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Aspects in Middleware

1: Adrian Coyler, Andrew Clement2: Charles Zhang, Hans-Arno Jacobsen

Presented by: Itay Maman

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Scope (1/2)

Middleware platforms offer great aspect opportunities Many policies Many applications from different vendors Must be highly customizable

On the other hand, there are many obstacles Customers are reluctant to switch to a new

platform Complexity of the code/System

=> New bugs due to aspectization

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Scope (2/2)

Our focus: lessons learnt while refactoring existing middleware products Original language: Java Aspect technology: AspectJ What makes middleware good AOP candidates? Refactoring process Discussion of benefits

We will NOT talk about developing an AOP-based middleware from scratch

Complex terminology Middleware, Three-tier application,

Components, ...

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Three-tier application

Three-tier application:An application that is organized into three

major distributed parts:User interface (client program),

Functional processing (business rules),and Data storage (Database)

A Common architecture for Enterprise applications E.g.: Banks, Hospitals, Phone companies, …

May get complex More than one database A Data storage may be a separate three-tier application itself Multiple types of clients Redundancy

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Example: A Three-tier application

Browser

Database

Server

HTTP

JDBC

First Tier Second Tier Third Tier

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The middle tier

The middle tier is the heart of the system The design of the other tiers is complex but

almost mechanical Includes most of the domain-specific

programming: Algorithms & calculations Decisions Behavior

Usually: Encapsulation using components (See next slide)

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Components

Components:

A reusable program building block that can be combined with other components in the same or other computers to form an application

Examples: Enterprise Java Beans, DCOM EJBs

Managed by a J2EE server Two classifications:

Session Bean/Entity Bean Stateless/Stateful

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Welcome to the world of J2EE

BrowserJSP

Accounts EJBDatabase

J2EE Server

HTTP

RMI

JDBC

First Tier Second Tier Third Tier

Servlet

Employees EJB

RMI

JDBC

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Middleware, Application Server (1/2)

Middleware:Software that mediates between two separate and often already existing

programs

Categories of middleware: Infrastructure middleware Distribution middleware Common middleware services

AKA: Application Server Domain Specific middleware

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Middleware, Application Server (2/2)

Application Server: A server program (typically running on

a dedicated computer) that is dedicated for running certain software

applications

The two terms describe different levels of the same concept An application server is just a sophisticated type

of middleware We will use both terms interchangeably

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Inspection of middleware needs

Two approaches1) Analytic: What is so unique in middleware

platforms?2) Programmatic: What is needed in a

typical component code?

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Analytic approach (1/3)

Let’s compare Application Servers (AS) with Operating Systems (OS).

It turns out that both are platforms, which:a) Launch applicationsb) Supply core services to these applicationsc) Let programmers focus on domain specific

tasks

On the other hand, there are some key differences…

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Analytic approach (2/3)

Application Server (AS) vs. Operating System (OS): Variability of applications

A single AS invokes a specific set of applications A desktop computer launches all sorts of programs

Interconnectivity Usually, components in an AS communicate with

each other In an OS most applications are stand-alone

Money invested A customer is willing to pay to customize his AS

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Analytic approach (3/3)

Summary of differences An AS is more coherent An AS must be highly customizable

“stop-install-reboot” is usually not an option

The solution: Aspects

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Programmatic approach (1/3)

Forces in Enterprise applications Availability/Fail Safety/Recoverability Multiplicity Transaction Management Scalability Concurrency Security Simplicity of the algorithms Backwards compatibility Heterogeneity

Of platforms (HW, OS), languages, tools

Non functional requirements (?!?)

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Programmatic approach (2/3)

Standard J2EE services Dynamic web pages: JSPs, Servlets Components: EJBs Naming: JNDI Messaging: JMS E-Mailing Transactions: JTA Authentication Imported from J2SE:

Remoting: RMI DB connectivity: JDBC XML

Other services may be provided by a specific implementations

Not part of the J2EE standard

Functional requirements (?!?)

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Programmatic approach (3/3)

What is left for the programmer to do? 1) Invoke J2EE services

Lookup,iterate,put,get,remove,send,receive

2) Write domain specific algorithms/behavior Which are typically not complicated

3) Address various concerns concerns which correspond to the non functional

requirements - or - “classical” aspects: Logging, error handling, …

The 3rd task (“Address various concerns”) is the most complex one

Due to its cross cutting nature Due to lack of support from the J2EE platform

The solution (again): Aspects

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Classification of concerns in middleware

Conformance to a policy (homogenous) “When to log?”, “When to release a resource?”, … Homogenous: Same treatment in all locations

Scattered behavior (heterogeneous) User authentication,Database connectivity, … Heterogenous: Treatment varies

Tier cutting concerns Compression, Encryption, … Very interesting, but out of this lecture’s scope

Relevant to: middleware code, components code

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“Large scale AOSD for Middleware”

Work of Coyler, Clement (IBM)

Used the “Websphere” application server An IBM product J2EE complaint

The idea: Identify cross cutting concerns Refactor them into aspects Compare the aspectized system with the

original

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Homogenous concerns (1/2)

The WebSphere source code should conform to several predefined standards (policies)

