Ruby Exception Handling

Ruby Exception Handling: FiberError

Our journey through our Ruby Exception Handling series continues, as today we’re taking a look at the FiberError. A FiberError occurs when attempting to make a call to Fiber class methods, after the fiber has been terminated.

In this article, we’ll examine the FiberError class in more detail, explore where it sits within Ruby’s Exception class hierarchy, and also investigate how to handle FiberErrors in your own code. Let’s get to it!

The Technical Rundown

  • All Ruby exceptions are descendants of the Exception class, or a subclass therein.
  • StandardError is a direct descendant of the Exception class, and is also a superclass with many descendants of its own.
  • FiberError is a direct descendant of the EncodingError class.

When Should You Use It?

As is often the case, to understand what a FiberError means and how it functions, we must first understand another aspect of Ruby coding; in this case, the Fiber class.

Fundamentally, fibers (known as continuations in older Ruby versions) are light-weight threads, used for implementing concurrency logic that is manually managed by the code (rather than the OS or virtual machine). While not commonly used, the basic purpose of fibers in Ruby is that a Fiber code block “remembers” its code state, and allows you to resume execution within that code block at any time.

Most commonly, this functionality is best used for creating Enumerators, which are just classes that allow for iteration over a series of objects. For example, imagine we want to create a Counter class, that allows us to enumerate a series of increasing integers. We can create it using the enum_for method within our own Counterclass iterator method, by then specifying our series of iterations using the yield call:

Calling Counter.new.iterator generates our new Enumerator object (which we can see by outputting the countervariable). Then, as expected, we can use our Enumerator behavior by calling the automatic .next method on it. For all Enumerators in Ruby, this calls the next yield to be iterated, so we output the next number in our list each time. Here’s the output we generated:

The obvious problem here is that we hard-coded each of our yield statements in our original Counter class iterator. This is a very poor practice, so we can clean it up by using a Fiber instead:

While this is a few more lines of code, this is far cleaner and, most importantly, we are no longer forced to manually indicate every single yield iteration that we expect. Instead, we’ve setup a Fiber.new block. Within that block, we’ve created an infinite loop that uses Fiber.yield to yield the next iteration of our count variable, before incrementing the value by +1.

The magic of using fibers is that we can then call the .resume method of our fiber from anywhere in code, and Ruby will find the Fiber.yield statement then resume processing from there, before continuing execution where that .resume was called. In other words, fiber allows us to execute code blocks “out of order”, as Ruby will remember where the fiber yield statement is waiting for our next .resume method call.

The result is that we can call the .next method for our counter as many times as we want, from anywhere, and it will execute @fiber.resume, which then yields the current count value within our class, iterates the count by one, then exits that loop block again until the next time we call .next. The output is as expected:

Whew! Now that we understand the basic purpose of Fiber, we can see how a FiberError might occur. Simply put, FiberErrors occur when using Fiber methods as above, but when the fiber can no longer call the .yieldmethod because it has been terminated. Termination occurs when the final Fiber.yield call has been made within the fiber code block. This causes any future attempts to call the fiber's .resume method to produce a FiberError, because that fiber is now dead or terminated.

In this example, we’ve modified the code above by removing the infinite loop. This forces our Fiber.yield calls to be finite (in fact, we only have one now):

There’s a bit of extra code that we’ll cover in a second, but let’s execute this without the infinite loop surrounding our Fiber.yield call and see what happens:

Sure enough, a FiberError is produced, but let’s examine the code a bit more to figure out what’s going on. One thing we did is add a call to the Fiber.alive? method within our Counter.next method. This simply checks if the specified fiber is still active/can be yielded, and returns a boolean indicating its status. With that in mind, let’s see what’s happening here. As before, we call the .next method three times in total, so let’s iterate over each to see why the FiberError is thrown.

counter.next Call #1

The first time we call .next, the output shows that Yes, @fiber is alive. It then calls .resume, which jumps to the Fiber.yield statement inside our fiber code block, which outputs the initial count value of zero:

counter.next Call #2

The second time we call .next, prior to calling @fiber.resume, our fiber is still alive. However, once Fiber.resume is called, the fiber cannot find the next Fiber.yield statement within its block (because there isn’t one), so it terminates itself. NOTE: Upon termination, a fiber will return the value of the last executed expression within the Fiber code block. This is why we added the “Fiber Terminated” line, so that the output from our secondcounter.next call indicates that our fiber has, in fact, been terminated:

counter.next Call #3

For our third and final counter.next call, even prior to calling .resume on our fiber, we have confirmed that it is no longer alive. As expected, the subsequent call to .resume is attempted on a fiber that has been terminated (is dead), so we get the expected FiberError in our output:

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