Overview

Pulsar is a Clojure library that provides high-performance lightweight threads and Erlang-like actors. It is a Clojure API for the Quasar Java library, with the addition of pattern matching and an Erlang-like syntax.

A good introduction to Pulsar (and Quasar) can be found in the blog post Erlang (and Go) in Clojure (and Java), Lightweight Threads, Channels and Actors for the JVM.

Pulsar and Quasar are developed by Parallel Universe and released as free software, dual-licensed under the Eclipse Public License and the GNU Lesser General Public License.

Dependencies

Aside from Pulsar’s dependency on Quasar and its dependent libraries, Pulsar makes use of the following open-source projects:

core.match - A pattern matching library for Clojure.

Gloss, by Zach Tellman - a byte-format DSL

News

August 7, 2016

Pulsar 0.7.6 has been released.

May 2, 2016

Pulsar 0.7.5 has been released.

January 18, 2016

Pulsar 0.7.4 has been released.

August 28, 2015

Pulsar 0.7.3 has been released.

June 25, 2015

Pulsar 0.7.2 has been released.

May 29, 2015

Pulsar 0.7.0 has been released.

December 23, 2014

Pulsar 0.6.2 has been released.

September 23, 2014

Pulsar 0.6.1 has been released.

July 23, 2014

Pulsar 0.6.0 has been released.

March, 2014

Pulsar 0.5.0 has been released.

January 22, 2014

Pulsar 0.4.0 has been released.

July 26, 2013

Distributed actors in Pulsar.

July 19, 2013

Quasar/Pulsar 0.2.0 has been released.

May 2, 2013

Introductory blog post: Erlang (and Go) in Clojure (and Java), Lightweight Threads, Channels and Actors for the JVM.

Getting Started

System Requirements

Java 7 and Clojure 1.5 are required to run Pulsar.

Using Leiningen

Add the following dependency to Leiningen’s project.clj:

[ co.paralleluniverse/pulsar "0.7.6" ]

Then, the following must be added to the project.clj file:

:java-agents [[ co.paralleluniverse/quasar-core "0.7.6" ]]

or, add the following to the java command line:

-javaagent:path-to-quasar-jar.jar

Building Pulsar

Clone the repository:

git clone git://github.com/puniverse/pulsar.git pulsar

and run:

lein midje

To build the documentation, you need to have Jekyll installed. Then run:

jekyll build

To generate the API documentation run

lein doc

You can run the examples like this:

lein -o run -m co.paralleluniverse.pulsar.examples.pingpong

For benchmarks, you should use lein trampoline , like so:

lein trampoline run -m co.paralleluniverse.pulsar.examples.ring-benchmark 1000 1000

User Manual

Pulsar Core

Quasar and Pulsar

Pulsar is a Clojure API to Quasar. Many of the concepts explained below are actually implemented in Quasar.

Fibers

Fibers are lightweight threads. They provide functionality similar to threads, and a similar API, but they’re not managed by the OS. They are lightweight (an idle fiber occupies ~400 bytes of RAM), and you can have millions of them in an application. If you are familiar with Go, fibers are like goroutines. Fibers in Pulsar (well, Quasar, actually) are scheduled by one or more ForkJoinPools.

One significant difference between Fibers and Threads is that Fibers are not preempted; i.e. a fiber is (permanently or temporarily) unscheduled by the scheduler only if it terminates, or if it calls one of a few specific Java methods that cause the fiber to become suspended. A function that calls a suspending operation is called a suspendable function, and a function that calls another suspendable function is itself suspendable.

Suspendable functions require special bytecode instrumentation (performed by an instrumentation agent), so they must be explicitly designated as such. The function suspendable! marks a given function as a suspendable function (this operation cannot be undone). The defsfn macro, with the same syntax as defn defines a suspendable function.

Note: All functions (i.e. fn s) passed to any of the Pulsar API functions and macros are automatically made suspendable, so in most simple cases you will never need to use sfn , defsfn or suspendable! .

Automatic instrumentation

Since version 0.6.3, Pulsar supports experimental automatic instrumentation of all Clojure code, so that suspendable! , sfn and defsfn are not required anymore. It can be enabled by setting the -Dco.paralleluniverse.pulsar.instrument.auto=all system property. Automatic instrumentation needs the ability to instrument some synchronized Clojure runtime methods, so the agent should be run with the =m options.

According to our benchmarks, the performance impact of automatic instrumentation should not exceed 20% in worst-case situations like articulated and math-intensive Clojure code, while in more common-case scenarios the slow-down should not be noticeable at all.

Spawning Fibers

To create a fiber that will start executing body , use

( fiber <body> )

Alternatively, to create a fiber of a function f that takes arguments arg1 and arg2 , run

( spawn-fiber f arg1 arg2 )

spawn-fiber automatically marks f as suspendable, so there’s no need to do so explicitly.

fiber and spawn-fiber takes optional keyword arguments:

:name - The fiber’s name.

- The fiber’s name. :fj-pool - The ForkJoinPool in which the fiber will run. If :fj-pool is not specified, then the pool used will be either the pool of the fiber calling spawn-fiber , or, if spawn-fiber is not called from within a fiber, a default pool.

- The in which the fiber will run. If is not specified, then the pool used will be either the pool of the fiber calling , or, if is not called from within a fiber, a default pool. :stack-size - The initial fiber data stack size.

The fiber will terminate when f completes execution.

Note: Spawning a fiber is a very cheap operation in both computation and memory. Do not fear creating many (thousands, tens-of-thousands or even hundereds-of-thousands) fibers.

Joining Fibers

To wait for the fiber’s termination, use

( join fiber )

If f returns a value, join will return that value. If f throws an exception, join will throw that exception.

You can also wait for a fiber’s termination for a given duration. The following will wait for half a second for the fiber to terminate:

( join 500 java.util.concurrent.TimeUnit/MILLISECONDS fiber )

The following will have the same effect:

( join 500 :ms fiber )

Bindings

Fibers behave well with Clojure bindings . A newly spawned fiber inherits the bindings in effect at the time of spawning, and bindings declared in a fiber last throughout the fiber’s lifetime. This is demonstrated in the following tests taken from the Pulsar test suite:

( def ^ :dynamic *foo* 40 ) ( facts "fiber-bindings" ( fact "Fiber inherits thread bindings" ( let [ fiber ( binding [ *foo* 20 ] ( spawn-fiber # ( let [ v1 *foo* ] ( Fiber/sleep 200 ) ( let [ v2 *foo* ] ( + v1 v2 )))))] ( join fiber )) => 40 ) ( fact "Bindings declared in fiber last throughout fiber lifetime" ( let [ fiber ( spawn-fiber # ( binding [ *foo* 15 ] ( let [ v1 *foo* ] ( Fiber/sleep 200 ) ( let [ v2 *foo* ] ( + v1 v2 )))))] ( join fiber )) => 30 ))

Compatibility with Clojure Concurrency Constructs

Code running in fibers may make free use of Clojure atoms and agents.

Spawning or dereferencing a future created with clojure.core/future is ok, but there’s a better alternative: you can turn a spawned fiber into a future with fiber->future and can then dereference or call regular future functions on the returned value, like realized? (In fact, you don’t even have to call fiber->future ; fibers already implement the Future interface and can be treated as futures directly, but this may change in the future, so, until the API is fully settled, we recommend using fiber->future ).

