A DAPT IST-2001-37126 Replication Framework: Progress Report Vance Maverick University of Bologna...

ADAPT IST-2001-37126

Replication Framework:Progress Report

Vance Maverick

University of Bologna

Dec. 11, 2003

ADAPT Bologna meeting, Dec. 20032 ADAPT

Outline

• Framework recap• EJB implementation• Web-service implementation• Transaction interception• Deployment support• Some open issues• Summary


Framework recap

• Generic API for interface between standard J2EE server and replication algorithm– Allow different replication algorithms to be plugged in

– Protect replication development from upgrades in the server

• Circulated paper designs from May onwards• Presented at Brussels meeting


ComponentMonitor API

• Approach: module barriers between J2EE container and replication algorithm

• Replication code implements interface ComponentMonitor

• Container provides services to replication code through interfaces– ContainerSupport provides top-level container services

– ComponentHandle stands for component


Intercepting invocations

ComponentMonitor

Handle

?

call()

call()

• Container diverts invocation to ComponentMonitor• Response call(ComponentHandle, Request)

• CM may perform arbitrary computation + communication

• To continue execution, it forwards call to ComponentHandle

InterceptedNormal


Communicating component state

ComponentMonitor

Handle

ComponentMonitor

Handle

Handle

State

StateState

• ComponentMonitor does not reference component directly

• Instead, uses serializable ComponentHandle

• Gets and sets state as opaque serializable “blob” (bytes)


EJB implementation of API

• Types of EJBs– Entity bean

– Stateless session bean

– Stateful session bean

• ComponentHandle contains– JNDI name of bean

– Session ID (for SFSB)

– Primary key object (for entity bean)

• Intercept persistence methods for entity bean


EJB Invocation Interception

Persistence Manager

EJB Container

Interceptors

EJBEJB

EJB


EJB Invocation Interception

Persistence Manager

EJB Container

Interceptors

EJBEJB

EJB

Component Monitor

Call Management

Persistence Management


EJB Persistence Invocation Interception

Persistence Manager

EJB Container

Interceptors

EJBEJB

EJB


EJB Persistence Invocation Interception

Persistence Manager

EJB Container

Interceptors

EJBEJB

EJB

Component Monitor

Call Management

Persistence Management

Wrapper


Axis web service objects

• Implement a web service (port type)• WSDL2Java tool

– Generates Java interface from WSDL definition

– Developer creates object as implementation of interface

• Deployed in Axis configuration file– “Scope”: instance per request, per client session, or per

server

– Request scope is stateless, others stateful


Web service implementation of API

• ComponentHandle– Service name

– Session ID (when session scope)• We generate this

• State transmission– Serialized form of web service object

• Automatic translation of EJB references

– When deserialized, new object replaces old


Handlers in Axis

• Handler model defined by Sun (JAX-RPC)

handleRequest()

handleResponse()

handleRequest()

handleResponse()

ServiceObject

HTTP


handleRequest()

handleResponse()

handleRequest()

handleResponse()

ServiceObject

static call()

call() call()

ComponentMonitor ComponentHandle

Proxy

AXIS

TxHandler WSMon Handler

“Normal”Axis chainHandlersdisabled

ComponentInterception


Container

Transactions

EJB

TransactionManager

PersistenceManager

DataSource(XAResource)

Read,Write

begin,commit

prepare, commit, rollback


Container

Transactions

TransactionManager

PersistenceManager


Read,Write

begin,commit


TransactionInterceptor

begin,commit

EJB


Container

Transactions

PersistenceManager


Read,Write

begin,commit


TransactionInterceptor

EJB


Remote Deployment (Farming)

Component Monitor

Container Support

Component Monitor

Container Support



Component Monitor

Container Support

Component Monitor

Container Support

Notification



Component Monitor

Container Support

Component Monitor

Container Support

Transmission



Component Monitor

Container Support

Component Monitor

Container Support

Deployment


Open issues I: Persistence

• Persistent creation vs. instantiation of existing object– Requires clarification in API

• So far, test applications based on entity beans, CMP• BMP should be equivalent

– ejbStore() and ejbLoad() have same timing, semantics

• In practice, developers don’t rely entirely on entity beans– Considered inefficient


Persistence through SFSBs

• Consider stateful session bean that accesses DB– Each method invocation may read and modify DB as well as

bean state

• Replication algorithm should wait till commit before synchronizing state on other replicas

• Bean should not access other state outside beans, DB– Standard development guideline

• With these cautions, we believe mechanism is adequate


Open issues II: Client-side invocation

ComponentMonitor

Stub

ComponentMonitor

Stub

ComponentMonitorComponentMonitor

Cluster 1

Cluster 2


What stub could do

• Select target host for invocation• Select means of communication

– Group communication instead of RMI

• Add information to request and response– Session ID

– List of alternate hosts

• Applies to J2EE invocation more than web services– Stub code is downloaded from server


Open Issues III: Message-Driven Beans

• Natural base for asynchronous service implementation • Stateless component with one method

– void onMessage(javax.jms.Message)

• Variant of existing invocation model– Response is posted to a JMS Session (not return value from

invocation)


Summary

• API has stabilized– Some loose ends to be tied up

– Core methods work correctly

• Demonstrated in “toy” replication algorithms• Replication algorithm development can begin

A DAPT IST-2001-37126 Replication Framework: Progress Report Vance Maverick University of Bologna...

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