Fix Rack::Timeout::RequestTimeoutException: Request ran for longer than 15000ms in Rails

This error means a request exceeded the configured Rack::Timeout threshold. Long-running requests block web server threads and degrade performance for all users. Move expensive operations to background jobs using Sidekiq or ActiveJob, optimize slow database queries, and add pagination to endpoints that return large datasets.

Reading the Stack Trace

Here's what each line means:

• rack-timeout (0.7.0) lib/rack/timeout/core.rb:120:in `call': Rack::Timeout raised this exception because the request exceeded the 15-second threshold.
• app/controllers/reports_controller.rb:15:in `generate': The generate action is performing expensive work inline instead of in a background job.
• app/models/report.rb:45:in `build_annual_report': The build_annual_report method runs complex queries that take too long for a web request.

Common Causes

def generate
  @report = Report.build_annual_report(params[:year])
  render json: @report
end

def index
  @orders = Order.all.map do |order|
    { order: order, items: order.line_items.map(&:name) }
  end
end

def verify
  response = Net::HTTP.get(URI('https://slow-api.example.com/verify'))
  render json: response
end

The Fix

Move the expensive report generation to a background job. Return a 202 Accepted response immediately and notify the user when the report is ready. This keeps web requests fast and prevents timeouts.

def generate
  @report = Report.build_annual_report(params[:year])
  render json: @report
end

def generate
  job = ReportJob.perform_later(params[:year], current_user.id)
  render json: { status: 'processing', job_id: job.provider_job_id }, status: :accepted
end

# app/jobs/report_job.rb
class ReportJob < ApplicationJob
  queue_as :reports

  def perform(year, user_id)
    report = Report.build_annual_report(year)
    user = User.find(user_id)
    ReportMailer.completed(user, report).deliver_later
  end
end

Testing the Fix

require 'rails_helper'

RSpec.describe ReportsController, type: :request do
  describe 'POST /reports/generate' do
    let(:user) { create(:user) }
    before { sign_in user }

    it 'returns accepted status' do
      post '/reports/generate', params: { year: 2024 }
      expect(response).to have_http_status(:accepted)
    end

    it 'enqueues a report job' do
      expect {
        post '/reports/generate', params: { year: 2024 }
      }.to have_enqueued_job(ReportJob)
    end
  end
end

Run your tests:

bundle exec rspec spec/requests/reports_spec.rb

Pushing Through CI/CD

git checkout -b fix/rails-rack-timeout,git add app/controllers/reports_controller.rb app/jobs/report_job.rb,git commit -m "fix: move report generation to background job to prevent timeout",git push origin fix/rails-rack-timeout

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']
      redis:
        image: redis:7
        ports: ['6379:6379']
    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. Move the expensive operation to a background job.
2. Add request specs verifying the async response.
3. Configure Sidekiq queues for the new job.
4. Open a pull request.
5. Merge and monitor job processing in staging.

Frequently Asked Questions