Running a dosync block inside a fiber is discouraged as it uses locks internally, but your mileage may vary.

Promises are supported and encouraged, but you should not make use of clojure.core/promise to create a promise that’s to be dereferenced in a fiber. Pulsar provides a different – yet completely compatible – form of promises, as you’ll see soon.

Transforming any Asynchronous Callback to A Fiber-Blocking Operation

Fibers are great as a replacement for callbacks. The await macro helps us easily turn any callback-based asynchronous operation to as simple fiber-blocking call. await assumes that an asynchronous function takes a callback of a single argument as its last parameter; await then blocks the current fiber until the callback is called, and the returns the value passed to the callback.

Here’s an example from the tests:

( let [ exec ( java.util.concurrent.Executors/newSingleThreadExecutor ) service ( fn [ a b clbk ] ; an asynchronous service ( .execute exec ^ Runnable ( fn [] ( sleep 50 ) ( clbk ( + a b ))))) fiber ( spawn-fiber ( fn [] ( await service 2 5 )))] ( join fiber )) ; => 7

Strands

Before we continue, one more bit of nomenclature: a single flow of execution in Quasar/Pulsar is called a strand. To put it more simply, a strand is either a normal JVM thread, or a fiber.

The strand abstraction helps you write code that works whether it runs in a fiber or not. For example, (Strand/currentStrand) returns the current fiber, if called from a fiber, or the current thread, if not. (Strand/sleep millis) suspends the current strand for a given number of milliseconds whether it’s a fiber or a normal thread. Also, join works for both fibers and threads (although for threads join will always return nil ).

Promises, Promises

Promises, also known as dataflow variables, are an especially effective, and simple, concurrency primitive.

A promise is a value that may only be set once, but read multiple times. If the promise is read before the value is set, the reading strand will block until the promise has been set, and then return its value.

Promises are defined in clojure.core , but ...pulsar.core provides its own, fully compatible version.

A promise is created simply like this:

( promise )

And is set with deliver . It can be read by dereferencing it with @ , and you can test whether it’s been set with realized? (other than the promise function itself, all other functions, like deliver and realized are those defined in clojure.core )

The promise function defined in Pulsar creates a promise, that, when dereferenced within a fiber, simply blocks the fiber and not the entire OS thread it’s running in.

Here’s an example of using promises from the tests:

( let [ v1 ( promise ) v2 ( promise ) v3 ( promise ) v4 ( promise ) f1 ( spawn-fiber # ( deliver v2 ( + @ v1 1 ))) t1 ( spawn-thread # ( deliver v3 ( + @ v1 @ v2 ))) f2 ( spawn-fiber # ( deliver v4 ( + @ v3 @ v2 )))] ( Strand/sleep 50 ) ( deliver v1 1 ) @ v4 ) ; => 5

This example shows how promises are set, and read, by both fibers and regular threads.

But Pulsar’s promises have one additional, quite nifty, feature. If you pass an optional function to promise , a new fiber running that function will be spawned, and the promise will receive the value returned from the function. Here’s another example from the tests:

( let [ v0 ( promise ) v1 ( promise ) v2 ( promise # ( + @ v1 1 )) v3 ( promise # ( + @ v1 @ v2 )) v4 ( promise # ( * ( + @ v3 @ v2 ) @ v0 ))] ( Strand/sleep 50 ) ( deliver v1 1 ) ( mapv realized? [ v0 v1 v2 v3 v4 ]) ; => [false true true true false] ( deliver v0 2 ) @ v4 ) ; => 10

Channels

Channels are queues used to pass messages between strands (remember, strands are a general name for threads and fibers). If you are familiar with Go, Pulsar channels are like Go channels. The call

( channel )

creates and returns a new channel.

A more general form of the channel function is:

( channel capacity overflow-policy single-producer? single-consumer? )

The channel’s capacity is the number of messages that can wait in the queue. A positive integer creates a bounded queue that can hold up to the given number of messages until they’re consumed. A capacity of -1 specifies an unbounded channel (unlimited number of pending messages), and a capacity of 0 specifies a transfer channel, one where the producer is blocked until a consumer requests a message and vice-versa.

overflow-policy specifies what happens to the producer (sender) of a message when the channel’s capacity is exhausted, and may be one of:

:throw - throws an exception

- throws an exception :drop - silently drops (discards) the message

- silently drops (discards) the message :block - blocks the sender until messages are consumed from the channel and it has remaining capacity

- blocks the sender until messages are consumed from the channel and it has remaining capacity :displace - removes the oldest message waiting in the channel to make room for the new message.

If you leave out the overflow-policy argument, the default policy of :block is used. Leaving both out (and simply calling (channel) is the same as (channel 0 :block) (obviously, a transfer channel (a channel of capacity 0), would only work with a :block policy).

Bounded channels are generally faster than unbounded channels.

Use of the :displace policy places an additional restriction on the channel: its messages may be consumed by a single strand only.

Finally you can specify if the channel will be single-producer (default: false ) and/or single-consumer (default: true ).

Sending and receiving messages

Sending a message to a channel is simple:

( snd channel message )

message can be any object, but not nil . Receiving a message from a channel is equally easy:

( rcv channel )

The rcv function returns the first message in the channel (the one that has waited there the longest), if there is one. If the channel is empty, the function will block until a message is sent to the channel, and will then return it.

Note: rcv is a suspendable function, so any function calling it must also be declared suspendable. But remember, the function passed to spawn-fiber is automatically made suspendable.

It is also possible to limit the amount of time rcv will wait for a message:

( rcv channel 10 java.util.concurrent.TimeUnit/MILLISECONDS )

or, equivalently:

( rcv channel 10 :ms )

These calls will wait for a message for 10 milliseconds before giving up and returning nil .

Closing the channel

After calling

( close! channel )

any future messages sent to the channel will be ignored. Any messages already in the channel will be received. Once the last message has been received, another call to rcv will return nil .

Channel Selection – sel and select

A powerful tool when working with channels is the ability to wait on a message from several channels at once.

The sel function takes a collection containing channel operation descriptors. A descriptor is either a channel or a pair (vector) of a channel and a message. Each channel in the sequence represents a rcv attempt, and each channel-message pair represents a snd attempt. The sel function performs at most one operation on the sequence, a rcv or a snd , which is determined by the first operation that can succeed. If no operation can be carried out immediately, sel will block until an operation can be performed.

For example, in the following call,

( sel [ ch1 [ ch2 msg1 ] ch3 [ ch4 msg2 ]])

a message will either be received from ch1 or ch3 , or one will be sent to either ch2 or ch4 . If, for instance, ch1 will become available for reading (i.e. it has been sent a message) first, than only a rcv will be performed on it. If ch2 becomes available for writing before that happens, then only that operation, a snd , will be performed. If two operations are available at the same time, one will be chosen randomly (unless the :priority option is set, as we’ll see later).

Note that if a channel’s overflow policy is anything by :block , then snd operations are always available.

The general form of the sel function is

( sel ports & opts )

sel returns a vector of two values describing the single operation that has been performed. The first is the message received if the operation is a rcv , or nil if it’s a snd ; the second is the channel on which the operation has been performed.

