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Manfred Bergmann




Introduction - Actor framework featuring actors and agents

Sento is a 'message passing' library/framework with actors similar to Erlang or Akka. It supports creating systems that should work reactive, require parallel computing and event based message handling.

sento features:

  • Actors with ask and tell operations. ask can be asynchronous or synchronous.
  • Agents: Agents are a specialization of Actors for wrapping state with a standardized interface of init, get and set. There are also specialized Agents for CLs array and hash-map.
  • Router: Router offers a similar interface as Actor with ask and tell but collects multiple Actors for load-balancing.
  • EventStream: all Actors and Agents are connected to an EventStream and can subscribe to messages or publish messages.
  • Tasks: a simple API for concurrency.

Version history

Version 2.0.0 (16.8.2022): Rename to "Sento". Incompatible change due to package names and system have changed.

Version 1.12.2 (29.5.2022): Removed the logging abstraction again. Less code to maintain. log4cl is featureful enough for users to either use it, or use something else in the applications that are based on sento.

Version 1.12.1 (25.5.2022): Shutdown and stop of actor, actor context and actor system can now wait for a full shutdown/stop of all actors to really have a clean system shutdown.

Version 1.12.0 (26.2.2022): Refactored and cleaned up the available actor-of facilities. There is now only one. If you used the macro before, you may have to adapt slightly.

Version 1.11.1 (25.2.2022): Minor additions to actor-of macro to allow specifying a destroy function.

Version 1.11.0 (16.1.2022): Changes to AC:FIND-ACTORS. Breaking API change. See API documentation for details.

Version 1.10.0: Logging abstraction. Use your own logging facility. sento doesn't lock you in but provides support for log4cl. Support for other logging facilities can be easily added so that the logging of sento will use your chosen logging library. See below for more details.

Version 1.9.0: Use wheel timer for ask timeouts.

Version 1.8.2: atomic add/remove of actors in actor-context.

Version 1.8.0: hash-agent interface changes. Added array-agent.

Version 1.7.6: Added cl:hash-table based agent with similar API interface.

Version 1.7.5: Allow agent to specify the dispatcher to be used.

Version 1.7.4: more convenience additions for task-async (completion-handler)

Version 1.7.3: cleaned up dependencies. Now sento works on SBCL, CCL, LispWorks, Allegro and ABCL

Version 1.7.2: allowing to choose the dispatcher strategy via configuration

Version 1.7.1: added possibility to create additional and custom dispatchers. I.e. to be used with tasks.

Version 1.7.0: added tasks abstraction facility to more easily deal with asynchronous and concurrent operations.

Version 1.6.0: added eventstream facility for building event based systems. Plus documentation improvements.

Version 1.5.0: added configuration structure. actor-system can now be created with a configuration. More configuration options to come.

Version 1.4.1: changed documentation to the excellent mgl-pax

Version 1.4: convenience macro for creating actor. See below for more details

Version 1.3.1: round-robin strategy for router

Version 1.3: agents can be created in actor-system

Version 1.2: introduces a breaking change

ask has been renamed to ask-s.

async-ask has been renamed to ask.

The proposed default way to query for a result from another actor should be an asynchronous ask. ask-s (synchronous) is of course still possible.

Version 1.0 of sento library comes with quite a few new features (compared to the previous 0.x versions). One of the major new features is that an actor is not bound to it's own message dispatcher thread. Instead, when an actor-system is set-up, actors can use a shared pool of message dispatchers which effectively allows to create millions of actors.

It is now possible to create actor hierarchies. An actor can have child actors. An actor now can also 'watch' another actor to get notified about it's termination.

It is also possible to specify timeouts for the ask-s and ask functionality.

This new version is closer to Akka (the actor model framework on the JVM) than to GenServer on Erlang. This is because Common Lisp from a runtime perspective is closer to JVM than to Erlang/OTP. Threads in Common Lisp are heavy weight OS threads rather than user-space low weight 'Erlang' threads (I'd like to avoid 'green threads', because threads in Erlang are not really green threads). While on Erlang it is easily possible to spawn millions of processes/threads and so each actor (GenServer) has its own process, this model is not possible when the threads are OS threads, because of OS resource limits. This is the main reason for working with the message dispatcher pool instead.

But let's jump right into it. I'll explain more later.

Getting hands-on

Creating an actor-system

To use the shared dispatcher pool we have to create an actor-system first.

(defvar *system* (asys:make-actor-system))

When we eval *system* in the repl we see a bit of the structure:

#<ACTOR-SYSTEM shared-workers: 4, user actors: 0, internal actors: 0>

So the actor-system has by default four shared message dispatcher workers. Depending on how busy the system tends to be this default can of course be increased.

