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A TOUR OF ASYNCHRONOUS PROGRAMMING IN SCALA
YIFAN XING?- 2018
ASYNCHRONOUS
PROGRAMMING
Yifan Xing
SCALA.CONCURRENT AND
MONIX!
@yifan_xing_e

01 Synchronous vs. Asynchronous
Differences
Futures & Promises
Debugging Async Programs
Monix
A TOUR OF ASYNCHRONOUS
PROGRAMMING IN SCALA
02
03
04
05Overview
ASYNCHRONOUS
PROGRAMMING:
SCALA.CONCURRENT
AND MONIX!

Concurrency
Synchronous
Asynchronous
Single-threaded
Multi-threaded
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Synchronous: Single-threaded
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Task 1 Task 2 Task 3
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Synchronous: Multi-threaded
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Task 1
Task 2
Task 3
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Asynchronous: Single-threaded
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Task 1
Task 2
Task 3
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Asynchronous: Multi-threaded
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Task 1
Task 2
Task 3
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Future
Promise
ExecutionContext
Examples
Scala.concurrent

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Scala.concurrent: Future
Computation Incomplete:
Computation Completed:
Future is not completed
Future is completed: with a Value => Success
Future is completed: with an Exception => Failure
An object holding a value which may become available at some point
Immutable
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Future

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Example:?Callbacks
Multiple
callbacks may
be registered;
NO guarantee
that they will be
executed in a
particular order.
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Example:?Callbacks
Chain callbacks
Allow one to enforce
callbacks to be
executed in a
specified order.
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Example:?Callbacks
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Exception in
callback:
NOT propagated
to the
subsequent
andThen
callbacks.

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Example:?Transformations
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More generic &
flexible
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If no match
cases or original future
contains a valid result:
New future will contain
the same result as the
original one.
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Evaluated only after
a Failure

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If both futures failed,
the resulting future
holds the throwable
object of the first
future.
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Scala.concurrent: Promises
A writable, single-assignment container
Can create a Future
Can complete a Future, only ONCE
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Promise:

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Example: Promises
A Promise can be
completed at most ONCE.
If the promise has already
been fulfilled, failed or
has timed out, calling this
method will throw an
IllegalStateException.
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Example: Promises
If the promise has
already been
completed returns
false, or true
otherwise.
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Problem in A Client-facing Interface
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p.future
Problem in A Client-facing Interface
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Monix
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What is Monix?
Asynchronous, event-based programs
Data type: Task, etc.
"Factory" of Future instances
Lazy & asynchronous computations

Monix: Task
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Lazily evaluated
Evaluated when
call runAsync
Trigger side effects
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Monix: Task Callback
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Eagerly evaluate task
Register callbacks
Similar to
Future#onComplete
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Monix:?Eager/ Lazy
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Evaluate eagerly
Equivalent to Future#successful
Defer eager evaluation
Similar to Task#eval
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Monix:?Memoization
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Evaluated lazily
Not memoized, recompute each
time the Task is executed.
Evaluate lazily
Evaluated exactly ONCE
Result is memoized
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Monix:?MemoizeOnSuccess
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Exceptions are not cached.
Only cache the first
successful result
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Monix:?Alternative Thread Pool
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Overrides default scheduler
Overriding the "default"
scheduler can only happen
ONCE
Task is immutable
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Monix:?CancelableFuture
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Calling it multiple
times has the same side-
effect as calling it only once.
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- A value
- Executed when constructed
- Eager evaluation
- ExecutionContext needed whenever use? ?
? ?operators
A function -?
Execution controllable by user?-?
Lazy evaluation -?
ExecutionScheduler needed whenever -?
?use runAsync? ?
MonixFuture
Eager/Lazy

- Memoized by default
- Evaluated once
- Store result
Explicitly use memoization operators -?
Evaluated whenever needed -?
Referential transparency -?
MonixFuture
Memoization

- Sometimes errors are? ? ? ? ? ? ? ? ? ? ? ? ?
? ?not?propagated
Scheduler.reportFailure -?
Defaults to System.err -?
Allow customized logging tools -?
MonixFuture
ErrorHandling

Future Monix

FutureMonix

Debugging
BEST PRACTICES
In Asynchronous Programs
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Writing Async Code
1. Rethink before
adding more code
2. Adding more code to
fix problems will
usually add more bugs
D e b u g
1. Test code in isolation
2. Observe what is
happening
3. Confirm behavior
before moving on
T e s t

Using Await & Print
TimeoutException if after
waiting for the specified
time awaitable is still not
ready
atMost may be:
A finite positive duration
Negative (no waiting is
done)
Duration.Inf for
unbounded waiting

ScalaTest: ScalaFutures
Default timeout
150ms
TestFailedException
if not finished after
timeout

Resources: Async Debugger & Capture
IntelliJ Capture (2017):
Iulian Dragos - Async Debugger in Scala IDE
IntelliJ IDEA 2017.1 EAP Extends Debugger with Async Stacktraces
Developer's Explanation
Debugging Tutorial
Async Stacktraces
Stack Retention
Scala IDE Doc
Demo: Jamie Allen
Rethink the Debugger: Scala Days 2014

Using?Async Debugger / Capture
Substituting its parts related
to the asynchronous code
execution with where it was
called
Sender: Async code executor
Receiver: executed code
Config exact signatures of
methods of the async code
Stores the stack and
variables info in a map
(1000 stacks), where the
key is a parameter with
the specified number.

( Sorry Alex :P )

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