The sel function takes two options. If :priority is set to true (thus: :priority true ), then if more than one operation becomes available at the same time, then the one that’s listed earlier in the channels collection will be performed.

The second option is :timeout , which takes an integer argument specifying the timeout in milliseconds. If the timeout elapses without any of the operations succeeding, sel will return nil . If the timeout value is 0 , then sel will never block. It will attempt to perform any of the requested operations, but if none are immediately available, it will return nil .

So, for example, calling

( sel [ ch1 ch2 ch3 ] :timeout 0 )

Will return, [msg ch] if any of the channels was immediately available for a rcv , or nil if none of them were.

The select macro performs a very similar operation as sel , but allows you to specify an action to perform depending on which operation has succeeded. It takes an even number of expressions, ordered as (ops1, action1, ops2, action2 …) with the ops being a channel operation descriptor (remember: a descriptor is either a channel for an rcv operation, or a vector of a channel and a message specifying a snd operation) or a collection of descriptors, and the actions are Clojure expressions. Like sel , select performs at most one operation, in which case it will run the operation’s respective action and return its result.

An action expression can bind values to the operations results. The action expression may begin with a vector of one or two symbols. In that case, the first symbol will be bound to the message returned from the successful receive in the respective ops clause (or nil if the successful operation is a snd ), and the second symbol, if present, will be bound to the successful operation’s channel.

Like sel , select blocks until an operation succeeds, or, if a :timeout option is specified, until the timeout (in milliseconds) elapses. If a timeout is specified and elapses, select will run the action in an optional :else clause and return its result, or, if an :else clause is not present, select will return nil .

Here’s an example:

( select :timeout 100 c1 ([ v ] ( println "received" v )) [[ c2 m2 ] [ c3 m3 ]] ([ v c ] ( println "sent to" c )) :else "timeout!" )

In the example, if a message is received from channel c1 , then it will be printed. If a message is sent to either c2 or c3 , then the identity of the channel will be printed, and if the 100 ms timeout elapses then “timeout!” will be printed.

Finally, just like sel , you can pass :priority true to select , in which case if more than one operation is available, the first one among them as listed in the select statement will be performed.

Topics

A topic is a send-port (a channel you can send to but not receive from), that broadcasts any message written to it to a number of subscriber channels.

A topic is created simply with:

( topic )

When a channel subscribes to the topic, it will receive all messages sent to the topic:

( subscribe! tpc ch )

You can also unsubscribe a channel:

( subscribe! tpc ch )

Note that a messages sent to the topic is essentially replicated to all subscribers, i.e. it will be received once in each channel.

Ticker Channels

A channel created with the :displace policy is called a ticker channel because it provides guarantees similar to that of a digital stock-ticker: you can start watching at any time, the messages you read are always read in order, but because of the limited screen size, if you look away or read to slowly you may miss some messages.

The ticker channel is useful when a program component continually broadcasts some information. The size channel’s circular buffer, its “screen” if you like, gives the subscribers some leeway if they occasionally fall behind reading.

As mentioned earlier, a ticker channel is single-consumer, i.e. only one strand is allowed to consume messages from the channel. On the other hand, it is possible, and useful, to create several views of the channel, each used by a different consumer strand. A view is created thus:

( ticker-consumer ch )

ch must be a channel of bounded capacity with the :displace policy. ticker-consumer returns a receive port (a channel that can only receive messages, not send them) that can be used to receive messages from ch . Each ticker-consumer will yield monotonic messages, namely no message will be received more than once, and the messages will be received in the order they’re sent, but if the consumer is too slow, messages could be lost.

Each consumer strand will use its own ticker-consumer , and each can consume messages at its own pace, and each ticker-consumer port will return the same messages (messages consumed from one will not be removed from the other views), subject possibly to different messages being missed by different consumers depending on their pace.

Primitive channels

It is also possible to create channels that carry messages of primitive JVM types. The analogous primitive channel functions to channel , snd and rcv , are, respectively:

int channels: int-channel , snd-int , rcv-int

channels: , , long channels: long-channel , snd-long , rcv-long

channels: , , float channels: float-channel , snd-float , rcv-float

channels: , , double channels: double-channel , snd-double , rcv-double

Because they don’t require boxing (for this reason snd-xxx and rcv-xxx are actually macros), primitive channels can provide better performance than regular channels. Primitive channels, however, are single-consumer, namely, only a single strand may read messages from any given channel.

Calling rcv-xxx on a closed channel will throw an exception.

Channel Transformation ( AKA Reactive Extensions)

The co.paralleluniverse.pulsar.rx namespace contains functions for transforming and combining channels. Known as “reactive extensions”, these transformations let you model your computation as a flow of data. These are the supported transformations:

map - returns a channel that transforms messages by applying a given mapping function. There are two versions of this operation: map which transforms messages as they are received from the channel, and snd-map which transforms the messages right before they are sent to the channel.

which transforms messages as they are received from the channel, and which transforms the messages right before they are sent to the channel. filter - returns a channel that only lets messages that satisfy a predicate through. Like map, there are two versions of this operation: filter , which filters messages as they are received from the channel, and snd-filter , which filters them right before they are sent. Either operation drops the messages that do not satisfy the predicate, and they are lost.

, which filters messages as they are received from the channel, and , which filters them right before they are sent. Either operation drops the messages that do not satisfy the predicate, and they are lost. zip - returns a channel that combines messages from a collection of channels into a combined vector message.

- returns a channel that combines messages from a collection of channels into a combined vector message. group - returns a channel that funnels messages from a set of given channels into one group channel.

Examples of using all channel transformations can be found in the rx test suite.

Dataflow (Reactive) Programming

Dataflow, or reactive programming, is a computation described by composing variables whose value may be set (and possibly changed) at any given time, without concern for when these values are set. Pulsar provides two dataflow primitives: vals, created with df-val , and vars, created with df-var , both in the co.paralleluniverse.pulsar.dataflow namespace.

A val is a dataflow constant. It can have its value set once, and read multiple times. Attempting to read the value of a val before it’s been set, will block until a value is set. Vals are like Pulsar promises, with the only difference being the behavior when attempting to set the val’s value more than once. A promise will return nil , while a val will throw an IllegalStateException .

A var is a dataflow variable. It can have it’s value set multiple times, and every new value can trigger the re-computation of other vars. You can set a var to retain historical values (consult the reference for more information).

Here is a simple example of using vals and vars:

( let [ a ( df-val ) x ( df-var ) y ( df-var # ( * @ a @ x )) ; this var has a formula z ( df-var # ( + @ a @ x )) r ( df-var # ( let [ v ( + @ a @ y @ z )] ; a formula with side-effects ( println "res: " v ) v )) f ( fiber ( loop [ i 0 ] ( when ( < i 5 ) ( sleep 50 ) ( x i ) ; sets the value of x ( recur ( inc i )))))] ( sleep 10 ) ( a 3 ) ; triggers everything by setting a ( join f ))

In this examples, vars y and z , are dependent on val a and var x , and will have their values recomputed – after a is set – whenever x changes.