An optional configuration can be passed to the actor-system factory function. See API documentation.

  1. Shutting down the system

    Shutting down an actor system may be necessary depending on how it's used. It can be done by:

    (ac:shutdown *system*)

    This will stop all dispatcher workers and all other actors that have been spawned in the system.

Creating actors

Actors kind of live within an actor-context. An actor-context contains a collection (of actors) and defines a Common Lisp protocol that defines a set of generic functions for creating, removing and finding actors in an actor-context.

There are two 'things' that host an actor-context. This is:

  1. the actor-system. Creating actors on the actor-system will create root actors.
  2. the actor. Creating actors on the context of an actor will create a child actor.

Let's create an actor.

(ac:actor-of *system* :name "answerer"
  (lambda (self msg state)
    (let ((output (format nil "Hello ~a" msg)))
      (format t "~a~%" output)
      (cons output state))))

This creates a root actor on the *system*. Notice that the actor is not assigned to a variable. It is now registered in the system. The :receive key argument to the actor-of macro is a function which does the main message processing of an actor. The parameters to the 'receive' function are the tuple:

  1. self - the instance of the actor
  2. msg - the received message of when this 'receive' function is called
  3. state - the current state of the actor

actor-of also allows to specify the initial state by using the :state key, a name, and a custom actor type. By default a standard actor of type 'actor is created. But you can subclass 'actor and specify your own. It is also possible to add 'after initialization' code using the :init key which takes a lambda with the actor instance as parameter.

The return value of the 'receive' function should also be familiar. It is the cons with car being sent back to sender (in case of ask/ask-s) and cdr set as the new state of the actor.

The actor-of macro still returns the actor as can be seen on the repl when this is executed. So it is of course possible to store the actor in a dynamic or lexical context. However, when the lexical context ends, the actor will still live as part of the actor context/system.

Here we see a few details of the actor. Among which is the name and also the type of message-box it uses. By default it is a message-box/dp which is the type of a shared message dispatcher message-box.

#<ACTOR answerer, running: T, state: NIL, message-box: #<MESSAGE-BOX/DP mesgb-9541, processed messages: 0, max-queue-size: 0, queue: #<QUEUE-UNBOUNDED #x3020029918FD>>>

Had we stored the actor to a variable, say *answerer* we can create a child actor of that by doing:

(ac:actor-of *answerer* :name "child-answerer"
    (lambda (self msg state)
        (let ((output (format nil "~a" "Hello-child ~a" msg)))
            (format t "~a~%" output)
            (cons output state))))

This will create a new actor on the context of the parent actor. The context can be specified with just the parent actor instance *answerer*.

Dispatchers :pinned vs. :shared

Dispatchers are somewhat alike thread pools. Dispatchers of the :shared type are a pool of workers. Workers are actors using a :pinned dispatcher. :pinned just means that an actor spawns its own mailbox thread.

So :pinned and :shared are types of dispatchers. :pinned spawns its own mailbox thread, :shared uses a worker pool to handle the mailbox messages.

By default an actor created using actor-of uses a :shared dispatcher type which uses the shared message dispatcher that is automatically setup in the system.
When creating an actor it is possible to specify the dispatcher-id. This parameter specifies which 'dispatcher' should handle the mailbox queue/messages.

Please see below for more info on dispatchers.

Finding actors in the context

If actors are not directly stored in a dynamic or lexical context they can still be looked up and used. The actor-context protocol contains a function find-actors which works like this:

(first (ac:find-actors *system* "answerer"))

See additional parameters and options here: API documentation

#<ACTOR answerer, running: T, state: NIL, message-box: #<MESSAGE-BOX/DP mesgb-9687, processed messages: 0, max-queue-size: 0, queue: #<QUEUE-UNBOUNDED #x30200263C95D>>>

This function only does a simple flat search. The functionality of looking up an actor in the system generally will be expanded upon.

tell, ask-s and ask

Let's send some messages.


tell is a fire-and-forget kind of send type. It doesn't expect a result in return.

And because of that, and in order to demonstrate it does something, it has to have a side-effect. So it dumps some string to the console using format, because we couldn't otherwise tell if the message was received and processed (see the *answerer* actor definitions above).

