jcc:TimedDefault cc Programmingin JAVA

Vijay Saraswat, IBM Research

Radha Jagadeesan, DePaul University

Vineet Gupta, Google

Concurrent programming in Java

Threads, shared heap, separate stack Synchronization based on locking

mutable objects in shared heap Rules to govern information flow between

thread-specific data structures (heap cache, stack) and shared store.

Concurrent programming in Java

`` Unfortunately the current specification has been found to be hard to understand and has subtle, often unintended, implications….. Several important issues [….] simply aren't discussed in the existing specification.''

Concurrent programming in Java

Concurrent execution in Java is the default.

Every line of Java code could be executed in parallel by multiple threads on the same object.

The Jcc model

jcc= JAVA −Threads +Vats +Promises +Agents

Jcc Design Goals

• Interoperability with Java: source code, JVM, and type system

• API for the programmer to add new constraint systems

• Support for reflective meta-programming• The implementation should be usable for

medium-sized programs.

Rest of the talk

An example Bank application The design elements of Jcc.

The bank application

Withdrawal

Deposit

The Bank application: Withdrawals

Withdrawals succeed if balance is large enough. If not, repeat withdrawal after each of the next n deposits. If balance still not large enough, it is rejected.

The bank application

RaviWithdrawalAccount1(zero)

Account2

Deposit(100) suspend

suspend

The bank application

RaviWithdrawalAccount1(25)

Account2

Deposit(100)Deposits

25

suspend

suspend

The bank application

RaviWithdrawalAccount1(100)

Account2

Deposit(100)Deposits

25 each

Rest of the talk

An example Bank application The design elements of Jcc.

Vats and ports

Stack

Heap

PortsLocal stack/heap

Single threaded

JVM = Multiple vats + Shared heap of immutable objects

Communication: Ports

Each port is located at a vat and is “read” only by code at that vat

Each port can have multiple writers/tellers at other vats

Objects written into ports are deep-copied from source to target vat

Vats and ports in the bank application

RaviWithdrawalAccount1(zero)

Account2

Deposit(100) suspend

suspend

Execution in a Vat

Process 1 message at a time to completion before getting another message: create new local heap objects invoke methods on objects in heap send objects to ports in another vat

All these operations do not block!

Bounded response guarantees?

Logical time

A logical time-step:

receive input

compute response

Correspondence to physical time if bounded response can be guaranteed cf. Esterel.

Logical time in the bank application

WithdrawalAccount1(zero)

Time steps: 0. Withdrawal(100) 1. Deposit(25) 2. Deposit(25) 3. Deposit(25) 4. Deposit(25)

Time based control constructs

1) next {S}. S is stored and executed in the next time instant.

2) always {S}: Run S at EVERY time instant

Oops! Many programs can be executing at the same time instant

Mutiple programs in the Account1 vat of the bank application

WithdrawalAccount1(zero)

Time steps: 0. Withdrawal(100): “withdrawal code” 1. Deposit(25): “withdrawal, deposit code” 2. Deposit(25): “withdrawal, deposit code” 3. Deposit(25): as above 4. Deposit(25): as above

Constraints for Intra-Vat communication

How to achieve determinate concurrency?Concurrent constraint programming!

Store

Constraint stores in Jcc

Each vat has its own store All items in a store are dropped at the end

of a time instant: programmer carries items into following time instant explicitly using next.

Constraints via promises (= typed logical variables) in Jcc.

Promises

Promise in Jcc = java.lang.Object in Java

1. Unrealized and unwatched. [new variables]

2. Realized [new constants]

3. Bound [o.equate(p) ]

4. Unrealized and watched

Watchers for promises

When (p) do S

If p is realized, run S.

Otherwise, suspend S on p. S is a watcher for p

Effect of p.equate(q)??

Watchers for promises

every (p) do S

run S in every time instant in which p is realized

Code for Account

public BankAccount() {

every (balanceUpdate) { // check if pending withdrawals // can be satisfied }

Code for Account

public Confirmation deposit(Integer amount){

Confirmation result = new Confirmation();when (amount) { // update balance result.equate(“Success”); }return result;}

Code for Account

public Confirmation withdraw(Integer amount){

Confirmation result = new Confirmation();when (amount) { // check balance // if balance sufficient

result.equate(“Success”); //if balance not sufficient, add to pending list

}return result;}

Conclusions

jcc= JAVA −Threads +Vats +Promises +Agents

Full power of Timed Default cc in Java.