Fix NoMemoryError: failed to allocate memory (NoMemoryError) in Rails

This error occurs when your Ruby process exhausts available memory. Common causes include loading too many records into memory at once, large file processing without streaming, and memory leaks from global caches or retained references. Use find_each for batch processing, stream large files, and monitor memory with tools like derailed_benchmarks.

Reading the Stack Trace

Here's what each line means:

• app/services/export_service.rb:12:in `generate_csv': The export service loads all records into memory to generate the CSV.
• activerecord (7.1.3) lib/active_record/relation.rb:288:in `to_a': ActiveRecord converts the entire result set to an array, consuming large amounts of memory.
• app/controllers/exports_controller.rb:8:in `create': The controller action triggers the memory-intensive export operation synchronously.

Common Causes

class ExportService
  def generate_csv
    orders = Order.all.to_a  # Loads millions of rows into memory
    CSV.generate do |csv|
      orders.each { |o| csv << [o.id, o.total, o.created_at] }
    end
  end
end

def build_report
  result = ''
  Order.all.each do |order|
    result += order.to_json + "\n"  # Creates new string on each iteration
  end
  result
end

CACHE = {}
def fetch_data(key)
  CACHE[key] ||= expensive_computation(key)  # Never evicts entries
end

The Fix

Use find_each with batch_size to load records in batches of 1000 instead of all at once. Stream the CSV output to an IO object instead of building a giant string in memory.

class ExportService
  def generate_csv
    orders = Order.all.to_a
    CSV.generate do |csv|
      orders.each { |o| csv << [o.id, o.total, o.created_at] }
    end
  end
end

class ExportService
  def generate_csv(io = StringIO.new)
    csv = CSV.new(io)
    csv << ['ID', 'Total', 'Created At']
    Order.find_each(batch_size: 1000) do |order|
      csv << [order.id, order.total, order.created_at]
    end
    io.rewind
    io
  end
end

Testing the Fix

require 'rails_helper'

RSpec.describe ExportService do
  describe '#generate_csv' do
    it 'generates CSV without loading all records' do
      create_list(:order, 5)
      service = ExportService.new
      result = service.generate_csv
      csv = CSV.parse(result.string, headers: true)
      expect(csv.size).to eq(5)
    end

    it 'includes headers' do
      service = ExportService.new
      result = service.generate_csv
      headers = CSV.parse(result.string).first
      expect(headers).to eq(['ID', 'Total', 'Created At'])
    end
  end
end

Run your tests:

bundle exec rspec spec/services/export_service_spec.rb

Pushing Through CI/CD

git checkout -b fix/rails-memory-bloat,git add app/services/export_service.rb,git commit -m "fix: use find_each and streaming CSV to prevent memory bloat",git push origin fix/rails-memory-bloat

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    services:
      postgres:
        image: postgres:16
        env:
          POSTGRES_PASSWORD: postgres
        ports: ['5432:5432']
    steps:
      - uses: actions/checkout@v4
      - uses: ruby/setup-ruby@v1
        with:
          ruby-version: '3.3'
          bundler-cache: true
      - run: bin/rails db:setup
      - run: bundle exec rspec

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

gem install bugstack

require 'bugstack'

Bugstack.init(api_key: ENV['BUGSTACK_API_KEY'])

Deploying the Fix (Manual Path)

1. Replace .all.to_a with find_each batch processing.
2. Add streaming for large file generation.
3. Profile memory usage with derailed_benchmarks.
4. Open a pull request.
5. Merge and monitor memory usage in production.

Frequently Asked Questions