The Pros & Cons of Asynchrony

Douglas C. Schmidtd.schmidt@vanderbilt.edu

www.dre.vanderbilt.edu/~schmidt

Professor of Computer Science

Institute for Software

Integrated Systems

Vanderbilt University

Nashville, Tennessee, USA

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Learning Objectives in this Part of the Lesson• Motivate the need for Java futures

by understanding the pros & consof synchrony

• Motivate the need for Java futuresby understanding the pros & consof asynchrony

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Overview of Asynchrony &Asynchronous Operations

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• Asynchronous operations can alleviate limitations with synchronous operations

Overview of Asynchrony & Asynchronous Operations

See en.wikipedia.org/wiki/Asynchrony_(computer_programming)

5See en.wikipedia.org/wiki/Asynchronous_method_invocation

• Asynchronous operations can alleviate limitations with synchronous operations

• Asynchrony is a means of concurrent programming where the caller does not block while waiting for the called code to finish

Overview of Asynchrony & Asynchronous Operations

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• Asynchronous operations can alleviate limitations with synchronous operations

• Asynchrony is a means of concurrent programming where the caller does not block while waiting for the called code to finish, e.g.

• Android AsyncTasks execute long-duration operations asynchronously in one or more background threads

Overview of Asynchrony & Asynchronous Operations

Background thread

See developer.android.com/reference/android/os/AsyncTask

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• Asynchronous operations can alleviate limitations with synchronous operations

• Asynchrony is a means of concurrent programming where the caller does not block while waiting for the called code to finish, e.g.

• Android AsyncTasks execute long-duration operations asynchronously in one or more background threads

• The caller (UI) thread can be notifiedupon completion, failure, or progressof the background task

Overview of Asynchrony & Asynchronous Operations

Calling thread

See developer.android.com/reference/android/os/AsyncTask

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The Pros of Asynchrony

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• Pros of asynchronous operations

The Pros of Asynchrony

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• Pros of asynchronous operations

• Responsiveness

• A calling thread needn’t block waiting for the async request to complete

The Pros of Asynchrony

See en.wikipedia.org/wiki/Asynchronous_method_invocation

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• Pros of asynchronous operations

• Responsiveness

• Elasticity

• Multiple requests can run scalably& concurrently on multiple cores

The Pros of Asynchrony

See en.wikipedia.org/wiki/Elasticity_(cloud_computing)

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• Pros of asynchronous operations

• Responsiveness

• Elasticity

• Multiple requests can run scalably& concurrently on multiple cores

• Elasticity is particularly usefulto auto-scale computationsin cloud environments

The Pros of Asynchrony

See en.wikipedia.org/wiki/Elasticity_(cloud_computing) & en.wikipedia.org/wiki/Autoscaling

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The Cons of Asynchrony

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• Cons of asynchronous operations

The Cons of Asynchrony

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• Cons of asynchronous operations

• Unpredictability

• Response times may not unpredictable dueto non-determinism of async operations

The Cons of Asynchrony

Non-determinism is a general problem with concurrency & not just asynchrony

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• Cons of asynchronous operations

• Unpredictability

• Response times may not unpredictable dueto non-determinism of async operations

• Results can occur in a different order than the original calls were made

The Cons of Asynchrony

CALLER CALLEE

searchForWord1

future result1

searchForWord2

future result3

searchForWord3

future result2

future1

future2

future3

future result1

future result3

future result2

Additional time & effort may be required if results must be ordered somehow

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• Cons of asynchronous operations

• Unpredictability

• Complicated programming & debugging

The Cons of Asynchrony

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• Cons of asynchronous operations

• Unpredictability

• Complicated programming & debugging

• The patterns & best-practicesof asynchronous programmingare not well understood

The Cons of Asynchrony

See dzone.com/articles/callback-hell

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• Cons of asynchronous operations

• Unpredictability

• Complicated programming & debugging

• The patterns & best-practicesof asynchronous programmingare not well understood

• Errors can be hard to trackdue to unpredictability

The Cons of Asynchrony

See www.jetbrains.com/help/idea/tutorial-java-debugging-deep-dive.html

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• Cons of asynchronous operations

• Unpredictability

• Complicated programming & debugging

• The patterns & best-practicesof asynchronous programmingare not well understood

• Errors can be hard to trackdue to unpredictability

The Cons of Asynchrony

Again, this non-determinism is a general problem with concurrent processing

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Weighing the Pros & Cons of Asynchrony

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• Two things are necessary for the pros of asynchrony to outweigh the cons

Weighing the Pros & Cons of Asynchrony

Pros

Cons

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• Two things are necessary for the pros of asynchrony to outweigh the cons

• Performance should improve tooffset the increased complexity of programming & debugging

Weighing the Pros & Cons of Asynchrony

Performance

Productivity

See upcoming lesson on “Java Completable Futures ImageStreamGang Example”

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• Two things are necessary for the pros of asynchrony to outweigh the cons

• Performance should improve tooffset the increased complexity of programming & debugging

• An asynchronous programming model should reflect the keyprinciples of the reactive paradigm

Weighing the Pros & Cons of Asynchrony

See earlier lesson on “Overview of Reactive Programming”

Responsive

Resilient

Message-driven

Elastic

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End of the Pros & Cons of Asynchrony