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With the recent, vivid trend towards multicore hardware and the ever growing application requirements, concurrency is no more a niche area it used to be, and is slowly becoming a norm. In this talk, we will talk about promises/futures, one of the concurrency models that has risen to the occasion. We will look at what they are, how they're implemented and used in Java and Javascript. We will see how Scala, with its functional paradigm and greater abstraction capabilities, avoids "callback hell" typically associated with the model, allows writing of concurrent code in "direct style", and thereby greatly reduces the cognitive burden, allowing you to focus on application logic better.
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Promise of a better future
Pooja AkshantalRahul Goma Phulore
Who are we?
What we are going to talk about
• How concurrency and parallelism are different ideas
• Why concurrency is important• Futures and promises • DEMO!• Pitfalls
Concurrency ≠ ParallelismConcurrency is a program-structuring technique in which there are multiple threads of control
A parallel program is one that uses a multiplicity of computational hardware (e.g. multiple processor cores)
Structure Execution
Dealing with lots of things at once
Doing lots of things at once
Orthogonality illustrated
Javascript promises, Python
generators
Java and Scala futures
Well… lots of things Parallel collections
P
C
NP
NC
Fall of Moore’s law
Age of multicore
• Multicore has become a norm!
• Need to exploit parallelism to perform
• Good concurrency abstractions a way to get there
Futures and promises
Futures and promises
• Can be thought of as a single concurrency abstraction
Future ≈ Cheap thread
• Futures are multiplexed dynamically onto threads as needed.
• Managed by ExecutionContext.• It’s practical to have hundreds of thousands
going at once.
• Spawn away!
Code Time
Async and non-blocking
• Goal: Do not block current thread while waiting for the result of the future.
• Callbacks:– Register callback which is invoked asynchronously
when future is completed– Async computations do not block
Callbacks?!
There is a solution!
• Higher-order functions / combinators• for-comprehensions• Other abstractions
Running example
• Collect statistics for a facebook page
• For every post on the page:– Collect all likes– Collect all comments
• Aggregate results into popularity score
Code Time
Other parallels
• Error recovery combinators
– recover– recoverWith
• Collection operations
– map: (Seq[A], A ⇒ B) ⇒ Seq[B]
mapF: (Seq[A], A ⇒ Future[B]) ⇒ Future[Seq[B]]
– filter: (Seq[A], A ⇒ Boolean) ⇒ Seq[A]
filterF: (Seq[A], A ⇒ Future[Boolean]) ⇒ Future[Seq[A]]
Pitfalls
• Interactions with thread-unsafe libraries• ThreadLocal variables• Retries, time-outs not very natural• Simpler than other concurrency models in
many ways, but still incur some cognitive cost
Credits
• Heather Miller, for her material on relationship between futures and promises
• Simon Marlow and Rob Pike, for their material on concurrency and parallelism
• Josh Suereth, for his Github example (Inspiration for our facebook example)
• Scala, Play, and Akka teams for all their amazing work
Thank You!
☺
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