Inside Rails Boot: Puma's Role in Initialization

Trace what happens when you run `rails server`! From Rails boot to Puma launch, uncover how components load, signals fire, plugins initialize, and requests are handled.

RubyConf India 2025

Transcript

1. 9 / 2 1 / 2 0 2 5 1 INSIDE THE RAILS BOOT PROCESS: UNPACKING PUMA’S ROLE IN RAILS SERVER INITIALIZATION
2. BELLO! Name: Udbhav Gambhir Role: Senior Engineer Company: ThoughtSpot /ngudbhav www.ngudbhav.com
3. H O W D O Y O U S TA R T T H E P U M A ? ❑ rails server ❑ foreman (via Procfile) ❑ puma-dev ❑ pumactl ❑ bundle exec puma ❑ Any other...? 9 / 2 1 / 2 0 2 5 3
4. P O S T-T E A M Y S T E R Y: W H AT H A P P E N S A F T E R R A I L S S E R V E R ? 9 / 2 1 / 2 0 2 5 4
5. 9 / 2 1 / 2 0 2 5 5 if Object.const_defined?(:Rackup) && ::Rackup.const_defined?(:Handler) module Rackup module Handler module Puma class << self include ::Puma::RackHandler end end register :puma, Puma end end else do_register = Object.const_defined?(:Rack) && ::Rack.release < '3' module Rack module Handler module Puma class << self include ::Puma::RackHandler end end end end ::Rack::Handler.register(:puma, ::Rack::Handler::Puma) if do_register end
6. 9 / 2 1 / 2 0 2 5 6 # rackup was removed in Rack 3, it is now a separate gem if Object.const_defined?(:Rackup) && ::Rackup.const_defined?(:Handler) module Rackup module Handler module Puma class << self include ::Puma::RackHandler end end register :puma, Puma end end else do_register = Object.const_defined?(:Rack) && ::Rack.release < '3' module Rack module Handler module Puma class << self include ::Puma::RackHandler end end end end ::Rack::Handler.register(:puma, ::Rack::Handler::Puma) if do_register end
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9. B U N D L E E X E C P U M A 9 / 2 1 / 2 0 2 5 9
10. P U M A C O N T R O L F L OW 9 / 2 1 / 2 0 2 5 10
11. P O S T-T E A M Y S T E R Y: W H AT H A P P E N S A F T E R R A I L S S E R V E R ? 9 / 2 1 / 2 0 2 5 11
12. P U M A C O N T R O L F L OW 9 / 2 1 / 2 0 2 5 12
13. L E T ' S K E E P I T S I M P L E ! ❑ Load the entire application ❑ Start all the background plugins ❑ Starts Control Server ❑ Start Server 9 / 2 1 / 2 0 2 5 13
14. 9 / 2 1 / 2 0 2 5 14 while true @requests_count += 1 handle_request(client, requests + 1)
15. 9 / 2 1 / 2 0 2 5 15 I T ' S A S C A L I N G P R O B L E M !
16. 9 / 2 1 / 2 0 2 5 16 while true @requests_count += 1 handle_request(client, requests + 1)
17. 9 / 2 1 / 2 0 2 5 17
18. M E M O R Y U S AG E ❑ memory_usage = static_memory + (threads * processing_memory) 9 / 2 1 / 2 0 2 5 18
19. P L E T H O R A O F D I S A D VAN TAG E S 9 / 2 1 / 2 0 2 5 19
20. 9 / 2 1 / 2 0 2 5 20 G L O BA L V I RT UA L M AC H I N E L O C K ( GV L )
21. 9 / 2 1 / 2 0 2 5 21 R A C E C O N D I T I O N S
22. 9 / 2 1 / 2 0 2 5 22 FA ULT TO L E R A N C E
23. 9 / 2 1 / 2 0 2 5 23 M ULT I -T H R E A D I N G V S M ULT I - P R O C E S S I N G
24. M E M O R Y U S AG E ❑ memory_usage = (static_memory + processing_memory) * processes 9 / 2 1 / 2 0 2 5 24
25. S O L U T I O N = > C oW ( C O P Y O N W R I T E ) 9 / 2 1 / 2 0 2 5 25 ❑ memory_usage = static_memory + (processes * processing_memory)
26. P U M A C O N T R O L F L OW 9 / 2 1 / 2 0 2 5 26
27. L E T ' S K E E P I T S I M P L E ! ❑ Load the entire application ❑ Start all the background plugins ❑ Starts Control Server ❑ Start Server 9 / 2 1 / 2 0 2 5 27
28. P U M A A N D C oW 9 / 2 1 / 2 0 2 5 28 def spawn_worker(idx, master) @config.run_hooks(:before_worker_fork, idx, @log_writer) pid = fork { worker(idx, master) } @config.run_hooks(:after_worker_fork, idx, @log_writer) pid end
29. T H A N K YO U ! 9 / 2 1 / 2 0 2 5 29