# class Fiber::ExecutionContext::Concurrent

- Fiber::ExecutionContext::Concurrent
- Fiber::ExecutionContext::Parallel
- Reference
- Object

##
Overview

Concurrent-only execution context.

Fibers running in the same context can only run concurrently and never in parallel to each others. However, they still run in parallel to fibers running in other execution contexts.

Fibers in this context can use simpler and faster synchronization primitives between themselves (for example no atomics or thread safety required), but data shared with other contexts needs to be protected (see `Sync`), and communication with fibers in other contexts requires safe primitives, for example `Channel`.

A blocking fiber blocks the entire context, and thus all the other fibers in the context.

For example: we can start a concurrent context to run consumer fibers, while the default context produces values. Because the consumer fibers will never run in parallel and don't yield between reading result then writing it, we are not required to synchronize accesses to the value:

    require "wait_group"

    consumers = Fiber::ExecutionContext::Concurrent.new("consumers")
    channel = Channel(Int32).new(64)
    wg = WaitGroup.new(32)

    result = 0

    32.times do
      consumers.spawn do
        while value = channel.receive?
          # safe, but only for this example:
          result = result + value
        end
      ensure
        wg.done
      end
    end

    1024.times { |i| channel.send(i) }
    channel.close

    # wait for all workers to be done
    wg.wait

    p result # => 523776

In practice, we still recommended to always protect shared accesses to a variable, for example using `Atomic#add` to increment result or a `Sync` primitive for more complex operations.

NOTE The `Concurrent` execution context isn't tied to a system thread, and may switch to another system thread, for example when a fiber is blocked on a syscall.

##
Defined in:

fiber/execution_context/concurrent.cr

##
Constructors

- .new(name : String) : self

  Creates a `Concurrent` context.

##
Instance Method Summary

- #resize(maximum : Int32) : Nil

  Always raises an `ArgumentError` exception because a concurrent context cannot be resized.

### Instance methods inherited from class `Fiber::ExecutionContext::Parallel`

capacity : Int32 capacity, inspect(io : IO) : Nil inspect, name : String name, resize(maximum : Int32) : Nil resize, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Fiber::ExecutionContext::Parallel`

new(name : String, maximum : Int32) : self
new(name : String, size : Range(Nil, Int32)) : self
new(name : String, size : Range(Int32, Int32)) : self new

### Instance methods inherited from module `Fiber::ExecutionContext`

spawn(*, name : String | Nil = nil, &block : -> ) : Fiber spawn

### Constructor methods inherited from module `Fiber::ExecutionContext`

current : ExecutionContext current

### Class methods inherited from module `Fiber::ExecutionContext`

current? : ExecutionContext | Nil current?, default : ExecutionContext::Parallel default, default_workers_count : Int32 default_workers_count, each(&) : Nil each, thread_keepalive : Time::Span thread_keepalive, thread_keepalive=(thread_keepalive : Time::Span) thread_keepalive=

### Instance methods inherited from class `Reference`

==(other : self)
==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup, hash(hasher) hash, initialize initialize, inspect(io : IO) : Nil inspect, object_id : UInt64 object_id, pretty_print(pp) : Nil pretty_print, same?(other : Reference) : Bool
same?(other : Nil) same?, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Reference`

new new, unsafe_construct(address : Pointer, *args, **opts) : self unsafe_construct

### Class methods inherited from class `Reference`

pre_initialize(address : Pointer) pre_initialize

### Instance methods inherited from class `Object`

! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil!(message)
not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io : String | IO, root : String)
from_json(string_or_io : String | IO) from_json, from_yaml(string_or_io : String | IO) from_yaml

### Macros inherited from class `Object`

class_getter(*names, &block) class_getter, class_getter!(*names) class_getter!, class_getter?(*names, &block) class_getter?, class_property(*names, &block) class_property, class_property!(*names) class_property!, class_property?(*names, &block) class_property?, class_setter(*names) class_setter, def_clone def_clone, def_equals(*fields) def_equals, def_equals_and_hash(*fields) def_equals_and_hash, def_hash(*fields) def_hash, delegate(*methods, to object) delegate, forward_missing_to(delegate) forward_missing_to, getter(*names, &block) getter, getter!(*names) getter!, getter?(*names, &block) getter?, property(*names, &block) property, property!(*names) property!, property?(*names, &block) property?, setter(*names) setter

##
Constructor Detail

def self.new(name : String) : self #

Creates a `Concurrent` context. The context will only really start when a fiber is spawned into it.

##
Instance Method Detail

def resize(maximum : Int32) : Nil #

Always raises an `ArgumentError` exception because a concurrent context cannot be resized.