Pulsar’s Actor System

To use the terms we’ve learned so far, an actor is a strand that owns a single channel with some added lifecycle management and error handling. But this reductionist view of actors does them little justice. Actors are fundamental building blocks that are combined to build a fault-tolerant application. If you are familiar with Erlang, Pulsar actors are just like Erlang processes.

An actor is a self-contained execution unit with well-defined inputs and outputs. Actors communicate with other actors (as well as regular program threads and fibers) by passing messages.

Note: Actors may write to and read from channels other than their own mailbox. In fact, actors can do whatever regular fibers can.

Spawning actors

Actors can run in any strand – fiber or thread - but for now, Pulsar only supports actors running in fibers (Quasar, the Java library that Pulsar wraps, allows running actors in regular threads).

An actor is basically a function that – if the actor is to do anything interesting – receives messages from the mailbox. To create and start an actor of a function f that takes arguments arg1 and arg2 , run

( spawn f arg1 arg2 )

This will create a new actor, and start running it in a new fiber.

spawn automatically marks f as suspendable, so there’s no need to do so explicitly.

spawn takes optional keyword arguments:

:name - The actor’s name (that’s also given to the fiber running the actor).

- The actor’s name (that’s also given to the fiber running the actor). :mailbox-size - The number of messages that can wait in the mailbox, or -1 (the default) for an unbounded mailbox.

- The number of messages that can wait in the mailbox, or -1 (the default) for an unbounded mailbox. :overflow-policy - What to do if a bounded mailbox overflows. Can be either: :throw , in which case an exception will be thrown into the receiving actor :drop , in which case the message will be silently discarded, or :block , in which case the sender will block until there’s room in the mailbox.

- What to do if a bounded mailbox overflows. Can be either: :trap - If set to true , linked actors’ death will send an exit message rather than throw an exception (see below).

- If set to , linked actors’ death will send an exit message rather than throw an exception (see below). :lifecycle-handle - A function that will be called to handle special messages sent to the actor. If set to nil (the default), the default handler is used, which is what you want in all circumstances, except for some actors that are meant to do some special tricks.

- A function that will be called to handle special messages sent to the actor. If set to (the default), the default handler is used, which is what you want in all circumstances, except for some actors that are meant to do some special tricks. :fj-pool - The ForkJoinPool in which the fiber will run. If :fj-pool is not specified, then the pool used will be either 1) the pool of the fiber calling spawn-fiber , or, if spawn-fiber is not called from within a fiber, a default pool.

- The in which the fiber will run. If is not specified, then the pool used will be either 1) the pool of the fiber calling , or, if is not called from within a fiber, a default pool. :stack-size - The initial fiber data stack size.

Of all the optional arguments, you’ll usually only use :name and :mailbox-size + :overflow-policy . As mentioned, by default the mailbox is unbounded. Bounded mailboxes provide better performance and should be considered for actors that are expected to handle messages at a very high rate.

An actor can be join ed, just like a fiber.

Note: Just like fibers, spawning an actor is a very cheap operation in both computation and memory. Do not fear creating many (thousands, tens-of-thousands or even hundreds-of-thousands) actors.

Sending and Receiving Messages

An actor’s mailbox is a channel that can be obtained with the mailbox-of function. You can therefore send a message to an actor like so:

( snd ( mailbox-of actor ) msg )

But there’s an easier way. Actors implement the SendPort interface, and so, are treated like a channel by the snd function. So we can simple call:

( snd actor msg )

While the above is a perfectly valid way of sending a message to an actor, this is not how it’s normally done. Instead of snd we normally use the ! (bang) function to send a message to an actor, like so:

( ! actor msg )

The bang operator has a slightly different semantic than snd . While snd will always place the message in the mailbox, ! will only do it if the actor is alive. It will not place a message in the mailbox if there is no one to receive it on the other end (and never will be, as mailboxes, like all channels, cannot change ownership).

In many circumstances, an actor sends a message to another actor, and expects a reply. In those circumstances, using !! instead of ! might offer reduced latency (but with the same semantics; both ! and !! always return nil )

The value @self , when evaluated in an actor, returns the actor’s own handle; for example it can itself be communicated and then used to send messages.

The value @mailbox instead, when evaluated in an actor, returns the receiving end of the actor’s own mailbox channel:

( rcv @ mailbox )

While an actor can be treated as a fiber with a channel, it has some extra features that give it a super-extra punch. Actors normally receive messages with the receive function, like so:

( receive )

receive has some features that make it very suitable for handling messages in actors. Its most visible feature is pattern matching. When an actor receives a message, it usually takes different action based on the type and content of the message. Making the decision with pattern matching is easy and elegant:

( let [ actor ( spawn # ( receive :abc "yes!" [ :why? answer ] answer :else "oy" ))] ( ! actor [ :why? "because!" ]) ( join actor )) ; => "because!"

As we can see in the example, receive not only picks the action based on the message, but also destructures the message and binds free variable, in our example – the answer variable. receive uses the core.match library for pattern matching, and you can consult its documentation to learn exactly how matching works.

Sometimes, we would like to assign the whole message to a variable. We do it by creating a binding clause in receive :

( receive [ m ] [ :foo val ] ( println "got foo:" val ) :else ( println "got" m ))

We can also match not on the raw message as its been received, but transform it first, and then match on the transformed value, like so, assuming transform is a function that takes a single argument (the message):

( receive [ m transform ] [ :foo val ] ( println "got foo:" val ) :else ( println "got" m ))

Now m – and the value we’re matching – is the the transformed value.

receive also deals with timeouts. Say we want to do something if a message has not been received within 30 milliseconds (all receive timeouts are specified in milliseconds):

( receive [ m transform ] [ :foo val ] ( println "got foo:" val ) :else ( println "got" m ) :after 30 ( println "nothing..." ))

Note: The :after clause in receive must be last.

Before we move on, it’s time for a short example. In this example, we will define an actor, adder , that receives an :add message with two numbers, and reply to the sender with the sum of those two numbers. In order to reply to the sender, we need to know who the sender is. So the sender will add a reference to itself in the message. In this request-reply pattern, it is also good practice to attach a random unique tag to the request, because messages are asynchronous, and it is possible that the adder will not respond to the requests in the order they were received, and the requester might want to send two requests before waiting for a response, so a tag is a good way to match replies with their respective requests. We can generate a random tag with the maketag function.

Here’s the adder actor:

( defsfn adder [] ( loop [] ( receive [ from tag [ :add a b ]] ( ! from tag [ :sum ( + a b )])) ( recur )))

And this is how we’ll use it from within another actor:

... ( let [ tag ( maketag ) a ... b ... ] ( ! adder-actor @ self tag [ :add a b ]) ( ->> ( receive [ tag [ :sum sum ]] sum :after 10 nil ) ( println "sum:" )) ...

Actors vs. Channels

One of the reasons of providing a different receive function for actors is because programming with actors is conceptually different from just using fibers and channels. I think of channels as hoses pumping data into a function, or as sort of like asynchronous parameters. A fiber may pull many different kinds of data from many different channels, and combine the data in some way.

Actors are a different abstraction. They are more like objects in object-oriented languages, assigned to a single thread. The mailbox serves as the object’s dispatch mechanism; it’s not a hose but a switchboard. It’s for this reason that actors often need to pattern-match their mailbox messages, while regular channels – each usually serving as a conduit for a single kind of data – don’t.

