replication
–  Through JBoss Cache
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
Troubles
•  If you are not able to successfully run JBoss
clustering, it does not mean that it does not work
!
–  Network issues (firewall, multicast disabled, etc.)
–  JGroups issues
•  JGroups comes with a handy set of Trouble Shooting tools
Università di Roma “La Sapienza”
Dipartimento di Informatica e Sistemistica
References
•  Java EE 6 Tutorial available at http://download.oracle.com/
javaee/6/tutorial/doc/
•  Bela Ban, “JBoss Clustering Overview”, 2006
•  JBoss Application Server 4.x, Chapter 16. Clustering, “High
Availability Enterprise Services via JBoss Clusters”,
available online at
–  http://docs.jboss.org/jbossas/jboss4guide/r4/html/cluster.chapt.html
•  Brian Stansberry, Galder Zamarreno JBoss Application
Server 5, “Clustering Guide”, November 2008
•  Any other problem?
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