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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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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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