CL-USER> (act:tell *answerer* "Foo")
Hello Foo

So we see that tell returns immediately with T. But to see the 'Hello Foo' it takes another hit on the return key, because the REPL is not asynchronous.

tell with sender

tell accepts a 'sender', which has to be an actor. So we can do like this:

CL-USER> (act:tell *child-answerer* "Foo" *answerer*)
Hello-child Foo
Hello Hello-child Foo

This sends "Foo" to *child-answerer*, but *child-answerer* sends the response to *answerer*. So we see outputs of both actors.


ask-s blocks until the message was processed by the actor. This call returns the car part of the cons return of the behavior function. Insofar an ask-s call is more resource intensive than just a tell.

(act:ask-s *answerer* "Bar")

Will respond with: 'Hello Bar'


ask combines both ask-s and tell. From ask-s it 'inherits' returning a result, even though it's a future result. Internally it is implemented using tell. In order to wait for a result a temporary actor is spawned that waits until it receives the result from the actor where the message was sent to. With this received result the future is fulfilled. So ask is async, it returns immediately with a future. That future can be queried until it is fulfilled. Better is though to setup an on-completed handler function on it.

So we can do:

          (act:ask *answerer* "Buzz")
          (lambda (result)
            (format t "Received result: ~a~%" result)))

Well, one step at a time:

(act:ask *answerer* "Buzz")

Returns with:

#<FUTURE promise: #<PROMISE finished: NIL errored: NIL forward: NIL #x302002EAD6FD>>

Then we can setup a completion handler on the future:

          (lambda (result)
            (format t "Received result: ~a~%" result)))

Remember '*' is the last result in the REPL which is the future here.

This will print after a bit:

Hello Buzz
Received result: Hello Buzz

ask-s and ask with timeout

A timeout (in seconds) can be specified for both ask-s and ask and is done like so:

To demonstrate this we could setup an example 'sleeper' actor:

(ac:actor-of *system* 
    (lambda (self msg state)
        (sleep 5)))

If we store this to *sleeper* and do the following, the ask-s will return a handler-error with an ask-timeout condition.

(act:ask-s *sleeper* "Foo" :time-out 2)

This works similar with the ask only that the future will be fulfilled with the handler-error cons.

To get a readable error message of the condition we can do:

CL-USER> (format t "~a" (cdr *))
A timeout set to 2 seconds occurred. Cause: 

Note that ask-s uses the calling thread for the timeout checks.
ask uses a wheel timer to handle timeouts. The default resolution for ask timeouts is 500ms with a maximum size of wheel slots (registered timeouts) of 1000. What this means is that you can have timeouts of a multiple of 500ms and 1000 ask operations with timeouts. This default can be tweaked when creating an actor-system, see API documentation for more details.

Long running operations in receive

Be careful with doing long running computations in the receive function message handler, because it will block message processing. It is advised to use a third-party thread-pool or a library like lparallel to do the computations with and return early from the receive message handler.

Considering the required cons return result of the receive function, in case a result computation is delegated to a thread-pool the receive function should return with (cons :no-reply <state>). The :no-reply will instruct the actor to not send a result to a sender automatically should a sender be available (for the cases of tell or ask). The computation result can be 'awaited' for in an asynchronous manner and a response to *sender* can be sent manually by just doing a (tell *sender* <my-computation-result>). The sender of the original message is set to the dynamic variable *sender*.

Due to an asynchronous callback of a computation running is a separate thread, the *sender* must be copied into a lexical environment because at the time of when the callback is executed the *sender* can have a different value.

This behavior must be part of the messaging protocol that is being defined for the actors at play.

Changing behavior

An actor can change behavior. The behavior is just a lambda that has to take three parameters:

  1. the actor's instance - usually called self
  2. the received message - maybe call msg?
  3. the current state of the actor

The behavior then can pattern match (or do some matching by other means) on the received message alone, or in combination with the current state.

The default behavior of the actor is given on actor construction using the default constructor make-actor.

During the lifetime of an actor the behavior can be changed using become.

So we remember the *answerer* which responds with 'Hello Foo' when we send (act:ask-s *answerer* "Foo"). We can now change the behavior with:

(act:become *answerer* 
            (lambda (self msg state)
              (cons (format nil "my new behavior for: ~a" msg) state)))

When we now send (act:ask-s *answerer* "Foo") we will get the response: 'my new behavior for: Foo'.

Reverting become / unbecome

To revert back to the default behavior as defined by the receive function of the constructor you may call unbecome.

Creating actors without a system

It is still possible to create actors without a system. This is how you do it:

;; make an actor
(defvar *my-actor* (act:make-actor (lambda (self msg state)
                                     (cons "Foo" state))
                                   :name "Lone-actor"))
;; setup a thread based message box
(setf (act-cell:msgbox *my-actor*) 
      (make-instance 'mesgb:message-box/bt))

You have to take care yourself about stopping the actor and freeing resources.


