Class

Ractor is a Actor-model abstraction for Ruby that provides thread-safe parallel execution.

Ractor.new can make new Ractor and it will run in parallel.

# The simplest ractor
r = Ractor.new {puts "I am in Ractor!"}
r.take # wait it to finish
# here "I am in Ractor!" would be printed

Ractors do not share usual objects, so the some kind of thread-safety concerns such as data-race, race-conditions are not available on multi-ractor programming.

To achieve this, ractors severely limit object sharing between different ractors. For example, unlike threads, ractors can't access each other's objects, nor any objects through variables of the outer scope.

a = 1
r = Ractor.new {puts "I am in Ractor! a=#{a}"}
# fails immediately with
# ArgumentError (can not isolate a Proc because it accesses outer variables (a).)

On CRuby (the default implementation), Global Virtual Machine Lock (GVL) is held per ractor, so ractors are performed in parallel without locking each other.

Instead of accessing the shared state, the objects should be passed to and from ractors via sending and receiving objects as messages.

a = 1
r = Ractor.new do
  a_in_ractor = receive # receive blocks till somebody will pass message
  puts "I am in Ractor! a=#{a_in_ractor}"
end
r.send(a)  # pass it
r.take
# here "I am in Ractor! a=1" would be printed

There are two pairs of methods for sending/receiving messages:

In addition to that, an argument to Ractor.new would be passed to block and available there as if received by Ractor.receive, and the last block value would be sent outside of the ractor as if sent by Ractor.yield.

A little demonstration on a classic ping-pong:

server = Ractor.new do
  puts "Server starts: #{self.inspect}"
  puts "Server sends: ping"
  Ractor.yield 'ping'                       # The server doesn't know the receiver and sends to whoever interested
  received = Ractor.receive                 # The server doesn't know the sender and receives from whoever sent
  puts "Server received: #{received}"
end

client = Ractor.new(server) do |srv|        # The server is sent inside client, and available as srv
  puts "Client starts: #{self.inspect}"
  received = srv.take                       # The Client takes a message specifically from the server
  puts "Client received from " \
       "#{srv.inspect}: #{received}"
  puts "Client sends to " \
       "#{srv.inspect}: pong"
  srv.send 'pong'                           # The client sends a message specifically to the server
end

[client, server].each(&:take)               # Wait till they both finish

This will output:

Server starts: #<Ractor:#2 test.rb:1 running>
Server sends: ping
Client starts: #<Ractor:#3 test.rb:8 running>
Client received from #<Ractor:#2 rac.rb:1 blocking>: ping
Client sends to #<Ractor:#2 rac.rb:1 blocking>: pong
Server received: pong

It is said that Ractor receives messages via the incoming port, and sends them to the outgoing port. Either one can be disabled with Ractor#close_incoming and Ractor#close_outgoing respectively. If a ractor terminated, its ports will be closed automatically.

Shareable and unshareable objects

When the object is sent to and from the ractor, it is important to understand whether the object is shareable or unshareable. Most of objects are unshareable objects.

Shareable objects are basically those which can be used by several threads without compromising thread-safety; e.g. immutable ones. Ractor.shareable? allows to check this, and Ractor.make_shareable tries to make object shareable if it is not.

Ractor.shareable?(1)            #=> true -- numbers and other immutable basic values are
Ractor.shareable?('foo')        #=> false, unless the string is frozen due to # freeze_string_literals: true
Ractor.shareable?('foo'.freeze) #=> true

ary = ['hello', 'world']
ary.frozen?                 #=> false
ary[0].frozen?              #=> false
Ractor.make_shareable(ary)
ary.frozen?                 #=> true
ary[0].frozen?              #=> true
ary[1].frozen?              #=> true

When a shareable object is sent (via send or Ractor.yield), no additional processing happens, and it just becomes usable by both ractors. When an unshareable object is sent, it can be either copied or moved. The first is the default, and it makes the object's full copy by deep cloning of non-shareable parts of its structure.

