Introduction to actor model with examples on Akka.NET

Arthur Shvetsov 1

Overview and history What is an actor model? Akka.NET main concepts Use-case scenarios

Agenda

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CPU clock speed vs number of cores

CPU clock speed (MHz) # of Cores3

Make classes and functions Find areas where you can do multiple things at once Queue work onto a new thread or thread pool Use synchronization mechanisms to protect shared memory Finish work on one thread Consume that work on another

How we write multithreaded code?

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Statefulness Concurrency Recovering from failures Bottlenecks Availability

Distributed programming pains

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Multithreading

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- If we don’t want to write multithread code?- Use concurrency abstractions instead 

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We won’t talk about TPL today 

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The Actor Model

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1973 - Concept formulated by Carl Hewitt et al.: “A universal modular ACTOR formalism for artificial intelligence”

1986 - Gul Agha, doctoral dissertation "Actors: A Model of Concurrent Computation in Distributed Systems“

1986 - Joe Armstrong and others in Ericsson - Erlang programming language and VM.

2009 - initial release of Akka framework (JVM, Scala) 2014.02 - initial release of Akka.NET - Aaron Stannard, Roger 

Actor model history

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The Actor Model provides a higher level of abstraction for writing concurrent and distributed systems.

Actor Model alleviates the developer from having to deal with explicit locking and thread management, making it easier to write correct concurrent and parallel systems.

Actor Model defines some general rules for how the system’s components should behave and interact with each other.

What is an actor model?

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No deadlocks No shared state No global mutable state Fault isolation – “let it crash” Encapsulation

Actor model benefits

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Akka.NET is a toolkit and runtime for building highly concurrent, distributed, and fault tolerant event-driven applications on .NET & Mono.

Akka.NET is a port of the popular Java/Scala framework Akka to .NET.

What is Akka.NET?

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Everything is an actor!

From OOP to Actor Model paradigm shift

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Actor is a code unit organization

What is actor?

OOP Actor

BehaviorState

Synchronous calls

BehaviorState

Asynchronous messages

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What is actor?

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All communication is done via message-passing All messages are immutable Sender and recipient are decoupled, asynchronous

Actors are fundamental units of work & concurrency

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Process messages Contain private state, invisible from the outside and changes it upon

reception of a message Change behavior between messages (i.e. state machine)

Actors can

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One message processed at a time Messages processed in FIFO order “At Most Once” delivery guarantied:

at-most-once delivery at-least-once delivery exactly-once delivery

Actors promise

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Example

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Switchable Behaviors

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Actor exists as hierarchies• Parent actors supervise child actors• Actors are resilient to failures

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One-For-One Strategy vs. All-For-One Strategy

Actor supervision

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Communicate with actors through “actor reference”

All actors are identified by a unique address

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Actor addresses have location transparency

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Location transparency Remote addressing Remote messaging Remote deployment

Akka.Remote

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Thread Instance of an object/component Publisher/Subscriber Singleton or service (e.g. data access layer) State machine Load balancer or router

Actor use cases

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http://getakka.net/ https://github.com/petabridge/akka-bootcamp

References

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Thank you!

Arthur Shvetsov

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