ruby 31 lines · 6 steps

A thread-safe counter with Mutex in Ruby

How a single mutex guards every read and write so concurrent threads never corrupt a shared counter.

Explained by highlit
Intermediate concurrency mutex thread-safety synchronization atomicity
1class ThreadSafeCounter
2 def initialize(initial = 0)
3 @count = initial
4 @mutex = Mutex.new
5 end
6 
7 def increment(by = 1)
8 @mutex.synchronize { @count += by }
9 end
10 
11 def decrement(by = 1)
12 @mutex.synchronize { @count -= by }
13 end
14 
15 def value
16 @mutex.synchronize { @count }
17 end
18 
19 def reset
20 @mutex.synchronize do
21 previous = @count
22 @count = 0
23 previous
24 end
25 end
26 
27 # Atomically read-modify-write under a single lock
28 def update
29 @mutex.synchronize { @count = yield(@count) }
30 end
31end
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Wrapping every access — reads included — in the same mutex is what makes the whole object thread-safe.
  2. 2`synchronize` returns its block's value, so guarded methods can read and return state in one atomic step.
  3. 3Passing a block lets callers run arbitrary read-modify-write logic atomically without exposing the lock.

Related explainers

ruby 
class ToastBroadcaster
  include ActionView::RecordIdentifier
 
  def self.broadcast_to(user, message:, type: :notice)

How Turbo Stream toasts broadcast in Rails

turbo-streams service-object real-time
Intermediate 6 steps
rust 
use axum::{extract::{Path, State}, http::StatusCode, Json};
use dashmap::DashMap;
use serde::Serialize;
use std::sync::Arc;

Request coalescing in an Axum handler

caching concurrency request-coalescing
Advanced 8 steps
ruby 
class TemplateInterpolator
  PLACEHOLDER = /\{\{\s*([\w.]+)\s*\}\}/
 
  def initialize(strict: false)

Interpolating templates with dotted keys in Ruby

regex string-interpolation hash-traversal
Intermediate 6 steps
ruby 
class KeyTransformer
  def self.camelize(data)
    new.camelize(data)
  end

Recursively camelizing nested Ruby data

recursion data-transformation pattern-matching
Intermediate 7 steps
ruby 
module Paginatable
  extend ActiveSupport::Concern
 
  private

A reusable pagination concern in Rails

pagination concerns http-headers
Intermediate 8 steps
go 
package config
 
import (
	"fmt"

A thread-safe config singleton in Go

singleton concurrency environment-variables
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

A thread-safe counter with Mutex in Ruby — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code