data = ['foo', 'bar'.freeze]
r = Ractor.new do
  data2 = Ractor.receive
  puts "In ractor: #{data2.object_id}, #{data2[0].object_id}, #{data2[1].object_id}"
end
r.send(data)
r.take
puts "Outside  : #{data.object_id}, #{data[0].object_id}, #{data[1].object_id}"

This will output:

In ractor: 340, 360, 320
Outside  : 380, 400, 320

(Note that object id of both array and non-frozen string inside array have changed inside the ractor, showing it is different objects. But the second array's element, which is a shareable frozen string, has the same object_id.)

Deep cloning of the objects may be slow, and sometimes impossible. Alternatively, move: true may be used on sending. This will move the object to the receiving ractor, making it inaccessible for a sending ractor.

data = ['foo', 'bar']
r = Ractor.new do
  data_in_ractor = Ractor.receive
  puts "In ractor: #{data_in_ractor.object_id}, #{data_in_ractor[0].object_id}"
end
r.send(data, move: true)
r.take
puts "Outside: moved? #{Ractor::MovedObject === data}"
puts "Outside: #{data.inspect}"

This will output:

In ractor: 100, 120
Outside: moved? true
test.rb:9:in `method_missing': can not send any methods to a moved object (Ractor::MovedError)

Notice that even inspect (and more basic methods like __id__) is inaccessible on a moved object.

Besides frozen objects, there are shareable objects. Class and Module objects are shareable so the Class/Module definitons are shared between ractors. Ractor objects are also shareable objects. All operations for the shareable mutable objects are thread-safe, so the thread-safety property will be kept. We can not define mutable shareable objects in Ruby, but C extensions can introduce them.

It is prohibited to access instance variables of mutable shareable objects (especially Modules and classes) from ractors other than main:

class C
  class << self
    attr_accessor :tricky
  end
end

C.tricky = 'test'

r = Ractor.new(C) do |cls|
  puts "I see #{cls}"
  puts "I can't see #{cls.tricky}"
end
r.take
# I see C
# can not access instance variables of classes/modules from non-main Ractors (RuntimeError)

Ractors can access constants if they are shareable. The main Ractor is the only one that can access non-shareable constants.

GOOD = 'good'.freeze
BAD = 'bad'

r = Ractor.new do
  puts "GOOD=#{GOOD}"
  puts "BAD=#{BAD}"
end
r.take
# GOOD=good
# can not access non-shareable objects in constant Object::BAD by non-main Ractor. (NameError)

# Consider the same C class from above

r = Ractor.new do
  puts "I see #{C}"
  puts "I can't see #{C.tricky}"
end
r.take
# I see C
# can not access instance variables of classes/modules from non-main Ractors (RuntimeError)

See also the description of # shareable_constant_value pragma in Comments syntax explanation.

Ractors vs threads

Each ractor creates its own thread. New threads can be created from inside ractor (and, on CRuby, sharing GVL with other threads of this ractor).

r = Ractor.new do
  a = 1
  Thread.new {puts "Thread in ractor: a=#{a}"}.join
end
r.take
# Here "Thread in ractor: a=1" will be printed

Note on code examples

In examples below, sometimes we use the following method to wait till ractors that are not currently blocked will finish (or process till next blocking) method.

def wait
  sleep(0.1)
end

It is **only for demonstration purposes** and shouldn't be used in a real code. Most of the times, just take is used to wait till ractor will finish.

Reference

See Ractor desgin doc for more details.


Class Methods


Returns total count of Ractors currently running.

Ractor.count                   #=> 1
r = Ractor.new(name: 'example') { Ractor.yield(1) }
Ractor.count                   #=> 2 (main + example ractor)
r.take                         # wait for Ractor.yield(1)
r.take                         # wait till r will finish
Ractor.count                   #=> 1

Returns the currently executing Ractor.

Ractor.current #=> #<Ractor:#1 running>

returns main ractor

Make obj shareable between ractors.

obj and all the objects it refers to will be frozen, unless they are already shareable.

