Iulian Dragos @jaguarul

Rethinking the debugger

Overview

• Async Debugger

• Scala IDE for Eclipse 4.0

Reactive applications

• Different parts of the application run in different logical threads

• ..the programmer must ensure correct coordination and communication

Concurrency

• Threads and Locks are replaced by

• Futures

• Actors

• parallel collections (fork-join frameworks)

Concurrency

• non-blocking

• often leads to callbacks

• flow of execution is not flow of program text

Futures

val fTweets = Future { getAllTweets(user) } ! // also a future val nrOfTweets = fTweets.map(ts => ts.size) ! nrOfTweets onSuccess { println }

Execution continues without

waiting for all tweets

Futures can be chained

Callback for successful completion

How do we debug asynchronous programs?

Detective work

• go from undesirable effects back to causes

• attempt a fix

• repeat

The Call Stack

java.lang.Exception at test.FuturesTest$$anonfun$simpleUse$1$$anonfun$2$$anonfun$apply$1.apply$mcI$sp(Futur.. at test.FuturesTest$$anonfun$simpleUse$1$$anonfun$2$$anonfun$apply$1.apply(FuturesTest.sca.. at test.FuturesTest$$anonfun$simpleUse$1$$anonfun$2$$anonfun$apply$1.apply(FuturesTest.sca.. at scala.concurrent.impl.Future$PromiseCompletingRunnable.liftedTree1$1(Future.scala:24) at scala.concurrent.impl.Future$PromiseCompletingRunnable.run(Future.scala:24) at scala.concurrent.impl.ExecutionContextImpl$$anon$3.exec(ExecutionContextImpl.scala:107) at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260) at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339) at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979) at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

Futures use a thread pool for execution!

The Problem

• The call-stack used to record the flow of control of sequential programs

• futures (and actors) execute each computation step on a different thread

• on a mostly uninteresting call stack

Can we recover the async stack?

Record call stack when a future is created

Under the hood

• Intercept Future instantiation

• Save stack contents (all of it)

• Resume

• When a breakpoint is hit

• check async stacks store

• (overhead is comparable to conditional breakpoint)

Chrome does it

Visual Studio does it, too

Debugging Actors

Debugging Actors

• Can we find out where a message was sent?

• what was the state at that point?

• IT’S THE SAME THING!

Not just futures

• message sends (Akka, Scala actors)

• Play iteratees

• any composable asynchronous structure

• user-defined/data-flow debugging

Debugging Actors

• Message-send is like an asynchronous method call

• “I’d really like to step-into that message send”

Step-with-a-bang

• resume execution until the next message sent is received by an actor

• the receiving actor might run on a different thread

• other messages might be exchanged in the meantime

Demo Time!

More tools

• Exception breakpoint: break on dead letters

• Logical Structure of an actor

• parent

• current sender

• supervisor strategy

Future

• user-extensible points-of-interest

• filter collected data (by package, frame)

Summary

Call Stack

Step Into

Exception breakpoint

Logical Structure

Async stack

Step-Message

Break on Dead Letters

Actor details

Scala IDE for Eclipse 4.0 preview and roadmap

Lithium release (4.0)

quiescence type-checking

quick fixes (implement abstract member)

multiple Scala version support

name hashing

stable API for plugins

Community steps up

4.0 community

Evaluate expression

Debugger improvements

Refactoring (extract..)

UI cleanups

wizards, hovers

completions

Quiescence Typing

3.0.x 4.0.0-M2

Multiple Scala Version support

• Starting with Scala 2.11.0 and IDE 4.0-M2

• Presentation compiler works in (2.10) compatibility mode

• The build compiler is 2.10

• Can have multiple Scala versions in the same workspace

What next?

Sbt-server

• Delegate build to an Sbt (shared) process

• Directly import Sbt project

• Run any task

• Launch/debug Play apps

GSoC project

• extensible IDE

• use Scala snippets to plug into the IDE

• Save Actions

• Debug Object rendering

• Auto-edits

Contribute!