But while the receive syntax is nice and all (it mirrors Erlang’s syntax), we could have achieved the same with rcv almost as easily:

( let [ m1 ( rcv 30 :ms )] ( if m1 ( let [ m ( transform m1 )] ( match ( transform ( rcv 30 :ms )) [ :foo val ] ( println "got foo:" val ) :else ( println "got" m ))) ( println "nothing..." )))

Pretty syntax is not the main goal of the receive function. The reason receive is much more powerful than rcv , is mostly because of a feature we will now introduce.

Note: Because actors implement the SendPort interface, the snd-map and snd-filter functions (in the rx namespace) can be applied to actors as well.

Selective Receive

An actor is a state machine. It usually encompasses some state and the messages it receives trigger state transitions. But because the actor has no control over which messages it receives and when (which can be a result of either other actors’ behavior, or even the way the OS schedules threads), an actor would be required to process any message and any state, and build a full state transition matrix, namely how to transition whenever any messages is received at any state.

This can not only lead to code explosion; it can lead to bugs. The key to managing a complex state machine is by not handling messages in the order they arrive, but in the order we wish to process them. If a message does not match any of the clauses in receive , it will remain in the mailbox. receive will return only when it finds a message that does. When another receive statement is called, it will again search the messages that are in the mailbox, and may match a message that has been skipped by a previous receive .

In this code snippet, we specifically wait for the :baz message after receiving :foo , and so process the messages in this order – :foo , :baz , :bar – even though :bar is sent before :baz :

( let [ res ( atom []) actor ( spawn # ( dotimes [ i 2 ] ( receive [ :foo x ] ( do ( swap! res conj x ) ( receive [ :baz z ] ( swap! res conj z ))) [ :bar y ] ( swap! res conj y ) [ :baz z ] ( swap! res conj z ))))] ( ! actor [ :foo 1 ]) ( ! actor [ :bar 2 ]) ( ! actor [ :baz 3 ]) ( join actor ) @ res ) ; => [1 3 2]

Another example demonstrates receiving messages in order of priority.

Selective receive is also very useful when communicating with other actors. Here’s an excerpt from this example:

( defsfn adder [] ( loop [] ( receive [ from tag [ :add a b ]] ( ! from tag [ :sum ( + a b )])) ( recur ))) ( defsfn computer [ adder ] ( loop [] ( receive [ m ] [ from tag [ :compute a b c d ]] ( let [ tag1 ( maketag )] ( ! adder [ @ self tag1 [ :add ( * a b ) ( * c d )]]) ( receive [ tag1 [ :sum sum ]] ( ! from tag [ :result sum ]) :after 10 ( ! from tag [ :error "timeout!" ]))) :else ( println "Unknown message: " m )) ( recur ))) ( defsfn curious [ nums computer ] ( when ( seq nums ) ( let [[ a b c d ] ( take 4 nums ) tag ( maketag )] ( ! computer @ self tag [ :compute a b c d ]) ( receive [ m ] [ tag [ :result res ]] ( println a b c d "->" res ) [ tag [ :error error ]] ( println "ERROR: " a b c d "->" error ) :else ( println "Unexpected message" m )) ( recur ( drop 4 nums ) computer )))) ( defn -main [] ( let [ ad ( spawn adder ) cp ( spawn computer ad ) cr ( spawn curious ( take 20 ( repeatedly # ( rand-int 10 ))) cp )] ( join cr ) :ok ))

In the example, we have three actors: curious , computer and adder . curious asks computer to perform a computation, and computer relies on adder to perform addition. Note the nested receive in computer : the actor waits for a reply from adder before accepting other requests (from curious ) in the outer receive (actually, because this pattern of sending a message to an actor and waiting for a reply is so common, it’s encapsulated by a construct call gen-server - yet another blatant theft from Erlang - which we’ll introduce later; if you want to see how this example looks using gen-server , take a look here.

There are several actor systems that do not support selective receive, but Erlang does, and so does Pulsar. The talk Death by Accidental Complexity, by Ulf Wiger, shows how using selective receive avoids implementing a full, complicated and error-prone transition matrix. In a different talk, Wiger compared non-selective (FIFO) receive to a tetris game where you must fit each piece into the puzzle as it comes, while selective receive turns the problem into a jigsaw puzzle, where you can look for a piece that you know will fit.

A word of caution: Using selective receive in your code may lead to deadlocks (because you’re essentially saying, I’m going to wait here until a specific message arrives). This can be easily avoided by always specifying a timeout (with the :after millis clause) when doing a selective receive. Selective receive is a powerful tool that can greatly help writing readable, maintainable message-handling code, but don’t over-use it.

Actor State

In Erlang, actor state is set by recursively calling the actor function with the new state as an argument. In Pulsar, we can do the same. Here’s an example:

( let [ actor ( spawn # ( loop [ i ( int 2 ) state ( int 0 )] ( if ( == i 0 ) state ( recur ( dec i ) ( + state ( int ( receive )))))))] ( ! actor 13 ) ( ! actor 12 ) ( join actor )) ; => 25

Clojure is all about managing state. It ensures that every computation has access to consistent data. Because actors communicate with other computation only by exchanging immutable messages, and because each actor runs in a single strand, it’s absolutely ok for an actor to have mutable state - only the actor has access to it.

Every Pulsar actor has a state field that can be read like this @state and written with set-state! . Here’s an example:

( let [ actor ( spawn # ( do ( set-state! 0 ) ( set-state! ( + @ state ( receive ))) ( set-state! ( + @ state ( receive ))) @ state ))] ( ! actor 13 ) ( ! actor 12 ) ( join actor )) ; => 25

Finally, what if we want several state fields? What if we want some or all of them to be of a primitive type? This, too, poses no risk of race conditions because all state fields are written and read only by the actor, and there is no danger of them appearing inconsistent to an observer. Pulsar supports this as an experimental feature (implemented internally with deftype ), like so:

( let [ actor ( spawn ( actor [ ^ int sum 0 ] ( set! sum ( int ( + sum ( receive )))) ( set! sum ( int ( + sum ( receive )))) sum ))] ( ! actor 13 ) ( ! actor 12 ) ( join actor )) ; => 25

These are three different ways of managing actor state. Eventually, we’ll settle on just one or two (and are open to discussion about which is preferred).

State Machines with Strampoline

As we’ve seen, the receive form defines which messages the actor is willing to accept and process. You can nest receive statements, or place them in other functions that the actor calls (in which case the must be defined with defsfn ). It is often useful to treat the actor as a state machine, going from one state to another, executing a different receive at each state (to define the acceptable transitions from the state). To change state, all we would have to do is call a different function, each with its own receive, but here we face a technical limitation of Clojure. As Clojure (due to JVM limitations) does not perform true tail-call optimization, every state transition (i.e. every function call), would add a frame to the stack, eventually throwing a stack overflow. Clojure solves it with the clojure.core/trampoline function. It takes a function and calls it. When the function returns, if the returned value is a function, trampoline calls it.

Pulsar comes with a version of trampoline for suspendable functions called strampoline (with the exact same API as trampoline ).

