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.
- Actors with
askcan be asynchronous or synchronous.
- Agents: Agents are a specialization of Actors for wrapping state with a standardized interface of
set. There are also specialized Agents for CLs array and hash-map.
- Router: Router offers a similar interface as Actor with
tellbut 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 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
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
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
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
async-ask has been renamed to
The proposed default way to query for a result from another actor should
be an asynchronous
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
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
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.
Creating an actor-system
To use the shared dispatcher pool we have to create an
(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>
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.
Shutting down the system
Shutting down an actor system may be necessary depending on how it's used. It can be done by:
This will stop all dispatcher workers and all other actors that have been spawned in the system.
Actors kind of live within 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
There are two 'things' that host an
actor-system. Creating actors on the
actor-systemwill create root actors.
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" :receive (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:
self- the instance of the actor
msg- the received message of when this 'receive' function is called
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
car being sent back to sender (in case of ask/ask-s) and
cdr set as the new state of the actor.
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
can create a child actor of that by doing:
(ac:actor-of *answerer* :name "child-answerer" :receive (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
Dispatchers are somewhat alike thread pools. Dispatchers of the
:shared type are a pool of workers. Workers are actors using a
:pinned just means that an actor spawns its own mailbox thread.
: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
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
format, because we couldn't otherwise
the message was received and processed (see the
*answerer* actor definitions above).
CL-USER> (act:tell *answerer* "Foo") T CL-USER> Hello Foo
So we see that
tell returns immediately with
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*) T CL-USER> Hello-child Foo Hello Hello-child Foo
This sends "Foo" to
sends the response to
*answerer*. So we see outputs of both
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
(act:ask-s *answerer* "Bar")
Will respond with: 'Hello Bar'
ask combines both
ask-s it 'inherits' returning
a result, even though it's a future result. Internally it is
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 can be queried until it is fulfilled. Better is
though to setup an
on-completed handler function on it.
So we can do:
(future:on-completed (act:ask *answerer* "Buzz") (lambda (result) (format t "Received result: ~a~%" result)))
Well, one step at a time:
(act:ask *answerer* "Buzz")
#<FUTURE promise: #<PROMISE finished: NIL errored: NIL forward: NIL #x302002EAD6FD>>
Then we can setup a completion handler on the future:
(future:on-completed * (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 and is done like so:
To demonstrate this we could setup an example 'sleeper' actor:
(ac:actor-of *system* :receive (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
(act:ask-s *sleeper* "Foo" :time-out 2)
(:HANDLER-ERROR . #<CL-GSERVER.UTILS:ASK-TIMEOUT #x30200319F97D>)
This works similar with the
ask only that the future will
be fulfilled with the
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: #<BORDEAUX-THREADS:TIMEOUT #x302002FAB73D>
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
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
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
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
Due to an asynchronous callback of a computation running is a separate
*sender* must be copied into a lexical environment because
at the time of when the callback is executed the
*sender* can have a
This behavior must be part of the messaging protocol that is being defined for the actors at play.
An actor can change behavior. The behavior is just a lambda that has to take three parameters:
- the actor's instance - usually called
- the received message - maybe call
- 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
During the lifetime of an actor the behavior can be changed using
So we remember the
*answerer* which responds with 'Hello Foo' when
(act:ask-s *answerer* "Foo"). We can now change the behavior
(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'.
To revert back to the default behavior as defined by the
receive function of the constructor you may call
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
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-agentcreates a new agent. Optionally specify an
actor-contextor define the kind of dispatcher the agent should use.
agent-getretrieves the current state of the agent. This directly delivers the state of the agent for performance reasons. There is no message handling involved.
agent-updateupdates the state of the agent
agent-update-and-getupdates 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-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
(defparameter *my-agent* (make-agent (lambda () 0)))
Now update the state several times (
agent-update is asynchronous and
(agent-update *my-agent* (lambda (state) (1+ state)))
Finally get the state:
(agent-get *my-agent* #'identity)
agent-get just uses the
identity function to return the state
So this simple agent represents a counter.
It is important to note that the retrieves state, i.e. with
should not be modified outside the agent.
Using an agent within an actor-system
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.
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
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
strategy can be configured when creating a router using
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:
Custom strategies can be implemented.
: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.
: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.
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.
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) (cond ((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)) :on-complete-fun (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 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
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.
- 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
an actor whose message-box worked using a single thread
:pinned) or a dispatched message queue
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.
Even though SBCL is by far the fastest one with
dispatched, it had massive
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 is fast overall. Not as fast as SBCL. But it seems the GC is more robust, in particular on the
dispatched - ask.
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.
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.