class Sync::Shared(T)

Overview

Safely share a value T across fibers and execution contexts using a RWLock to control when the access to a value can be shared (read-only) or must be exclusive (replace or mutate the value).

For example:

require "sync/shared"

class Queue
  @@running : Sync::Shared.new([] of Queue)

  def self.on_started(queue)
    @@running.write(&.push(queue))
  end

  def self.on_stopped(queue)
    @@running.write(&.delete(queue))
  end

  def self.each(&)
    @@running.read do |list|
      list.each { |queue| yield queue }
    end
  end
end

Consider a Shared(T) if your workload mostly consists of immutable reads of the value, with only seldom writes or inner mutations of the value's inner state.

Included Modules

Defined in:

shared.cr

Constructors

Instance Method Summary

Constructor Detail

def self.new(value : T, type : Type = :checked) #

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Instance Method Detail

def clone_value : T #

Locks in shared mode and returns a deep copy of the value. The lock is released before returning the new value.

@[Experimental("The method may not have much value over #read(&.clone)")]


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def dup_value : T #

Locks in shared mode and returns a shallow copy of the value. The lock is released before returning the new value.

Sometimes it may not be needed to own or borrow the value (no need to mutate it, or the value is returned by copy anyway), and taking a copy can help release the lock early, instead of keeping the lock acquired for a while, preventing progress of fibers trying to lock exclusively.

@[Experimental("The method may not have much value over #read(&.dup)")]


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def read(& : T -> U) : U forall U #

Locks in shared mode and yields the value. The lock is released before returning.

The value is owned in shared mode for the duration of the block, and thus shouldn't be mutated for example, unless T can be safely mutated (it should be Sync::Safe).

WARNING The value musn't be retained and accessed after the block has returned.


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def replace(& : T -> T) : Nil #

Locks in exclusive mode, yields the current value and eventually replaces the value with the one returned by the block. The lock is released before returning.

The current value is now owned: it can be safely retained and mutated even after the block returned.

WARNING The new value musn't be retained and accessed after the block has returned.


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def set(value : T) : Nil #

Locks in exclusive mode and sets the value.


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def unsafe_get : T #

Returns the value without any synchronization.

WARNING Breaks the safety constraints! Only use when you can guarantee that the current fiber acquired the lock or to access a value that can be read in a single load operation from memory.


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def unsafe_set(value : T) : T #

Sets the value without any synchronization.

WARNING Breaks the safety constraints! Only use when you can guarantee that the current fiber acquired the lock or to access a value that can be written in a single store operation into memory.


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def value : T #

Locks in shared mode and returns the value. Unlocks before returning.

Always acquires the lock, so reading the value is synchronized in relation with the other methods. However, safely accessing the returned value entirely depends on the safety of T, which should be Sync::Safe.

Prefer #dup_value or #clone_value to get a shallow or deep copy of the value instead.

WARNING Breaks the shared/exclusive guarantees since the returned value outlives the lock, the value can be accessed concurrently to the synchronized methods.

@[Experimental("The method may not have much value over #read(&.itself)")]


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def write(& : T -> U) : U forall U #

Locks in exclusive mode and yields the value. The lock is released before returning.

The value is owned in exclusive mode for the duration of the block, as such it can be safely mutated.

WARNING The value musn't be retained and accessed after the block has returned.


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