An Agent is a specialized Actor. It is meant primarily for maintaining state and comes with some conveniences to do that.

To use an Agent import sento.agent package.

There is no need to subclass an Agent. Rather create a facade to customize an agent. See below.

An Agent provides three functions to use it.

  • make-agent creates a new agent. Optionally specify an actor-context or define the kind of dispatcher the agent should use.
  • agent-get retrieves the current state of the agent. This directly delivers the state of the agent for performance reasons. There is no message handling involved.
  • agent-update updates the state of the agent
  • agent-update-and-get updates the agent state and returns the new state.

All four take a lambda. The lambda for make-agent does not take a parameter. It should return the initial state of the agent. agent-get and agent-update both take a lambda that must support one parameter. This parameter represents the current state of the agent.

Let's make a simple example:

First create an agent with an initial state of 0.

(defparameter *my-agent* (make-agent (lambda () 0)))

Now update the state several times (agent-update is asynchronous and returns t immediately):

(agent-update *my-agent* (lambda (state) (1+ state)))

Finally get the state:

(agent-get *my-agent* #'identity)

This agent-get just uses the identity function to return the state as is.

So this simple agent represents a counter.

It is important to note that the retrieves state, i.e. with identity should not be modified outside the agent.

Using an agent within an actor-system

The make-agent constructor function allows to provides an optional system argument that, when given, makes the constructor create the agent within the given actor-system. This implies that the systems shared messages dispatcher is used for the agent and no separate thread is created for the agents message box.

It also implies that the agent is destroyed then the actor-system is destroyed.

However, while actors can create hierarchies, agents can not. Also the API for creating agents in systems is different to actors. This is to make explicit that agents are treated slightly differently than actors even though under the hood agents are actors.

Wrapping an agent

While you can use the agent as in the example above it is usually advised to wrap an agent behind a more simple facade that doesn't work with lambdas.

For example could a facade for the counter above look like this:

(defvar *counter-agent* nil)

(defun init-agent (initial-value)
  (setf *counter-agent* (make-agent (lambda () initial-value))))

(defun increment () (agent-update *counter-agent* #'1+))
(defun decrement () (agent-update *counter-agent* #'1-))
(defun counter-value () (agent-get *counter-agent* #'identity))

Alternatively, one can wrap an agent inside a class and provide methods for simplified access to it.


A Router is a facade over a set of actors. Routers are either created with a set of actors using the default constructor router:make-router or actors can be added later.

Routers implement part of the actor protocol, so it allows to use tell, ask-s or ask which it forwards to a 'routee' (one of the actors of a router) by passing all of the given parameters. The routee is chosen by applying a strategy. The built-in default strategy a routee is chosen randomly.

The strategy can be configured when creating a router using the constructors &key parameter :strategy. The strategy is just a function that takes the number of routees and returns a routee index to be chosen for the next operation.

Currently available strategies: :random and :round-robin.

Custom strategies can be implemented.



A :shared dispatcher is a separate facility that is set up in the actor-system. It consists of a configurable pool of 'dispatcher workers' (which are in fact actors). Those dispatcher workers execute the message handling in behalf of the actor and with the actors message handling code. This is protected by a lock so that ever only one dispatcher will run code on an actor. This is to ensure protection from data race conditions of the state data of the actor (or other slots of the actor).

Using this dispatcher allows to create a large number of actors. The actors as such are generally very cheap.


The :pinned dispatcher is represented by a thread that operates on the actors message queue. It handles one message after the other with the actors message handling code. This also ensures protection from data race conditions of the state of the actor.

This variant is slightly faster (see below) but requires one thread per actor.

custom dispatcher

It is also possible to create additional dispatcher of type :shared. A name can be freely chosen, but by convention it should be a global symbol, i.e. :my-dispatcher.

When creating actors using act:actor-of, or when using the tasks api it is possible to specify the dispatcher (via the 'dispatcher-id' i.e. :my-dispatcher) that should handle the actor, agent, or task messages.

A custom dispatcher is in particular useful when using tasks for longer running operations. Longer running operations should not be used for the :shared dispatcher because it (by default) is responsible for the message handling of most actors.


The eventstream allows messages (or events) to be posted on the eventstream in a fire-and-forget kind of way. Actors can subscribe to the eventstream if they want to get notified for particular messages or generally on all messages posted.
This allows to create event-based systems.