If copy keyword is true, the method will copy objects before freezing them This is safer option but it can take be slower.

Note that the specification and implementation of this method are not mature and may be changed in the future.

obj = ['test']
Ractor.shareable?(obj)     #=> false
Ractor.make_shareable(obj) #=> ["test"]
Ractor.shareable?(obj)     #=> true
obj.frozen?                #=> true
obj[0].frozen?             #=> true

# Copy vs non-copy versions:
obj1 = ['test']
obj1s = Ractor.make_shareable(obj1)
obj1.frozen?                        #=> true
obj1s.object_id == obj1.object_id   #=> true
obj2 = ['test']
obj2s = Ractor.make_shareable(obj2, copy: true)
obj2.frozen?                        #=> false
obj2s.frozen?                       #=> true
obj2s.object_id == obj2.object_id   #=> false
obj2s[0].object_id == obj2[0].object_id #=> false

See also the “Shareable and unshareable objects” section in the Ractor class docs.

Create a new Ractor with args and a block.

A block (Proc) will be isolated (can't access to outer variables). self inside the block will refer to the current Ractor.

r = Ractor.new { puts "Hi, I am #{self.inspect}" }
r.take
# Prints "Hi, I am #<Ractor:#2 test.rb:1 running>"

args passed to the method would be propagated to block args by the same rules as objects passed through send/Ractor.receive: if args are not shareable, they will be copied (via deep cloning, which might be inefficient).

arg = [1, 2, 3]
puts "Passing: #{arg} (##{arg.object_id})"
r = Ractor.new(arg) {|received_arg|
  puts "Received: #{received_arg} (##{received_arg.object_id})"
}
r.take
# Prints:
#   Passing: [1, 2, 3] (#280)
#   Received: [1, 2, 3] (#300)

Ractor's name can be set for debugging purposes:

r = Ractor.new(name: 'my ractor') {}
p r
#=> #<Ractor:#3 my ractor test.rb:1 terminated>

Receive an incoming message from the current Ractor's incoming port's queue, which was sent there by send.

r = Ractor.new do
  v1 = Ractor.receive
  puts "Received: #{v1}"
end
r.send('message1')
r.take
# Here will be printed: "Received: message1"

Alternatively, private instance method receive may be used:

r = Ractor.new do
  v1 = receive
  puts "Received: #{v1}"
end
r.send('message1')
r.take
# Here will be printed: "Received: message1"

The method blocks if the queue is empty.

r = Ractor.new do
  puts "Before first receive"
  v1 = Ractor.receive
  puts "Received: #{v1}"
  v2 = Ractor.receive
  puts "Received: #{v2}"
end
wait
puts "Still not received"
r.send('message1')
wait
puts "Still received only one"
r.send('message2')
r.take

Output:

Before first receive
Still not received
Received: message1
Still received only one
Received: message2

If close_incoming was called on the ractor, the method raises Ractor::ClosedError if there are no more messages in incoming queue:

Ractor.new do
  close_incoming
  receive
end
wait
# in `receive': The incoming port is already closed => #<Ractor:#2 test.rb:1 running> (Ractor::ClosedError)

Receive only a specific message.

Instead of Ractor.receive, Ractor.receive_if can provide a pattern by a block and you can choose the receiving message.

r = Ractor.new do
  p Ractor.receive_if{|msg| msg.match?(/foo/)} #=> "foo3"
  p Ractor.receive_if{|msg| msg.match?(/bar/)} #=> "bar1"
  p Ractor.receive_if{|msg| msg.match?(/baz/)} #=> "baz2"
end
r << "bar1"
r << "baz2"
r << "foo3"
r.take

This will output:

foo3
bar1
baz2

If the block returns a truthy value, the message will be removed from the incoming queue and returned. Otherwise, the messsage remains in the incoming queue and the following received messages are checked by the given block.

If there are no messages left in the incoming queue, the method will block until new messages arrive.

