jcc:TimedDefault cc Programming in 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 Ravi Withdrawal Account1 (zero) Account2 Deposit(100)suspend

The bank application Ravi Withdrawal Account1 (25) Account2 Deposit(100) Deposits 25 suspend

The bank application Ravi Withdrawal Account1 (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 Ports Local 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 Ravi Withdrawal Account1 (zero) Account2 Deposit(100)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 Withdrawal Account1 (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 Withdrawal Account1 (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.