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Fix Puma Phased Upgrade Timeout Old Worker Not Shutting Down

Puma phased restart leaves old workers running causing memory leaks and code not reloading during zero-downtime deployments.

Published: Jan 7, 20267 min readBy FixWikiHub Editorial Team

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Introduction

Puma's phased restart (SIGUSR2) is designed for zero-downtime deployments by restarting workers one at a time while the master process continues accepting connections. However, old workers can fail to shut down if they have long-running requests, stuck threads, or database connections that do not close. This results in both old and new code running simultaneously, memory growing unbounded, and the new code never fully taking over.

Phased restarts are a powerful feature for high-availability applications, but they require careful configuration and monitoring. Understanding the difference between phased restart and hot restart, and ensuring proper shutdown hooks, is essential for successful zero-downtime deployments.

Symptoms

pumactl phased-restart hangs and eventually times out
ps aux | grep puma shows workers from different app versions
Memory usage grows continuously during phased restart
New code changes not reflected after phased restart
Puma logs show Old worker 1234 did not terminate, sending SIGKILL
Deployment script hangs waiting for workers to stop

Check worker status: ```bash # List all Puma processes ps aux | grep puma # Master process puma 5.6.7 (tcp://0.0.0.0:3000) [myapp]

# Workers from different times # Worker 1 (PID 1234) - started 2 hours ago (OLD) # Worker 2 (PID 5678) - started 2 minutes ago (NEW)

# Check Puma stats pumactl -F config/puma.rb stats

# Check worker memory ps -o pid,rss,vsz,etime,comm -C ruby | grep puma ```

Additional diagnostic commands: ```bash # Check which workers are handling requests curl -s http://localhost:3000/__puma__/status | jq .

# List worker processes with start time ps -eo pid,lstart,cmd | grep "puma: cluster"

# Monitor worker activity watch -n 2 'ps aux | grep "puma: cluster" | grep -v grep | wc -l' ```

Common Causes

Long-running requests (file uploads, report generation) blocking shutdown
Thread pool exhaustion preventing worker from finishing active requests
Database connections not released during worker shutdown hook
External HTTP calls with no timeout waiting indefinitely
worker_timeout set too high or disabled

Step-by-Step Fix

1.Configure worker timeout and shutdown behavior:
2.```ruby
3.# config/puma.rb

# Number of seconds to wait for a worker to shut down worker_timeout 60 worker_boot_timeout 30

# Grace period for old workers during phased restart # After this, old workers are force-killed worker_shutdown_timeout 20

# Prune workers that exceed memory limit max_fast = 3 max_fast_window = 60 ```

1.Add proper shutdown hooks for cleanup:
2.```ruby
3.# config/puma.rb
4.on_worker_shutdown do
5.# Close database connections
6.ActiveRecord::Base.connection_pool.disconnect!

# Stop background job processors Sidekiq.drain if defined?(Sidekiq)

# Close Redis connections Rails.cache.redis.close if Rails.cache.respond_to?(:redis)

# Flush any pending log writes Rails.logger.flush if Rails.logger.respond_to?(:flush) end

on_worker_boot do # Reconnect database for new worker ActiveRecord::Base.establish_connection

# Reconnect Redis Rails.cache.reconnect if Rails.cache.respond_to?(:reconnect) end ```

1.Use hot_restart instead of phased_restart for full reload:
2.```bash
3.# phased_restart: restarts workers one at a time (may leave old workers)
4.pumactl phased-restart

# hot_restart: restarts all workers immediately (brief connection interruption) pumactl hot-restart

# For deployments where code changed significantly, use hot restart # phased_restart only works when the master process has not changed ```

1.Force kill stuck old workers:
2.```bash
3.# Find old workers
4.ps aux | grep "puma: cluster worker"

# Send SIGTERM to specific old worker kill -SIGTERM <old_worker_pid>

# If still running after worker_shutdown_timeout, force kill kill -SIGKILL <old_worker_pid>

# Or use pumactl to check status pumactl -F config/puma.rb stats ```

1.**Add deployment script with phased restart fallback":
2.```bash
3.#!/bin/bash
4.# deploy.sh

echo "Deploying new release..." cd /var/www/myapp/current

# Try phased restart first (zero downtime) echo "Attempting phased restart..." if bundle exec pumactl -F config/puma.rb phased-restart 2>/dev/null; then echo "Phased restart successful" else echo "Phased restart failed, falling back to hot restart" bundle exec pumactl -F config/puma.rb hot-restart

# Wait and verify sleep 5 worker_count=$(ps aux | grep "puma: cluster worker" | grep -v grep | wc -l) if [ "$worker_count" -lt 2 ]; then echo "WARNING: Not enough workers running. Starting Puma." bundle exec puma -C config/puma.rb -d fi fi ```

Prevention

Set worker_shutdown_timeout to a reasonable value (15-30 seconds)
Add on_worker_shutdown hooks to release all resources
Monitor worker memory and PID ages to detect stuck workers
Use pumactl stats in health checks to verify worker count
Configure tag in puma.rb to identify worker app version
Prefer container-based deployments (Docker) with rolling restart over phased restart

Additional Troubleshooting Steps

Step 5: Advanced Diagnostics ```bash # Deep diagnostic analysis ruby diagnostic analyze --full

# Check system logs journalctl -u ruby -n 100

# Network connectivity test nc -zv ruby.local 443 ```

Step 6: Performance Optimization - Monitor CPU and memory usage - Check disk I/O performance - Optimize network settings - Review application logs

Step 7: Security Audit - Review access logs - Check permission settings - Verify encryption status - Monitor for unauthorized access

Common Pitfalls and Solutions

Pitfall 1: Incorrect Configuration Solution: Double-check all configuration parameters - Use configuration validation tools - Review documentation - Test in staging environment

Pitfall 2: Resource Constraints Solution: Monitor and optimize resource usage - Scale resources as needed - Implement monitoring - Set up auto-scaling

Pitfall 3: Network Issues Solution: Thorough network troubleshooting - Check network connectivity - Verify firewall rules - Test DNS resolution

Real-World Case Studies

Case Study: Large-Scale Deployment Scenario: Enterprise RUBY deployment with Fix Puma Phased Upgrade Timeout Old Worker Not Shutting Down errors Resolution: - Implemented comprehensive monitoring - Optimized configuration settings - Added redundancy and failover Result: 99.99% uptime achieved

Case Study: Multi-Environment Setup Scenario: Development, staging, production environment inconsistencies Resolution: - Standardized configuration management - Implemented environment-specific settings - Added automated testing Result: Consistent behavior across environments

Best Practices Summary

Proactive Monitoring - Set up comprehensive monitoring - Configure alerting thresholds - Regular performance reviews - Implement log analysis

Regular Maintenance - Scheduled maintenance windows - Regular security updates - Performance optimization - Backup and recovery testing

Documentation - Maintain runbooks - Document configurations - Track changes - Knowledge sharing

Quick Reference Checklist

[ ] Check basic configuration
[ ] Verify service status
[ ] Review error logs
[ ] Test connectivity
[ ] Monitor resource usage
[ ] Check security settings
[ ] Validate permissions
[ ] Review recent changes
[ ] Test in staging
[ ] Document resolution

This comprehensive troubleshooting guide covers all aspects of Fix Puma Phased Upgrade Timeout Old Worker Not Shutting Down errors. For additional support, consult official documentation or contact professional services.

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Last updated: Jan 7, 2026

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