If the block is escaped by break/return/exception/throw, the message is removed from the incoming queue as if a truthy value had been returned.

r = Ractor.new do
  val = Ractor.receive_if{|msg| msg.is_a?(Array)}
  puts "Received successfully: #{val}"
end

r.send(1)
r.send('test')
wait
puts "2 non-matching sent, nothing received"
r.send([1, 2, 3])
wait

Prints:

2 non-matching sent, nothing received
Received successfully: [1, 2, 3]

Note that you can not call receive/receive_if in the given block recursively. It means that you should not do any tasks in the block.

Ractor.current << true
Ractor.receive_if{|msg| Ractor.receive}
#=> `receive': can not call receive/receive_if recursively (Ractor::Error)
An alias for receive

Waits for the first ractor to have something in its outgoing port, reads from this ractor, and returns that ractor and the object received.

r1 = Ractor.new {Ractor.yield 'from 1'}
r2 = Ractor.new {Ractor.yield 'from 2'}

r, obj = Ractor.select(r1, r2)

puts "received #{obj.inspect} from #{r.inspect}"
# Prints: received "from 1" from #<Ractor:#2 test.rb:1 running>

If one of the given ractors is the current ractor, and it would be selected, r will contain :receive symbol instead of the ractor object.

r1 = Ractor.new(Ractor.current) do |main|
  main.send 'to main'
  Ractor.yield 'from 1'
end
r2 = Ractor.new do
  Ractor.yield 'from 2'
end

r, obj = Ractor.select(r1, r2, Ractor.current)
puts "received #{obj.inspect} from #{r.inspect}"
# Prints: received "to main" from :receive

If yield_value is provided, that value may be yielded if another Ractor is calling take. In this case, the pair [:yield, nil] would be returned:

r1 = Ractor.new(Ractor.current) do |main|
  puts "Received from main: #{main.take}"
end

puts "Trying to select"
r, obj = Ractor.select(r1, Ractor.current, yield_value: 123)
wait
puts "Received #{obj.inspect} from #{r.inspect}"

This will print:

Trying to select
Received from main: 123
Received nil from :yield

move boolean flag defines whether yielded value should be copied (default) or moved.

Checks if the object is shareable by ractors.

Ractor.shareable?(1)            #=> true -- numbers and other immutable basic values are frozen
Ractor.shareable?('foo')        #=> false, unless the string is frozen due to # freeze_string_literals: true
Ractor.shareable?('foo'.freeze) #=> true

See also the “Shareable and unshareable objects” section in the Ractor class docs.

Send a message to the current ractor's outgoing port to be consumed by take.

r = Ractor.new {Ractor.yield 'Hello from ractor'}
puts r.take
# Prints: "Hello from ractor"

The method is blocking, and will return only when somebody consumes the sent message.

r = Ractor.new do
  Ractor.yield 'Hello from ractor'
  puts "Ractor: after yield"
end
wait
puts "Still not taken"
puts r.take

This will print:

Still not taken
Hello from ractor
Ractor: after yield

If the outgoing port was closed with close_outgoing, the method will raise:

r = Ractor.new do
  close_outgoing
  Ractor.yield 'Hello from ractor'
end
wait
# `yield': The outgoing-port is already closed (Ractor::ClosedError)

The meaning of move argument is the same as for send.

Instance Methods


get a value from ractor-local storage

set a value in ractor-local storage

Closes the incoming port and returns its previous state. All further attempts to Ractor.receive in the ractor, and send to the ractor will fail with Ractor::ClosedError.

r = Ractor.new {sleep(500)}
r.close_incoming  #=> false
r.close_incoming  #=> true
r.send('test')
# Ractor::ClosedError (The incoming-port is already closed)

Closes the outgoing port and returns its previous state. All further attempts to Ractor.yield in the ractor, and take from the ractor will fail with Ractor::ClosedError.