Consider this example:

( let [ state2 ( sfn [] ( receive :bar :foobar )) state1 ( sfn [] ( receive :foo state2 )) actor ( spawn ( fn [] ( strampoline state1 )))] ( ! actor :foo ) ( Thread/sleep 50 ) ; or (Strand/sleep 50) ( ! actor :bar ) ( join actor )) ; => :foobar

The actor starts at state1 (represented by the function with the same name), by calling (strampoline state1) . In state1 we expect to receive the message :foo . When it arrives, we transition to state2 by returning the state2 function (which will immediately be called by strampoline ). In state1 we await the :bar message, and then terminate.

What happens if the messages :foo and :bar arrive in reverse order? Thanks to selective receive the result will be exactly the same! state1 will skip the :bar message, and transition to state2 when :foo arrives; the receive statement in state2 will then find the :bar message waiting in the mailbox:

( let [ state2 ( sfn [] ( receive :bar :foobar )) state1 ( sfn [] ( receive :foo state2 )) actor ( spawn ( fn [] ( strampoline state1 )))] ( ! actor :bar ) ( Thread/sleep 50 ) ; or (Strand/sleep 50) ( ! actor :foo ) ( join actor )) ; => :foobar

Error Handling

The actor model does not only make concurrency easy; it also helps build fault-tolerant systems by compartmentalizing failure. Each actor is it’s own execution context - if it encounters an exception, only the actor is directly affected (like a thread, only actors are lightweight). Unlike regular functions/objects, where an exception has to be caught and handled immediately on the call stack, with actors we can completely separate code execution from error handling.

In fact, when using actors, it is often best to to follow the philosophy laid out by Joe Armstrong, Erlang’s chief designer, of “let it crash”. The idea is not to try and catch exceptions inside an actor, because attempting to catch and handle all exceptions is futile. Instead, we just let the actor crash, monitor its death elsewhere, and then take some action.

The principle of actor error handling is that an actor can be asked to be notified of another actor’s death. This is done through linking and watching.

Linking actors

You link two actors with the link! function like this:

( link! actor1 actor2 )

Better yet, is to call the function from within one of the actors, say actor1 , in which case it will be called like so:

( link! actor2 )

A link is symmetrical. When two actors are linked, when one of them dies, the other throws an exception which, unless caught, kills it as well.

Here’s an example from the tests:

( let [ actor1 ( spawn # ( Fiber/sleep 100 )) actor2 ( spawn ( fn [] ( link! actor1 ) ( try ( loop [] ( receive [ m ] :foo :bar ) ( recur )) ( catch co.paralleluniverse.actors.LifecycleException e true ))))] ( join actor1 ) ( join actor2 )) ; => true

Remember, linking is symmetrical, so if actor2 were to die, actor1 would get the exception.

What if actor2 wants to be notified when actor1 dies, but doesn’t want to die itself? The :trap flag for the spawn macro, tells is to trap lifecycle exceptions and turn them into messages:

( let [ actor1 ( spawn # ( Strand/sleep 100 )) actor2 ( spawn :trap true ( fn [] ( link! actor1 ) ( receive [ m ] [ :exit _ actor reason ] actor )))] ( join actor1 ) ( join actor2 )) ; => actor1

Now, when actor1 dies, actor2 receives an :exit message, telling it which actor has died and how. We’ll look into the :exit message in a second.

We can undo the link by calling

( unlink! actor1 actor2 )

or

( unlink! actor2 )

from within actor1 .

Watching actors

A more robust way of being notified of actor death than linking is with a watch (called monitor in Erlang; this is one of the very few occasions we have abandoned the Erlang function names):

( let [ actor1 ( spawn # ( Fiber/sleep 200 )) actor2 ( spawn # ( let [ w ( watch! actor1 )] ( receive [ :exit w actor reason ] actor )))] ( join actor1 ) ( join actor2 )) ; => actor1

Watches are asymmetrical. Here, actor2 watches for actor1 ’s death, but not vice-versa. When actor1 dies, actor2 gets an :exit message, of the exact same structure of the message sent when we used a link and a :trap flag.

The watch! function returns a watch object. Because an actor can potentially set many watches on another actor (say, it calls a library function which calls watch! ), we could potentially get several copies of the exit message, each for a different watch.

The message is a vector of 4 elements:

:exit The watch interested in the message (or nil when linking). Note how in the example we pattern-match on the second element (with the w value, which contains the watch object), to ensure that we only process the message belonging to our watch. The actor that just died. The dead actor’s death cause: nil for a natural death (no exception thrown, just like in our example), or the throwable responsible for the actor’s death.

We can remove a watch by calling

( unwatch! actor1 actor2 )

or

( unwatch! actor2 )

from within actor1 .

Actor Registration

Registering an actor gives it a public name that can be used to locate the actor. You register an actor like so:

( register! name actor )

or:

( register! actor )

in which case the name will be the one given to the actor when it was spawn ed. name can be a string, or any object with a nice string representation (like a keyword).

You obtain a reference to a registered actor with:

( whereis name )

but most actor-related functions can work directly with the registered name. For example, instead of this:

( register! :foo actor ) ( ! ( whereis :foo ) "hi foo!" )

you can write:

( register! :foo actor ) ( ! :foo "hi foo!" )

You unregister an actor like so:

( unregister! actor )

Registration and Monitoring

When you register an actor, Pulsar automatically creates a JMX MBean to monitor it. Look for it using JConsole or VisualVM.

Details TBD.

Registration and Clustering

If you’re running in a Galaxy cluster, registering an actor will make it globally available on the cluster (so the name must be unique to the entire cluster).

Details TBD.

Behaviors

Erlang’s designers have realized that many actors follow some common patterns - like an actor that receives requests for work and then sends back a result to the requester. They’ve turned those patterns into actor templates, called behaviors, in order to save people work and avoid some common errors. Some of these behaviors have been ported to Pulsar.

Behaviors have two sides. One is the provider side, and is modeled in Pulsar as a protocols. You implement the protocol, and Pulsar provides the full actor implementation that uses your protocol. The other is the consumer side – functions used by other actors to access the functionality provided by the behavior.

All behaviors (gen-server, gen-event and supervisors) support the shutdown! function, which requests an orderly shutdown of the actor:

( shutdown! behavior-actor )

gen-server

gen-server is a template for a server actor that receives requests and replies with responses. The consumer side for gen-server consists of the following functions:

( call! actor request )

This would send the request message to the gen-server actor, and block until a response is received. It will then return the response. If the request triggers an exception in the actor, that exception will be thrown by call! .

There’s also a timed version of call! , which gives up and returns nil if the timeout expires. For example, :

( call-timed! actor 100 :ms request )

would wait up to 100ms for a response.

You can also send a gen-server messages that do not require a response with the cast! function:

( cast! actor message )

Finally, you can shutdown a gen-server with the shutdown function:

( shutdown! actor )

In order to create a gen-server actor(the provider side), you need to implement the following protocol:

( defprotocol Server ( init [ this ]) ( handle-call [ this ^ Actor from id message ]) ( handle-cast [ this ^ Actor from id message ]) ( handle-info [ this message ]) ( handle-timeout [ this ]) ( terminate [ this ^ Throwable cause ]))

init – will be called when the actor starts

– will be called when the actor starts terminate – will be called when the actor terminates.

