class Ractor

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

Ractor.new makes a new Ractor, which can run in parallel.

# The simplest ractor
r = Ractor.new {puts "I am in Ractor!"}
r.take # wait for it to finish
# Here, "I am in Ractor!" is printed

Ractors do not share all objects with each other. There are two main benefits to this: across ractors, thread-safety concerns such as data-races and race-conditions are not possible. The other benefit is parallelism.

To achieve this, object sharing is limited across ractors. For example, unlike in threads, ractors can’t access all the objects available in other ractors. Even objects normally available through variables in the outer scope are prohibited from being used across ractors.

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).)

The object must be explicitly shared:

a = 1
r = Ractor.new(a) { |a1| puts "I am in Ractor! a=#{a1}"}

On CRuby (the default implementation), Global Virtual Machine Lock (GVL) is held per ractor, so ractors can perform in parallel without locking each other. This is unlike the situation with threads on CRuby.

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

a = 1
r = Ractor.new do
  a_in_ractor = receive # receive blocks until somebody passes a 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" is printed

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

In addition to that, any arguments passed to Ractor.new are passed to the block and available there as if received by Ractor.receive, and the last block value is sent outside of the ractor as if sent by Ractor.yield.

A little demonstration of a classic ping-pong:

server = Ractor.new(name: "server") 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 to the client, and available as srv
  puts "Client starts: #{self.inspect}"
  received = srv.take                       # The client takes a message 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 to the server
end

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

This will output something like:

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

Ractors receive their messages via the incoming port, and send them to the outgoing port. Either one can be disabled with Ractor#close_incoming and Ractor#close_outgoing, respectively. When a ractor terminates, its ports are closed automatically.

Shareable and unshareable objects ↑

When an object is sent to and from a ractor, it’s important to understand whether the object is shareable or unshareable. Most Ruby objects are unshareable objects. Even frozen objects can be unshareable if they contain (through their instance variables) unfrozen objects.

Shareable objects are those which can be used by several threads without compromising thread-safety, for example numbers, true and false. Ractor.shareable? allows you to check this, and Ractor.make_shareable tries to make the object shareable if it’s not already, and gives an error if it can’t do it.

Ractor.shareable?(1)            #=> true -- numbers and other immutable basic values are shareable
Ractor.shareable?('foo')        #=> false, unless the string is frozen due to # frozen_string_literal: true
Ractor.shareable?('foo'.freeze) #=> true
Ractor.shareable?([Object.new].freeze) #=> false, inner object is unfrozen

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 occurs on it. 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 copies the object fully by deep cloning (Object#clone) the 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 something like:

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

Note that the object ids of the array and the non-frozen string inside the array have changed in the ractor because they are different objects. The second array’s element, which is a shareable frozen string, is the same object.

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

Class and Module objects are shareable so the class/module definitions are shared between ractors. Ractor objects are also shareable. All operations on shareable 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 (get) instance variables of shareable objects in other ractors if the values of the variables aren’t shareable. This can occur because modules/classes are shareable, but they can have instance variables whose values are not. In non-main ractors, it’s also prohibited to set instance variables on classes/modules (even if the value is shareable).

class C
  class << self
    attr_accessor :tricky
  end
end

C.tricky = "unshareable".dup

r = Ractor.new(C) do |cls|
  puts "I see #{cls}"
  puts "I can't see #{cls.tricky}"
  cls.tricky = true # doesn't get here, but this would also raise an error
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'.dup

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 has its own main Thread. New threads can be created from inside ractors (and, on CRuby, they share the 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 the examples below, sometimes we use the following method to wait for ractors that are not currently blocked to finish (or to make progress).

def wait
  sleep(0.1)
end

It is **only for demonstration purposes** and shouldn’t be used in a real code. Most of the time, take is used to wait for ractors to finish.

Reference ↑

See Ractor design doc for more details.

Public Class Methods

count ()

Returns the number of Ractors currently running or blocking (waiting).

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 until r will finish
Ractor.count                   #=> 1

# File ractor.rb, line 302
def self.count
  __builtin_cexpr! %q{
    ULONG2NUM(GET_VM()->ractor.cnt);
  }
end
current ()

Returns the currently executing Ractor.

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

# File ractor.rb, line 288
def self.current
  __builtin_cexpr! %q{
    rb_ractor_self(rb_ec_ractor_ptr(ec));
  }
end
main ()

returns main ractor

# File ractor.rb, line 848
def self.main
  __builtin_cexpr! %q{
    rb_ractor_self(GET_VM()->ractor.main_ractor);
  }
end
make_shareable(obj, copy: false) → shareable_obj

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, it will copy objects before freezing them, and will not modify obj or its internal objects.

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.

# File ractor.rb, line 825
def self.make_shareable obj, copy: false
  if copy
    __builtin_cexpr! %q{
      rb_ractor_make_shareable_copy(obj);
    }
  else
    __builtin_cexpr! %q{
      rb_ractor_make_shareable(obj);
    }
  end
end
new(*args, name: nil) {|*args| block } → ractor

Create a new Ractor with args and a block.