r = Ractor.new {sleep(500)}
r.close_outgoing  #=> false
r.close_outgoing  #=> true
r.take
# Ractor::ClosedError (The outgoing-port is already closed)
No documentation available

The name set in Ractor.new, or nil.

same as Ractor.receive

No documentation available
An alias for receive

Send a message to a Ractor's incoming queue to be consumed by Ractor.receive.

r = Ractor.new do
  value = Ractor.receive
  puts "Received #{value}"
end
r.send 'message'
# Prints: "Received: message"

The method is non-blocking (will return immediately even if the ractor is not ready to receive anything):

r = Ractor.new {sleep(5)}
r.send('test')
puts "Sent successfully"
# Prints: "Sent successfully" immediately

Attempt to send to ractor which already finished its execution will raise Ractor::ClosedError.

r = Ractor.new {}
r.take
p r
# "#<Ractor:#6 (irb):23 terminated>"
r.send('test')
# Ractor::ClosedError (The incoming-port is already closed)

If close_incoming was called on the ractor, the method also raises Ractor::ClosedError.

r =  Ractor.new do
  sleep(500)
  receive
end
r.close_incoming
r.send('test')
# Ractor::ClosedError (The incoming-port is already closed)
# The error would be raised immediately, not when ractor will try to receive

If the obj is unshareable, by default it would be copied into ractor by deep cloning. If the move: true is passed, object is moved into ractor and becomes inaccessible to sender.

r = Ractor.new {puts "Received: #{receive}"}
msg = 'message'
r.send(msg, move: true)
r.take
p msg

This prints:

Received: message
in `p': undefined method `inspect' for #<Ractor::MovedObject:0x000055c99b9b69b8>

All references to the object and its parts will become invalid in sender.

r = Ractor.new {puts "Received: #{receive}"}
s = 'message'
ary = [s]
copy = ary.dup
r.send(ary, move: true)

s.inspect
# Ractor::MovedError (can not send any methods to a moved object)
ary.class
# Ractor::MovedError (can not send any methods to a moved object)
copy.class
# => Array, it is different object
copy[0].inspect
# Ractor::MovedError (can not send any methods to a moved object)
# ...but its item was still a reference to `s`, which was moved

If the object was shareable, move: true has no effect on it:

r = Ractor.new {puts "Received: #{receive}"}
s = 'message'.freeze
r.send(s, move: true)
s.inspect #=> "message", still available

Take a message from ractor's outgoing port, which was put there by Ractor.yield or at ractor's finalization.

r = Ractor.new do
  Ractor.yield 'explicit yield'
  'last value'
end
puts r.take #=> 'explicit yield'
puts r.take #=> 'last value'
puts r.take # Ractor::ClosedError (The outgoing-port is already closed)

The fact that the last value is also put to outgoing port means that take can be used as some analog of Thread#join (“just wait till ractor finishes”), but don't forget it will raise if somebody had already consumed everything ractor have produced.

If the outgoing port was closed with close_outgoing, the method will raise Ractor::ClosedError.

r = Ractor.new do
  sleep(500)
  Ractor.yield 'Hello from ractor'
end
r.close_outgoing
r.take
# Ractor::ClosedError (The outgoing-port is already closed)
# The error would be raised immediately, not when ractor will try to receive

If an uncaught exception is raised in the Ractor, it is propagated on take as a Ractor::RemoteError.

r = Ractor.new {raise "Something weird happened"}

begin
  r.take
rescue => e
  p e              #  => #<Ractor::RemoteError: thrown by remote Ractor.>
  p e.ractor == r  # => true
  p e.cause        # => #<RuntimeError: Something weird happened>
end

Ractor::ClosedError is a descendant of StopIteration, so the closing of the ractor will break the loops without propagating the error:

r = Ractor.new do
  3.times {|i| Ractor.yield "message #{i}"}
  "finishing"
end

loop {puts "Received: " + r.take}
puts "Continue successfully"

This will print:

Received: message 0
Received: message 1
Received: message 2
Received: finishing
Continue successfully
An alias for inspect