Aspects in Middleware

1. Aspects in Middleware 1: Adrian Coyler, Andrew Clement 2: Charles Zhang, Hans-Arno Jacobsen Presented by: Itay Maman

2. 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

3. 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, ...

4. 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

5. Browser Database Example: A Three-tier application First Tier Second Tier Third Tier HTTP JDBC Server

6. 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)

7. 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

8. Browser Database Welcome to the world of J2EE First Tier Second Tier Third Tier JSP HTTP Servlet RMI RMI Accounts EJB JDBC Employees EJB JDBC J2EE Server

9. 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

10. 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

11. Inspection of middleware needs • Two approaches • Analytic: What is so unique in middleware platforms? • Programmatic: What is needed in a typical component code?

12. Analytic approach (1/3) • Let’s compare Application Servers (AS) with Operating Systems (OS). • It turns out that both are platforms, which: • Launch applications • Supply core services to these applications • Let programmers focus on domain specific tasks On the other hand, there are some key differences…

13. 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

14. 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

15. 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 (?!?)

16. 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 (?!?)

17. Programmatic approach (3/3) • What is left for the programmer to do? • Invoke J2EE services • Lookup,iterate,put,get,remove,send,receive • Write domain specific algorithms/behavior • Which are typically not complicated • 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

18. 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

19. “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

20. 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

21. 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

22. 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)

23. 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]

24. 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

25. 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"; }

26. Refactoring (of EJB support) public class A extends B { public void init() { one(); add(); two(); } void add() { } } publicaspect EJBAspect1 { EJBContainer ejbc; pointcut registration(A ci) : execution(* add()) && this(ci); before(A ci) : registration(ci) { ejbc = newContainer(); if(ejbc != null) register(ejbc) } } • Original code public class A extends B { private EJBContainer ejbc; public void init() { one(); ejbc = newContainer(); if(ejbc != null) register(ejbc) two(); } } • Refactored code

27. 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

28. 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

29. “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

30. “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

31. “Quantifying Aspects…”(3/3) • Results: • 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

32. 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

33. -The End-

34. First Tier Second Tier Third Tier Browser JSP HTTP Servlet Database RMI EJB JDBC J2EE Server Welcome to the world of Middleware

35. Welcome to the world of Middleware