Aspects Made Explicit for

Safe Transactional Semantics

Kevin Hoffman / Patrick Eugster

-2-Kevin Hoffman and Patrick Eugster

Properties of Transactions

A tomicity – all or nothing

C onsistency – changes respect specifications

I solation – effects of concurrency masked

D urability – committed results never lost

-3-Kevin Hoffman and Patrick Eugster

Implementation Approaches

Library approach

- Safety

- Elegance

+Flexibility

+Interoperability

Language approach

+Safety

+Elegance

- Flexibility

- Interoperability

-4-Kevin Hoffman and Patrick Eugster

The Pursuit of Elegance

Cross-cutting concerns tangle/scatter code

Aspect Oriented Programming (AOP) / AspectJ

Exposes structural points (e.g. method call, member

accesses) of a program’s primary decomposition as

language features

Provides language features (aspects) to superimpose

logic (advice) systematically onto the mainline code

(via pointcuts)

AOP improves elegance but complicates safety

-5-Kevin Hoffman and Patrick Eugster

AOP/AspectJ in 60 seconds

void buy(…){

…}

void cancel(…){

…}

void return(…){

…}

Store class

…DB.buy(order);…

…DB.cancel(o);…

…DB.return(o);…

pointcut tranOp():call(* * Store.*(..));

around(): tranOp() { //adviceT = thisJoinP.getTarget();TransMan.beginTrans(T);try{

Object ret = proceed();TransMan.commit(T);return ret;

} catch (Exception e){TransMan.abort(T);throw e;

}}

JSP Pages Transactionalizer aspect

-6-Kevin Hoffman and Patrick Eugster

AOP/AspectJ Safety Issues

void buy(…){

…}

void cancel(…){

…}

void return(…){

…}

Store class

…DB.buy(order);…

…DB.cancel(o);…

…DB.return(o);…

JSP Pages Transactionalizer aspect

pointcut tranOp():call(* * Store.*(..));

around(): tranOp() { //adviceT = thisJoinP.getTarget();TransMan.beginTrans(T);try{

Object ret = proceed();TransMan.commit(T);return ret;

} catch (Exception e){TransMan.abort(T);throw e;

}}

Aspects can throw

unexpected

exceptions

due to new logic.

But there is no way

to constrain the

base code to catch

these exceptions…

-7-Kevin Hoffman and Patrick Eugster

void buy(…){TA.jpTrans() {

…

}}void cancel(…){

TA.jpTrans() {…

}}void return(…){

TA.jpTrans() {…

}}

Our Idea: Explicit Join PointsStore class

…DB.buy(order);…

…DB.cancel(o);…

…DB.return(o);…

scoped joinpoint jpTrans()throws TranException;

pointcut tranOp():joinpoint(jpTrans);

before (): tranOp() {T = thisJoinP.getTarget();TransMan.beginTrans(T);

}after () throwing: tranOp() {

TransMan.abort(T);}after () returning: tranOp() {

TransMan.commit(T);}

JSP PagesTransactionalizer aspect [TA]

-8-Kevin Hoffman and Patrick Eugster

EJP Larger Example [Aspect]aspect Transactionalizer {

scoped joinpoint jpTrans(Serializable[ ] objects)throws TransactionException; /* EJPs makes this constraint possible */

pointcut pcTrans(Serializable[ ] objects) :joinpoint(jpTrans) && args(objects);

before(Serializable[ ] objs): pcTrans(objs) {TransMan.begin(objs, ...);

}

after() throwing: pcTrans() { // if mainline code fails,

TransMan.abort(...); // abort transaction

}after() returning: pcTrans() { // after normal completion,

TransMan.commit(...); // commit transaction

}}

-9-Kevin Hoffman and Patrick Eugster

EJP Larger Example [Mainline]/* EJPs explicitly require object to be Serializable to be Transactionalized */

class Agent implements Serializable {

void createTrip(Person p, Flight f, Hotel h, RentalCar c){

boolean bCharged = false;

try {

/* EJPs provide bilateral communication with aspects */

Serializable [ ] objects = {this, p, f, h, c};

joinpoint Transactionalizer.jpTrans(objects) {

/* Scope of transaction specified by scope of EJP */

f.ReserveSeat(p); h.ReserveRoom(p);

c.ReserveVehicle(c); m_CC.Debit(p.getCC(), …);

bCharged=true;

}

} catch (TransactionException e) { /* Required by EJP */

if (bCharged){ m_CC.Credit(p.getCC(), …); }

}

}

-10-Kevin Hoffman and Patrick Eugster

Why is an EJP not a method call?

Multiple aspects may advise the EJP

Scoped EJPs provide new code structure

which can be advised, increasing safety

Aspect instance can depend on context of

joinpoint (via pertarget, percflow, …)

Pointcuts may be passed as parameters,

utilizing the full power of AOP features

-11-Kevin Hoffman and Patrick Eugster

How EJP Measures Up

+ Safety: EJP constraints enforced at

compile time

+ Elegance: EJP increases AspectJ power

(bilaterial communication, constraints)

+ Flexibility: Change implementation easily

+ Interoperability: Superset of AspectJ;

works with any other Java program

-12-Kevin Hoffman and Patrick Eugster

The End of the Beginning

kjhoffma@cs.purdue.edu

p@cs.purdue.edu