IdempotencyLock

A simple, robust idempotency solution for Ruby on Rails applications. Ensures operations run exactly once using database-backed locks with support for TTL expiration, multiple error handling strategies, and clean return value handling.

Installation

Add this line to your application's Gemfile:

gem "idempotency_lock"

Then execute:

bundle install

Generate the migration:

rails generate idempotency_lock:install
rails db:migrate

Usage

Basic Usage

Wrap any operation that should only run once:

result = IdempotencyLock.once("send-welcome-email-user-#{user.id}") do
  UserMailer.welcome(user).deliver_now
end

if result.executed?
  puts "Email sent!"
else
  puts "Email already sent (skipped)"
end

The once method returns a Result object that tells you what happened:

result.executed?  # true if the block ran
result.skipped?   # true if skipped due to existing lock
result.success?   # true if executed without error
result.error?     # true if an error occurred
result.value      # the return value of the block
result.error      # the exception if one was raised

TTL / Expiration

Sometimes you want "run once per hour" instead of "run once forever":

# Run daily report once per day
IdempotencyLock.once("daily-report", ttl: 24.hours) do
  ReportGenerator.daily_summary
end

# Run cleanup once per hour
IdempotencyLock.once("hourly-cleanup", ttl: 1.hour) do
  TempFile.cleanup_old
end

# TTL also accepts integer seconds
IdempotencyLock.once("periodic-task", ttl: 3600) do
  some_periodic_work
end

Error Handling

Control what happens when an error occurs inside the block:

# :keep (default) - Keep the lock, don't allow retry
IdempotencyLock.once("risky-op", on_error: :keep) do
  might_fail
end

# :unlock - Release the lock so another process can retry
IdempotencyLock.once("retriable-op", on_error: :unlock) do
  external_api_call
end

# :raise - Re-raise the exception after releasing the lock
IdempotencyLock.once("critical-op", on_error: :raise) do
  must_succeed_or_alert
end

# Custom handler with a Proc
IdempotencyLock.once("custom-error", on_error: ->(e) { Bugsnag.notify(e) }) do
  something_important
end

Manual Lock Management

# Check if an operation is locked
IdempotencyLock.locked?("my-operation")  # => true/false

# Release a lock manually (useful for testing or manual intervention)
IdempotencyLock.release("my-operation")

# Clean up all expired locks (good for a periodic job)
IdempotencyLock.cleanup_expired

Alias

The wrap method is available as an alias for once:

IdempotencyLock.wrap("operation-name") { do_something }

Use Cases

Preventing Duplicate Webhook Processing

class WebhooksController < ApplicationController
  def stripe
    event_id = params[:id]

    result = IdempotencyLock.once("stripe-webhook-#{event_id}") do
      StripeWebhookProcessor.process(params)
    end

    head :ok
  end
end

Ensuring One-Time Migrations

IdempotencyLock.once("backfill-user-preferences-2024-01") do
  User.find_each do |user|
    user.create_preferences! unless user.preferences.present?
  end
end

Rate-Limited Operations

# Only send one reminder per user per day
IdempotencyLock.once("reminder-user-#{user.id}", ttl: 24.hours) do
  ReminderMailer.daily_reminder(user).deliver_later
end

Preventing Duplicate Sidekiq Jobs

class ImportantJob
  include Sidekiq::Job

  def perform(record_id)
    IdempotencyLock.once("important-job-#{record_id}", on_error: :unlock) do
      # If this fails, another job can retry
      ImportantService.process(record_id)
    end
  end
end

Database Schema

The gem creates an idempotency_locks table with:

Column	Type	Description
name	string (max 255)	Unique identifier for the lock
expires_at	datetime	When the lock expires (null = never)
executed_at	datetime	When the operation was executed
created_at	datetime	Standard Rails timestamp
updated_at	datetime	Standard Rails timestamp

Indexes:

• Unique index on name (provides atomicity)
• Index on expires_at (partial index on PostgreSQL/SQLite, regular index on MySQL)

Note on name length: Lock names are limited to 255 characters to ensure compatibility with database unique index limits. MySQL users with utf8mb4 encoding may need to reduce this to 191 characters by modifying the migration before running it.

How It Works

1. Lock Acquisition: Attempts to create a record with the given name. The unique index ensures only one process succeeds.

2. TTL Support: If a TTL is provided, the lock includes an expires_at timestamp. Expired locks can be claimed by new operations via an atomic UPDATE.

3. Error Handling: Different strategies control whether the lock is released on error, allowing for retry semantics.

4. Return Values: The block's return value is captured and returned in the Result object.

Thread Safety

The gem relies on database-level uniqueness constraints for atomicity. This means:

• ✅ Safe across multiple processes (web servers, job workers)
• ✅ Safe across multiple machines
• ✅ Safe in multi-threaded environments
• ✅ Survives application restarts

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests.

Contributing

Bug reports and pull requests are welcome on GitHub.

License

The gem is available as open source under the terms of the MIT License.