The given block (Proc) will be isolated (can’t access any 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>"

Any args passed are propagated to the block arguments by the same rules as objects sent via send/Ractor.receive. If an argument in args is not shareable, it 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') {}; r.take
p r
#=> #<Ractor:#3 my ractor test.rb:1 terminated>

# File ractor.rb, line 273
def self.new(*args, name: nil, &block)
  b = block # TODO: builtin bug
  raise ArgumentError, "must be called with a block" unless block
  if __builtin_cexpr!("RBOOL(ruby_single_main_ractor)")
    warn("Ractor is experimental, and the behavior may change in future versions of Ruby! " \
         "Also there are many implementation issues.", uplevel: 0, category: :experimental)
  end
  loc = caller_locations(1, 1).first
  loc = "#{loc.path}:#{loc.lineno}"
  __builtin_ractor_create(loc, name, args, b)
end
receive → msg

Receive a message from the incoming port of the current ractor (which was sent there by send from another ractor).

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

Alternatively, the private instance method receive may be used:

r = Ractor.new do
  v1 = receive
  puts "Received: #{v1}"
end
r.send('message1')
r.take
# This prints: "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 the 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)

# File ractor.rb, line 430
def self.receive
  __builtin_cexpr! %q{
    ractor_receive(ec, rb_ec_ractor_ptr(ec))
  }
end
Also aliased as: recv
receive_if {|msg| block } → msg

Receive only a specific message.

Instead of Ractor.receive, Ractor.receive_if can be given a pattern (or any filter) in a block and you can choose the messages to accept that are available in your ractor’s incoming queue.

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 is removed from the incoming queue and returned. Otherwise, the message remains in the incoming queue and the next 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. You should not do any tasks in the block other than message filtration.

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

# File ractor.rb, line 509
def self.receive_if &b
  Primitive.ractor_receive_if b
end
recv
Alias for: receive
select(*ractors, [yield_value:, move: false]) → [ractor or symbol, obj]

Wait for any ractor to have something in its outgoing port, read from this ractor, and then return 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>
# But could just as well print "from r2" here, either prints could be first.

If one of the given ractors is the current ractor, and it is selected, r will contain the :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}"
# Could print: 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] is 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 will be copied (default) or moved.

# File ractor.rb, line 358
def self.select(*ractors, yield_value: yield_unspecified = true, move: false)
  raise ArgumentError, 'specify at least one ractor or `yield_value`' if yield_unspecified && ractors.empty?

  if ractors.delete Ractor.current
    do_receive = true
  else
    do_receive = false
  end

  __builtin_ractor_select_internal ractors, do_receive, !yield_unspecified, yield_value, move
end
shareable?(obj) → true | false

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 # frozen_string_literal: true
Ractor.shareable?('foo'.freeze) #=> true

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

# File ractor.rb, line 784
def self.shareable? obj
  __builtin_cexpr! %q{
    RBOOL(rb_ractor_shareable_p(obj));
  }
end
yield(msg, move: false) → nil

Send a message to the current ractor’s outgoing port to be accepted by take.

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

This 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 the move argument is the same as for send.

# File ractor.rb, line 643
def self.yield(obj, move: false)
  __builtin_cexpr! %q{
    ractor_yield(ec, rb_ec_ractor_ptr(ec), obj, move)
  }
end

Public Instance Methods

<<
Alias for: send
[] (sym)

get a value from ractor-local storage

# File ractor.rb, line 838
def [](sym)
  Primitive.ractor_local_value(sym)
end
[]= (sym, val)

set a value in ractor-local storage

# File ractor.rb, line 843
def []=(sym, val)
  Primitive.ractor_local_value_set(sym, val)
end
close_incoming → true | false

Closes the incoming port and returns whether it was already closed. 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)

# File ractor.rb, line 749
def close_incoming
  __builtin_cexpr! %q{
    ractor_close_incoming(ec, RACTOR_PTR(self));
  }
end
close_outgoing → true | false

Closes the outgoing port and returns whether it was already closed. 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)

# File ractor.rb, line 767
def close_outgoing
  __builtin_cexpr! %q{
    ractor_close_outgoing(ec, RACTOR_PTR(self));
  }
end
inspect () 
# File ractor.rb, line 716
def inspect
  loc  = __builtin_cexpr! %q{ RACTOR_PTR(self)->loc }
  name = __builtin_cexpr! %q{ RACTOR_PTR(self)->name }
  id   = __builtin_cexpr! %q{ UINT2NUM(rb_ractor_id(RACTOR_PTR(self))) }
  status = __builtin_cexpr! %q{
    rb_str_new2(ractor_status_str(RACTOR_PTR(self)->status_))
  }
  "#<Ractor:##{id}#{name ? ' '+name : ''}#{loc ? " " + loc : ''} #{status}>"
end
Also aliased as: to_s
name ()

The name set in Ractor.new, or nil.

# File ractor.rb, line 729
def name
  __builtin_cexpr! %q{RACTOR_PTR(self)->name}
end
recv ()
Alias for: receive
send(msg, move: false) → self

Send a message to a Ractor’s incoming queue to be accepted 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

An attempt to send to a 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 is raised immediately, not when the ractor tries to receive

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

# File ractor.rb, line 599
def send(obj, move: false)
  __builtin_cexpr! %q{
    ractor_send(ec, RACTOR_PTR(self), obj, move)
  }
end
Also aliased as: <<
take → msg

Get a message from the ractor’s outgoing port, which was put there by Ractor.yield or at ractor’s termination.

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 sent to the outgoing port means that take can be used as an analog of Thread#join (“just wait until ractor finishes”). However, it will raise if somebody has already consumed that message.

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 by 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 termination of the ractor will break out of any loops that receive this message 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
# File ractor.rb, line 710
def take
  __builtin_cexpr! %q{
    ractor_take(ec, RACTOR_PTR(self))
  }
end
to_s ()
Alias for: inspect