The document discusses the actor model as an alternative approach to concurrency, traceable back to Carl Hewitt's 1973 work and further developed into tools like Akka and Erlang. Key features of the actor model include the ability of actors to send messages, create new actors, and maintain a state without external modification, ensuring inherent thread-safety. It also outlines the supervisory relationship between actors, handling failures consistently, and highlights various enterprise integration patterns supported by the actor model.

1. The Actor Model
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An alternative approach to concurrency
Lorenzo Nicora
Senior Consultant @ OpenCredo
@nicusX
https://opencredo.com/author/lorenzo/

2. ✓ (1973) Carl Hewitt: “A Universal Modular Actor Formalism for Artificial Intelligence”
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A bit of History…
1973
2009
✓ (2009) Akka toolkit, Scala/Java
✓ (1986) Ericsson: Erlang
Lorenzo Nicora Actor Model

3. According to Alan Kay,
author of Smalltalk
and inventor of the “Object-Orientation”:
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The original idea of Object
An “Object”
✓ Is the basic unit of object-oriented system
✓ Has its own state
✓ Communicate with others
by sending and receiving messages*
* in Java, C++, C# (derived from Simula, not Smalltalk)
Objects interact by invoking methods
1980
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4. An “Actor”
is a computational entity that,
in response to a message it receives,
can concurrently:
a) Send a finite number of messages to other actors
b) Create a finite number of new actors
c) Designate the behaviour to be used for the next message
it receives. [implies a state]
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The Actor
Actor
Actor
Actor
Mailbox Mailbox
Mailbox
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5. Actors interacts
only by messaging
Actors react to
messages
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Actors are Reactive
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6. An Actor handles one message a time
Never concurrently
An Actor has a state
State cannot be modified from outside the actor
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Thread-safe
An Actor is inherently thread-safe
No Lock/Synchronisation on State
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Supervision
Actors are created by Actors
Parent -> Child
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Supervision
Failure Handling
• If a Child throws an Exception, the Supervisor get notified
• Supervision Strategy: e.g. relaunch, abandon…
Failure is always handled consistently
Parent = Supervisor
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Lightweight
Actor is lightweight
Creating an Actor has little overhead
beyond creating an Object
Lorenzo Nicora Actor Model

10. Non-blocking implementation
of Request/Response pattern
with multiple downstream collaborators
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Example: Non-blocking Request/Response
• One-Actor-per-Request pattern
• Enterprise Integration Patterns:
✴ Request/Reply
✴ Return Address
✴ Scatter-Gather, Aggregators
✴ Round-robin Message Router
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Example: Non-blocking Request/Response
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Conclusions
Actor model:
an alternative approach to concurrency
Actors, Threads
+ Business logic implementation +
inherently thread-safe
- A new programming paradigm -
to learn
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Lorenzo Nicora Actor Model