63c6bc7d88
- completely decouple Java/Scala and remove heading suffixes - move all images into images/ - move cluster docs into “official” place
75 lines
2.6 KiB
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75 lines
2.6 KiB
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.. _stm-scala:
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#######################################
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Software Transactional Memory
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#######################################
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Overview of STM
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===============
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An `STM <http://en.wikipedia.org/wiki/Software_transactional_memory>`_ turns the
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Java heap into a transactional data set with begin/commit/rollback
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semantics. Very much like a regular database. It implements the first three
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letters in `ACID`_; ACI:
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* Atomic
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* Consistent
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* Isolated
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.. _ACID: http://en.wikipedia.org/wiki/ACID
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Generally, the STM is not needed very often when working with Akka. Some
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use-cases (that we can think of) are:
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- When you really need composable message flows across many actors updating
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their **internal local** state but need them to do that atomically in one big
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transaction. Might not be often, but when you do need this then you are
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screwed without it.
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- When you want to share a datastructure across actors.
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The use of STM in Akka is inspired by the concepts and views in `Clojure`_\'s
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STM. Please take the time to read `this excellent document`_ about state in
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clojure and view `this presentation`_ by Rich Hickey (the genius behind
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Clojure).
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.. _Clojure: http://clojure.org/
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.. _this excellent document: http://clojure.org/state
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.. _this presentation: http://www.infoq.com/presentations/Value-Identity-State-Rich-Hickey
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Scala STM
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=========
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The STM supported in Akka is `ScalaSTM`_ which will be soon included in the
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Scala standard library.
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.. _ScalaSTM: http://nbronson.github.com/scala-stm/
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The STM is based on Transactional References (referred to as Refs). Refs are
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memory cells, holding an (arbitrary) immutable value, that implement CAS
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(Compare-And-Swap) semantics and are managed and enforced by the STM for
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coordinated changes across many Refs.
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Persistent Datastructures
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=========================
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Working with immutable collections can sometimes give bad performance due to
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extensive copying. Scala provides so-called persistent datastructures which
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makes working with immutable collections fast. They are immutable but with
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constant time access and modification. They use structural sharing and an insert
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or update does not ruin the old structure, hence "persistent". Makes working
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with immutable composite types fast. The persistent datastructures currently
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consist of a `Map`_ and `Vector`_.
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.. _Map: http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Map
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.. _Vector: http://www.scala-lang.org/api/current/index.html#scala.collection.immutable.Vector
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Integration with Actors
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=======================
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In Akka we've also integrated Actors and STM in :ref:`agents-scala` and
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:ref:`transactors-scala`.
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