Asynchrony for JVM in 2010¹

• com.twitter.util.Future
• scala.actors.Future
• java.concurrent.Future
• Scala 2.8 (2.5 years before 2.10 and SIP-14)
• Java 6 (4 years before CompletableFuture)

1. Initial Twitter Future implementation from 2 Aug 2010

Why not switch after SIP-14?

• Finagle favours local scheduling
• Root compression (available in 2.12)
• Interrupts
• First-class locals

Scheduling

• ExecutionContext vs. Scheduler
• Finagle's threading model is dictated by Netty (shared event loop)
• Event loop takes care to keep good thread affinity¹
• It makes sense to just run promises there
• Twitter Futures use LocalScheduler by default²

1. com.twitter.concurrent.Scheduler
2. Wrapping blocking code with Future {}

Root Compression / Promise Linking

• Allows for (unbounded) space-leaks-free Future recursion
• Think of tail-call-elimination for Future.flatMap
• Heavily used in Finagle¹ and many Twitter services
• Available in Scala 2.12^2,3

1. Ping-based Failure Detector (infinite future recursion)
2. Async Continuations Passing Style
3. SI-7336

Interrupts

• Propagate an interrupt through a Future chain
• Think of future cancellation
• Not yet supported in Scala Futures¹
• Enable many Finagle features (i.e., timeouts²)

1. Cancellable Scala Future
2. Finagle Timeout filter

Locals

• A "thread" local that follows an (asynchronous) application flow
• Not yet supported in Scala Futures^{1, 2}
• Empower Finagle's contexts³ (propagated across network boundaries)

1. ThreadLocal variables and Scala Futures
2. SI-9835
3. Finagle Contexts