Fibers are primitives for implementing light weight cooperative concurrency in Ruby. Basically they are a means of creating code blocks that can be paused and resumed, much like threads. The main difference is that they are never preempted and that the scheduling must be done by the programmer and not the VM.
As opposed to other stackless light weight concurrency models, each fiber comes with a small 4KB stack. This enables the fiber to be paused from deeply nested function calls within the fiber block.
When a fiber is created it will not run automatically. Rather it must be be explicitly asked to run using the Fiber#resume method. The code running inside the fiber can give up control by calling Fiber.yield in which case it yields control back to caller (the caller of the Fiber#resume).
Upon yielding or termination the Fiber returns the value of the last executed expression
For instance:
fiber = Fiber.new do
Fiber.yield 1
2
end
puts fiber.resume
puts fiber.resume
puts fiber.resume
produces
1 2 FiberError: dead fiber called
The Fiber#resume method accepts an arbitary number of parameters, if it is the first call to resume then they will be passed as block arguments. Otherwise they will be the return value of the call to Fiber.yield
Example:
fiber = Fiber.new do |first|
second = Fiber.yield first + 2
end
puts fiber.resume 10
puts fiber.resume 14
puts fiber.resume 18
produces
12 14 FiberError: dead fiber called
Returns the current fiber. You need to require ‘fiber‘ before using this method. If you are not running in the context of a fiber this method will return the root fiber.
Yields control back to the context that resumed the fiber, passing along any arguments that were passed to it. The fiber will resume processing at this point when resume is called next. Any arguments passed to the next resume will be the value that this Fiber.yield expression evaluates to.
Returns true if the fiber can still be resumed (or transferred to). After finishing execution of the fiber block this method will always return false.
Resumes the fiber from the point at which the last Fiber.yield was called, or starts running it if it is the first call to resume. Arguments passed to resume will be the value of the Fiber.yield expression or will be passed as block parameters to the fiber‘s block if this is the first resume.
Alternatively, when resume is called it evaluates to the arguments passed to the next Fiber.yield statement inside the fiber‘s block or to the block value if it runs to completion without any Fiber.yield
Transfer control to another fiber, resuming it from where it last stopped or starting it if it was not resumed before. The calling fiber will be suspended much like in a call to Fiber.yield.
The fiber which recieves the transfer call is treats it much like a resume call. Arguments passed to transfer are treated like those passed to resume.
You cannot resume a fiber that transferred control to another one. This will cause a double resume error. You need to transfer control back to this fiber before it can yield and resume.