Each policy is defined by a specification document

Three such policies were investigated Tracing and logging First Failure Data Capture (FFDC) Monitoring and statistics

Each policy creates a homogenous concern

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Homogenous concerns (2/2)

The investigation process Encapsulate each policy in an abstract aspect

Defines how the policy is to be applied Write a concrete sub-aspect for each component

Provides concrete specification of pointcuts Weave the advices into the program Compare the augmented program with the original

(No details were given about the comparison process)

Results No numerical data is reported The authors claim that many locations were found

where the policy was not followed by the original program

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Heterogeneous concerns (1/6)

The motivation: Decompose a certain service (feature) off the middleware Find the code which is part of the service Place the code in dedicated classes/aspects Use AspectJ to build two different flavors of the

system: Feature turned on Feature turned off

The problem: How to find all pieces of code which implement a feature? (see next slide)

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Heterogeneous concerns (2/6)

(The problem: How to find all pieces of code which implement a feature?)

The solution: An iterative semi-manual algorithm

[1] Choose an initial set of classes: S (These classes are known to be part of the feature)

[2] for each code site that calls methods of classes in S:

[3] If the site takes part in the implementation of the feature, add its class to S (Move the site to a new class if needed)

[4] Go back to [2] if S has changed in the last pass

The challenge: Automation of step [2]

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Heterogeneous concerns (3/6)

Q: What’s the easiest way to implement step [2]?

public aspect EJBSeparation { pointcut inEjb() { within(T1) || within(T2) || ...; }

pointcut ejbCall() { call(* T1.*(..)) || call(* T2.*(..)) || ...; }

declare warning : ejbCall() && !inEjb() : "Link to EJB Support found";}

A: AspectJ! This aspect finds all location which use the EJB service

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Heterogeneous concerns (4/6)

The process: Run Query (Aspect) Inspect results (warnings) Refactor Add classes to pointcut definitions

public aspect EJBSeparation { pointcut inEjb() { within(T1) || within(T2) || ...; }

pointcut ejbCall() { call(* T1.*(..)) || call(* T2.*(..)) || ...; }

declare warning : ejbCall() && !inEjb() : "Link to EJB Support found";}

AnalyzeQueryReport

Alter EJB Support pointcut definition

Run Query

RefactorComponent

AnalyzeQueryReport

Alter EJB Support pointcut definition

Run Query

RefactorComponent

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Refactoring (of EJB support)

Original code

public class A extends B {

public void init() { one(); add(); two(); }

void add() { }}

public aspect EJBAspect1 { EJBContainer ejbc;

pointcut registration(A ci) : execution(* add()) && this(ci);

before(A ci) : registration(ci) { ejbc = newContainer(); if(ejbc != null) register(ejbc) }}

public class A extends B { private EJBContainer ejbc; public void init() { one(); ejbc = newContainer(); if(ejbc != null) register(ejbc) two(); }}

Refactored code

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Heterogeneous concerns (5/6)

The concern which was factored out: EJB support

Results The two flavors passed all J2EE conformance

tests Except for EJB related tests in the “off” flavor

Slight improvements when EJB support is off Performance Footprint Startup time

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Heterogeneous concerns (6/6)

Why was EJB support chosen? This is one of the “heaviest” services offered by

WebSphere. As such, we would expect one to choose a

simpler feature for this type of research A speculation:

EJB support is one of the primary functional requirements of a J2EE server

=> The system was designed “around” this service

=> The System’s primary decomposition does not cut the EJB service

=> It is relatively easy to refactor it

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“Quantifying Aspects…” (1/3)

Work of Zhang, Jacobsen (University of Toronto) Full name: “Quantifying Aspects in Middleware

Platforms”

Used the “CORBA” middleware A Distribution Middleware Less sophisticated than an application server

(WebSphere)

Methodology: Similar to the previous work But, OO metrics were used to evaluate the benefits of

AOP

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“Quantifying Aspects…” (2/3)

Investigated aspects Homogenous: Logging, Error handling, Synchronization,

pre/post conditions Heterogeneous: Dynamic programming model, Portable

interceptors

OO Metrics Size: Total number of executable lines Weight: Number of methods per class CCN: Number of unique execution paths per method Coupling: Average number of other classes “known” by

a class

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“Quantifying Aspects…” (3/3)

Results:

CCNSizeWeightCoupling

Original4.2589316.521.13

Refactored

4.0448991618.33

Analysis: All metrics decreased in the refactored program This indicates that the primary code became simpler

Though the complete program code is just as complex It is difficult to evaluate the significance of the

improvement

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Summary

Various concerns in middleware Homogenous Heterogenous Tier-cutting

AOP refactoring Using aspects (!) to find concerns Changing the original code to make it aspect-

friendly

Numerical indications to the benefits of the refactroring process

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-The End-

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Welcome to the world of Middleware

BrowserJSP

EJB Database

J2EE Server

HTTP

RMI

JDBC

First Tier Second Tier Third Tier

Servlet

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Welcome to the world of Middleware

AnalyzeQueryReport

Alter EJB Support pointcut definition

Run Query

RefactorComponent

AnalyzeQueryReport

Alter EJB Support pointcut definition

Run Query

RefactorComponent