– will be called when the actor terminates. handle-call – called when the call function has been called on the actor :). This is where the gen-server’s functionality usually lies. The value returned from handle-call will be sent back to the actor making the request, unless nil is returned, in which case the response has to be sent manually as we’ll see later.

– called when the function has been called on the actor :). This is where the gen-server’s functionality usually lies. The value returned from will be sent back to the actor making the request, unless is returned, in which case the response has to be sent manually as we’ll see later. handle-cast – called to handle messages sent with cast! .

– called to handle messages sent with . handle-info – called whenever a message has been sent to the actor directly (i.e., with ! ) rather than through call! or cast! .

– called whenever a message has been sent to the actor directly (i.e., with ) rather than through or . handle-timeout – called whenever the gen-server has not received any messages for a configurable duration of time. The timeout can be configured using either the :timeout option to the gen-server function, or by calling the set-timeout! function, as we’ll immediately see.

You spawn a gen-server actor like so:

( spawn ( gen-server <options?> server ))

where options can now only be :timeout millis . Here’s an example from the tests:

( let [ gs ( spawn ( gen-server ( reify Server ( init [ _ ]) ( terminate [ _ cause ]) ( handle-call [ _ from id [ a b ]] ( Strand/sleep 50 ) ( + a b )))))] ( call! gs 3 4 ) ; => 7

And here’s one with server timeouts:

( let [ times ( atom 0 ) gs ( spawn ( gen-server :timeout 20 ( reify Server ( init [ _ ]) ( handle-timeout [ _ ] ( if ( < @ times 5 ) ( swap! times inc ) ( shutdown! ))) ( terminate [ _ cause ]))))] ( join 200 :ms gs ) @ times ) ; => 5

You can set (and reset) the timeout from anywhere within the protocol’s methods by calling, say

( set-timeout! 100 :ms )

A timeout value of 0 or less means no timeout.

If the handle-call function returns nil , then no response is sent to the caller. The call! function remains blocked until a response is sent manually. This is done with the reply! function, which takes, along with the response message, the identity of the caller and the request ID, both passed to handle-call . Here’s an example:

( let [ gs ( spawn ( gen-server :timeout 50 ( reify Server ( init [ _ ] ( set-state! {})) ( terminate [ _ cause ]) ( handle-call [ _ from id [ a b ]] ( set-state! ( assoc @ state :a a :b b :from from :id id )) nil ) ( handle-timeout [ _ ] ( let [{ :keys [ a b from id ]} @ state ] ( when id ( reply! from id ( + a b ))))))))] ( call-timed! gs 100 :ms 5 6 )) ; => 11

In the example, handle-call saves the request in the actor’s state, and later, in handle-timeout sends the response using reply! . The response is returned by call-timed! .

If an error is encountered during the generation of the delayed repsonse, an exception can be returned to the caller (and will be thrown by call! ), using reply-error! :

( reply-error! to id ( Exception. "does not compute" ))

where to is the identity of the caller passed as from to handle-call .

gen-event

gen-event is an actor behavior that receives messages (events) and forwards them to registered event handlers.

You spawn a gen-event like this:

( spawn ( gen-event init ))

init is an initializer function called from within the gen-event actor.

You can then add event handlers:

( add-handler! ge handler )

with ge being the gen-event actor (returned by the call to spawn ), and handler being a function of a single argument that will be called whenever an event is generated.

You generate an event with the notify! function:

( notify! ge event )

with ge being the gen-event actor, and event is the event object (which can be any object). The event object is then passed to all registered event handlers.

An event handler can be removed like so:

( remove-handler! ge handler )

Here’s a complete example, taken from the tests:

( let [ ge ( spawn ( gen-event # ( add-handler! @ self handler1 )))] ( add-handler! ge handler2 ) ( notify! ge "hello" ))

In this example, handler1 is added in the init function (note how @self refers to the gen-event actor itself, as the init function is called from within the actor), and handler2 is added later.

When notify! is called, both handlers will be called and passed the event object (in this case, the "hello" string).

gen-fsm

gen-fsm is an actor behavior that helps manage actor state transitions.

You spawn a gen-fsm like this:

( spawn ( gen-fsm initial-state ))

initial-state serves as the actor’s first state. A state is a function that executes as part of the actor body, and returns the next actor state. If a state returns :done , the actor terminates.

Here is a complete example:

( letfn [( state1 [] ( receive :a state2 )) ( state2 [] ( receive :b :done ))] ( let [ gfsm ( spawn ( gen-fsm state1 ))] ( ! gfsm :b ) ; deferred until state2 ( ! gfsm :qqq ) ; ignored ( ! gfsm :a ) ; received by state1, which transitions to state2, which then processes the :b message ( join gfsm )))

Supervisors

A supervisor is an actor behavior designed to standardize error handling. Internally it uses watches and links, but it offers a more structured, standard, and simple way to react to errors.

The general idea is that actors performing business logic, “worker actors”, are supervised by a supervisor actor that detects when they die and takes one of several pre-configured actions. Supervisors may, in turn, be supervised by other supervisors, thus forming a supervision hierarchy that compartmentalizes failure and recovery.

A supervisor works as follows: it has a number of children, worker actors or other supervisors that are registered to be supervised wether at the supervisor’s construction time or at a later time. Each child has a mode, :permanent , :transient or :temporary that determines whether its death will trigger the supervisor’s recovery event. When the recovery event is triggered, the supervisor takes action specified by its restart strategy, or it will give up and fail, depending on predefined failure modes.

When a child actor in the :permanent mode dies, it will always trigger its supervisor’s recovery event. When a child in the :transient mode dies, it will trigger a recovery event only if it has died as a result of an exception, but not if it has simply finished its operation. A :temporary child never triggers it supervisor’s recovery event.

A supervisor’s restart strategy determines what it does during a recovery event: A strategy of :escalate means that the supervisor will shut down (“kill”) all its surviving children and then die; a :one-for-one strategy will restart the dead child; an :all-for-one strategy will shut down all children and then restart them all; a :rest-for-one strategy will shut down and restart all those children added to the supervisor after the dead child.

A supervisor is spawned so:

( spawn ( supervisor restart-strategy init ))

where restart-strategy is one of: :escalate , :one-for-one , :all-for-one , or :rest-for-one , and init is a function that returns a sequence of child specs that will be used to add children to the supervisor when it’s constructed.

A child spec is a vector of the following form:

[ id mode max-restarts duration unit shutdown-deadline-millis actor-fn & actor-args ]

where:

id is an optional identifier (usually a string) for the child actor. May be nil .

is an optional identifier (usually a string) for the child actor. May be . mode is one of :permanent , :transient or :temporary .

is one of , or . max-restarts , duration and unit are a triplet specifying how many times is the child allowed to restart in a given period of time before the supervisor should give up, kill all its children and die. For example 20 5 :sec means at most 20 restarts in 5 seconds.

