The ultimate guide to Ruby concurrency interviews. Developed by FAANG engineers, this course covers Monitors, Deferred Callbacks and problem-solving strategies for Reader-Writer and Dining Philosopher Problems.

Concurrency in Ruby is one of the most complex and advanced topics brought up during senior engineering interviews. Knowledge of concurrency and multithreading can put interviewees at a considerable advantage. This course lays the foundations of advanced concurrency and multithreading and explains concepts such as Monitors and Deferred Callbacks in depth. It also builds simple and complete solutions to popular concurrency problems that can be asked about in interviews like the Reader-Writer Problem and the Dining Philosopher Problem. While prior knowledge of concurrency is not required to follow through with this course, familiarity with the basics of Ruby would be helpful.

Ruby Concurrency for Senior Engineering Interviews

<div class="card">
   # Problem
    <div class="ans">

<p>Suppose we have a machine that creates molecules by combining atoms. We are creating water molecules by joining one oxygen and two hydrogen atoms. The atoms are represented by threads. The machine will wait for the required atoms (threads), then group one oxygen and two hydrogen threads to simulate the creation of a molecule. The molecule then exists the machine. You have to ensure that one molecule is completed before moving onto the next molecule. If more than the required number of threads arrive, they will have to wait. The figure below explains the working of our machine:</p>

<div style="width:50%;margin-right:auto;margin-left:auto;">

![](/api/collection/5307417243942912/6328791100030976/page/6525953858600960/image/5386284286607360)
</div>

<p>Two Hydrogen threads are admitted in the machine as they arrive but when the third thread arrives in step 3, it is made to wait. When an Oxygen thread arrives in step 4, it is allowed to enter the machine. A water molecule is formed in step 5 which exists the machine in step 6. That is when the waiting Hydrogen thread is notified and the process of creating more molecules continues. The threads can arrive in any order which means that HHO, OHH and HOH are all valid outputs.</p>

<p>The code for the class is as follows:</p>

```ruby
class H2OMachine

    def initialize
    end
    
    def HydrogenAtom
    end

    def OxygenAtom    
    end
end
```

<p>The input to the machine can be in any order. Your program should enforce a 2:1 ratio for Hydrogen and Oxygen threads, and stop more than the required number of threads from entering the machine.</p>


</div>
</div>

<div>
   # Problem
    <div>

<p>Suppose we have a machine that creates molecules by combining atoms. We are creating water molecules by joining one oxygen and two hydrogen atoms. The atoms are represented by threads. The machine will wait for the required atoms (threads), then group one oxygen and two hydrogen threads to simulate the creation of a molecule. The molecule then exists the machine. You have to ensure that one molecule is completed before moving onto the next molecule. If more than the required number of threads arrive, they will have to wait. The figure below explains the working of our machine:</p>

<div>

![](/api/collection/5307417243942912/6328791100030976/page/6525953858600960/image/5386284286607360)
</div>

<p>Two Hydrogen threads are admitted in the machine as they arrive but when the third thread arrives in step 3, it is made to wait. When an Oxygen thread arrives in step 4, it is allowed to enter the machine. A water molecule is formed in step 5 which exists the machine in step 6. That is when the waiting Hydrogen thread is notified and the process of creating more molecules continues. The threads can arrive in any order which means that HHO, OHH and HOH are all valid outputs.</p>

<p>The code for the class is as follows:</p>

```ruby
class H2OMachine

    def initialize
    end
    
    def HydrogenAtom
    end

    def OxygenAtom    
    end
end
```

<p>The input to the machine can be in any order. Your program should enforce a 2:1 ratio for Hydrogen and Oxygen threads, and stop more than the required number of threads from entering the machine.</p>


</div>
</div>

This problem simulates the creation of water molecule by grouping three threads representing Hydrogen and Oxygen atoms.

The Basics

API Overview

Brief Intro to Asynchronous Programming

Interview Practice Problems

Bonus Questions

Epilogue

Build a Molecule

Problem