Here is a simple example:

(defparameter *sys* (asys:make-actor-system))

(ac:actor-of *sys* :name "listener"
  :init (lambda (self)
          (ev:subscribe self self 'string))
  :receive (lambda (self msg state)
               ((string= "my-message" msg)
                (format t "received event: ~a~%" msg)))
             (cons :no-reply state)))

(ev:publish *sys* "my-message")

This subscribes to all 'string based events and just prints the message when received.
The subscription here is done using the :init hook of the actor. The ev:subscribe function requires to specify the eventstream as first argument. But there are different variants of the generic function defined which allows to specofy an actor directly. The eventstream is retrieve from the actor through its actor-context.

received event: my-message

See the API documentation for more details.


'tasks' is a convenience package that makes dealing with asynchronous and concurrent operations very easy.

Here is a simple example:

(defparameter *sys* (make-actor-system))

(with-context (*sys*)
  // run something without requiring a feedback
  (task-start (lambda () (do-lengthy-IO))
  // run asynchronous - with await
  (let ((task (task-async (lambda () (do-a-task)))))
    // do some other stuff
    // eventually we need the task result
    (+ (task-await task) 5))
  // run asynchronous with completion-handler (continuation)
  (task-async (lambda () (some-bigger-computation))
              (lambda (result)
                (do-something-with result)))

  // concurrently map over the given list
    '(1 2 3 4 5)
    (task-async-stream #'1+)
    (reduce #'+)))

=> 20 (5 bits, #x14, #o24, #b10100)

All functions available in 'tasks' package require to be wrapped in a with-context macro. This macro removes the necessity of an additional argument to each of the functions which is instead supplied by the macro.

What happens in this example is that the list '(1 2 3 4 5) is passed to task-async-stream. task-async-stream then spawns a 'task' for each element of the list and applies the given function (here 1+) on each list element. The function though is executed by a worker of the actor-systems :shared dispatcher. task-async-stream then also collects the result of all workers. In the last step (reduce) the sum of the elements of the result list are calculated.

It is possible to specify a second argument to the with-context macro to specify the dispatcher that should be used for the tasks.
The concurrency here depends on the number of dispatcher workers.

Be also aware that the :shared dispatcher should not run long running operations as it blocks a message processing thread. Create a custom dispatcher to use for tasks when you plan to operate longer running operations.

See the API documentation for more details.


Some words on immutability. sento does not make deep copies of the actor states. So whatever is returned from receive function as part of the (cons back-msg state) is just setfed to the actor state. The user is responsible to make deep copies if necessary in an immutable environment. The user is responsible to not implictly modify the actor state outside of the actor.


sento does its own logging using different log levels from 'trace' to 'error'. It uses log4cl. If you wish to also use log4cl in your application but find that sento is too noisy in debug and trace logging you can change the log level for the 'sento package only by:

(log:config '(sento) :warn)

This will tell log4cl to do any logging for sento in warn level.


Hardware specs:

  • iMac Pro (2017) with 8 Core Xeon, 32 GB RAM


The benchmark was created by having 8 threads throwing each 125k (1m alltogether) messages at 1 actor. The timing was taken for when the actor did finish processing those 1m messages. The messages were sent by either all tell, ask-s, or ask to an actor whose message-box worked using a single thread (:pinned) or a dispatched message queue (:shared / dispatched) with 8 workers.

Of course a tell is in most cases the fastest one, because it's the least resource intensive and there is no place that is blocking in this workflow.

SBCL (v2.0.10)

Even though SBCL is by far the fastest one with tell on both :pinned and dispatched, it had massive problems on dispatched - ask-s where I had to lower the number of messages to 200k alltogether. Beyond that value SBCL didn't get it worked out.

LispWorks (8.0)

LispWorks is fast overall. Not as fast as SBCL. But it seems the GC is more robust, in particular on the dispatched - ask.

CCL (v1.12)

CCL is on acceptable average speed. The problems CCL had was heap exhaustion for both the ask tasks where the number of messages had to be reduced to 80k. Which is not a lot. Beyond this value the runtime would crash. However, CCL for some reason had no problems where SBCL was struggling with the dispatched - ask-s.

ABCL (1.8)

The pleasant surprise was ABCL. While not being the fastest it is the most robust. Where SBCL and CCL were struggling you could throw anything at ABCL and it'll cope with it. I'm assuming that this is because of the massively battle proven Java Runtime.

Dependencies (13)

  • alexandria
  • atomics
  • binding-arrows
  • blackbird
  • bordeaux-threads
  • cl-mock
  • cl-speedy-queue
  • cl-str
  • fiveam
  • log4cl
  • lparallel
  • mgl-pax
  • timer-wheel

Dependents (0)

    • GitHub
    • Quicklisp