, and are a triplet specifying how many times is the child allowed to restart in a given period of time before the supervisor should give up, kill all its children and die. For example means at most 20 restarts in 5 seconds. shutdown-deadline-millis is the maximal amount of time, in milliseconds that the child is allowed to spend from the time it’s requested to shut down until the time it is terminated. Whenever a the supervisor shuts down a child, it does so by sending it the message [:shutdown sup] , with sup being the supervisor. If the shutdown deadline elapses, the supervisor will forcefully shut it down by interrupting the child’s strand.

is the maximal amount of time, in milliseconds that the child is allowed to spend from the time it’s requested to shut down until the time it is terminated. Whenever a the supervisor shuts down a child, it does so by sending it the message , with being the supervisor. If the shutdown deadline elapses, the supervisor will forcefully shut it down by interrupting the child’s strand. actor-fn & actor-args are the (suspendable) function (with optional arguments) that’s to serve as the child actor’s body.

It is often useful to pass the supervisor to a child (so it could later dynamically add other children to the supervisor, for example). This is easily done because the init function is called inside the supervisor; therefore, any reference to @self inside the init function returns the supervisor. If you pass @self , then, as an argument to a child actor, it will receive the supervisor.

Other than returning a sequence of child specs from the init function, you can also dynamically add a child to a supervisor by simply calling

( add-child! sup id mode max-restarts duration unit shutdown-deadline-millis actor-fn & actor-args )

with sup being the supervisor, and the rest of the arguments comprising the child spec for the actor, with the difference that if actor-fn , instead of an actor function, is a spawned actor (the value returned from spawn ), then supervisor will supervise an already-spawned actor. Otherwise, (if it is a function), a new actor will be spawned.

A supervised actor may be removed from the supervisor by calling

( remove-child! sup id )

with id being the one given to the actor in the child spec or the arguments to add-child .

Hot Code Swapping

Hot code swapping is the ability to change your program’s code while it is running, with no need for a restart. Pulsar actors support a limited form of hot code swapping. Hot code swapping in Pulsar generally entails rebinding or redefining vars.

An example of hot code swapping is found in the codeswap.clj program.

Swapping plain actors

In the actor’s main loop, using the recur-swap macro rather than recur , would use the new definition of the actor function, if one is found. The only difference in syntax between recur-swap and recur is that recur-swap takes the name of the function is the first parameter.

In the following example (taken from codeswap.clj) if a is redefined, its new definition will be used in the next iteration.

( defsfn a [ n ] ( when-let [ m ( receive-timed 1000 )] ( println "message:" m )) ( recur-swap a ( inc n )))

Swapping gen-server

gen-server s don’t need any special action to support hot code swapping. If the implementation of the Server protocol passed to gen-server is a var and that var is redefined, the new definition will be used when processing the next request. This is an example (taken from codeswap.clj) of a gen-server that supports hot code swapping when s is redefined:

( def s ( sreify Server ( init [ _ ]) ( terminate [ _ cause ]) ( handle-call [ _ from id [ a b ]] ( sleep 50 ) ( + a b )))) ( spawn ( gen-server s ))

core.async

core.async is a new asynchronous programming library for Clojure built by Rich Hickey and other contributors. It provides something akin to fibers through go blocks (though more limited than fibers) and channels, and is also available in ClojureScript. Because core.async provides a subset of Pulsar’s capability, Pulsar provides an optional API that’s compatible with core.async which some people may prefer. The original core.async test suite has been ported as well in order to ensure API-level compatibility.

The core.async implementation is found in the co.paralleluniverse.pulsar.async namespace. At present the support level is up to version 0.1.346.0-17112a-alpha , excluding deprecated and experimental features.

These definitions are no more than thin wrappers around Pulsar functions and macros:

(chan) is the same as calling (channel) .

is the same as calling . (chan (buffer n)) or (chan n) are the same as (channel n :block) or (channel n) .

or are the same as or . (chan (dropping-buffer n)) is the same as (channel n :drop)

is the same as (chan (sliding-buffer n)) is the same as (channel n :displace)

is the same as close! is the same as close!

is the same as >! and >!! are the same as snd

and are the same as <! and <!! are the same as rcv

The core.async API and the Pulsar API may be used interchangeably, so you can call >! on a channel created with channel or snd on a channel created with chan . They are one and the same.

go simply performs its body within a newly spawned fiber.

These are the differences between the Pulsar implementation of the core.async API, and the original implementation:

Channels created with (chan (sliding-buffer n)) are single-consumer.

are single-consumer. You may use the ! and the !! defs interchangeably (the original implementation forces the use of the single-bang defs in go-blocks and the double-bang defs in regular threads). This means that in the Pulsar implementation all of the double-bang names ( >!! , <!! , alts!! and alt!! ) are actually redundant (but included for compatibility).

and the defs interchangeably (the original implementation forces the use of the single-bang defs in go-blocks and the double-bang defs in regular threads). This means that in the Pulsar implementation all of the double-bang names ( , , and ) are actually redundant (but included for compatibility). In addition to thread and thread-call there are fiber and fiber-call as well which are not part of the original core.async API.

In addition, there are performance differences, mostly resulting from the fact that Pulsar uses fork-join pools to schedule fibers, while core.async uses regular thread pools. In short, if there is little interaction between go blocks (say, each go block writes something to a channel and completes), you can expect better performance from the original implementation. If there is a lot of interaction between go blocks (there’s non-trivial message passing among them), then the Pulsar implementation will yield better performance.

Clustering

Pulsar is able to run on a cluster, thereby letting actors and channels communicate across machines. The Pulsar/Quasar cluster runs on top of Galaxy, Parallel Universe’s in-memory data grid.

In this version, clustering is pretty rudimentary, but essential features should work: actors can be made discoverable on the network, messages can be passed among actors on different nodes, and an actor on a failing node will behave as expected of a dying actor with respect to exit messages sent to other, remote, watching it or linked to it.

Enabling Clustering

First, you will need to add quasar-galaxy as a dependency to your project:

[ co.paralleluniverse/quasar-galaxy "0.7.6" ]

To make an actor discoverable, all you need to do is register it:

( register! :global-actor1 actor )

or, if the actor already has a name (set in spawn ), simply call:

( register! )

That’s it. The actor is now known throughout the cluster. If you want to send a message to it, call

( ! :global-actor1 [ :hi-from @ self ])

Though this call looks up the actor in the registry every time it’s called. A better way might be:

( let [ a ( whereis :global-actor1 )] ( ! a [ :hi-from @ self ]))

An actor doesn’t have to be registered in order to be reachable on the network. Registering it simply makes it discoverable. If we pass a local actor in a message to a remote actor, the remote actor will be able to send messages to the local actor as well. In the simple example above, we are sending @self to :global-actor1; :global-actor1 will be able to send messages back to us.

Example

The best way to get started is by running the distributed pingpong example.

Here’s the code for ping and for pong. You run them like this:

lein with-profile cluster update-in :jvm-opts conj '"-Dgalaxy.nodeId=2"' '"-Dgalaxy.port=7052"' '"-Dgalaxy.slave_port=8052"' -- run -m co.paralleluniverse.pulsar.examples.cluster.ping lein with-profile cluster update-in :jvm-opts conj '"-Dgalaxy.nodeId=1"' '"-Dgalaxy.port=7051"' '"-Dgalaxy.slave_port=8051"' -- run -m co.paralleluniverse.pulsar.examples.cluster.pong

Cluster Configuration

For instructions on how to configure the Galaxy cluster, please refer to Galaxy’s getting started guide.

Examples

The Pulsar source code contains several examples:

In addition, the test suite contains many more small examples.