Akka 和 Erlang 比起来怎么样？

我最近一直在观察 Akka，它给我留下了深刻的印象。它似乎具有 erlang 的大部分杀手特性——位置透明性、监督层次结构等等。二郎有没有什么特点是阿卡没有的？

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For me, hot code swapping in an entire Erlang cluster without downtime (for example: make:all([netload]) is one of the Erlang killer features.

But let's reverse your question: What does akka have that Erlang doesn't? Of course you can add dozens of extensions and libraries (scala, akka, spring, osgi, ...) to Java to try to come close to Erlang. But where is the point? In sum all these extensions are much more complex than learning the simple Erlang language that now has proven for over 2 decades that it can do the job offering top scalability with zero downtime.

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Disclaimer: I am the PO for Akka

  • Erlang does copy-on-send - Akka uses shared memory (immutable objects) for in-VM sends
  • Erlang does per-process GC - Akka uses JVM GCs
  • Erlang has OTP - Akka integrates with the entire Java ecosystem (Apache Camel, JAX-RS, etc etc)
  • Erlang does the process scheduling for you - Akka allows you to use many different Dispatchers with endless configuration opportunities
  • Erlang does hot code reload - Akka can support it, but it's less flexible because of JVM classloading

Those are the ones from the top of my head.

On the other hand, using Akka means that you can use Scala, Java, Groovy or JRuby to write your applications.

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Nearly nobody mentions process isolation. Without guarantees of "your thread cannot mess with my junk", distributed systems are much more difficult to reason about. (They're already difficult enough with Erlang's processes.)

AFAIK (which isn't far, given my limited direct experience with the JVM), only Erlang actually gets process isolation "right" on the JVM. Mr. Google can give some hints on where to find research by Fox and Candea (?) on research systems that use a "micro-reboot" technique ("recovery-oriented computing"). An Erlang developer reads that research and says a couple of things:

  1. Welcome to the club, what took you so long?
  2. The JVM makes it awfully, awfully hard to join, though. :-)
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In Erlang processes are guaranteed to be switched approximately each 1000 reductions. In such a naive framework as Scala/Akka agent owns a scheduler until it finishes work in receive. Checkmate. Game over. Hasta la vista:) People, do not waste your time on pseudo techs. I shocked that guys here compare Scala with Erlang.

Also there are many other so called "killer features", but here is my advice, do not think in terms of features, think about idioms that enables particular language. Scala steals "best features", Erlang enables/implements you with right idioms to build systems reliably, with high level language that driven from those right idioms. When you learn Erlang you are rebuilding your mind, your way of thinking about distributed reliable system, Erlang teaches you and upgrades you. Scala is just another one imperative (oh, sorry, multiparadigmal, funny word) language that tries to steal good features from other languages.

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Probably Erlang is better for bigger distributed systems (following vjache's answer) but for a normal server when you just want use the full power of multiple CPUs then Akka is good choice— provides good abstraction, performance and integration with the Java ecosystem.


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