semmitmondo
Hi,
I just found a very strange and mysterious error here. I initially thought it was me doing something the wrong way, but I managed to strip it down to a bare, out of the box Phoenix app. Here’s how to reproduce it:
mix phx.hew test --no-ecto
cd test/
iex -S mix
Then start Observer and kill the TestWeb.Endpoint process. What you will see is the whole application is exiting. The reason is that the top level supervisor cannot restart the Endpoint. Well, that’s not quite true, because if you trace the top level supervisor, it seems that the restart succeeded, still the supervisor thinks it failed:
iex> :sys.trace Process.whereis(Test.Supervisor), true
Here’s the full output.
*DBG* 'Elixir.Test.Supervisor' got {'EXIT',<0.235.0>,killed}
*DBG* 'Elixir.Test.Supervisor' new state {state,
{local,'Elixir.Test.Supervisor'},
one_for_one,
{['Elixir.TestWeb.Endpoint'],
#{'Elixir.TestWeb.Endpoint' =>
{child,
{restarting,<0.235.0>},
'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}}},
undefined,3,5,
[-576460567],
0,'Elixir.Supervisor.Default',
{ok,
{#{intensity => 3,period => 5,
strategy => one_for_one},
[{'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}]}}}
*DBG* 'Elixir.Test.Supervisor' got {'EXIT',<0.336.0>,
{shutdown,
{failed_to_start_child,
'Elixir.Phoenix.PubSub.PG2',
{already_started,<0.237.0>}}}}
*DBG* 'Elixir.Test.Supervisor' new state {state,
{local,'Elixir.Test.Supervisor'},
one_for_one,
{['Elixir.TestWeb.Endpoint'],
#{'Elixir.TestWeb.Endpoint' =>
{child,
{restarting,<0.235.0>},
'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}}},
undefined,3,5,
[-576460567],
0,'Elixir.Supervisor.Default',
{ok,
{#{intensity => 3,period => 5,
strategy => one_for_one},
[{'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}]}}}
*DBG* 'Elixir.Test.Supervisor' got cast {try_again_restart,
'Elixir.TestWeb.Endpoint'}
*DBG* 'Elixir.Test.Supervisor' new state {state,
{local,'Elixir.Test.Supervisor'},
one_for_one,
{['Elixir.TestWeb.Endpoint'],
#{'Elixir.TestWeb.Endpoint' =>
{child,
{restarting,<0.235.0>},
'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}}},
undefined,3,5,
[-576460567,-576460567],
0,'Elixir.Supervisor.Default',
{ok,
{#{intensity => 3,period => 5,
strategy => one_for_one},
[{'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}]}}}
*DBG* 'Elixir.Test.Supervisor' got {'EXIT',<0.337.0>,
{shutdown,
{failed_to_start_child,
'Elixir.Phoenix.PubSub.PG2',
{already_started,<0.237.0>}}}}
*DBG* 'Elixir.Test.Supervisor' new state {state,
{local,'Elixir.Test.Supervisor'},
one_for_one,
{['Elixir.TestWeb.Endpoint'],
#{'Elixir.TestWeb.Endpoint' =>
{child,
{restarting,<0.235.0>},
'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}}},
undefined,3,5,
[-576460567,-576460567],
0,'Elixir.Supervisor.Default',
{ok,
{#{intensity => 3,period => 5,
strategy => one_for_one},
[{'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}]}}}
*DBG* 'Elixir.Test.Supervisor' got cast {try_again_restart,
'Elixir.TestWeb.Endpoint'}
*DBG* 'Elixir.Test.Supervisor' new state {state,
{local,'Elixir.Test.Supervisor'},
one_for_one,
{['Elixir.TestWeb.Endpoint'],
#{'Elixir.TestWeb.Endpoint' =>
{child,
{restarting,<0.235.0>},
'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}}},
undefined,3,5,
[-576460567,-576460567,-576460567],
0,'Elixir.Supervisor.Default',
{ok,
{#{intensity => 3,period => 5,
strategy => one_for_one},
[{'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}]}}}
*DBG* 'Elixir.Test.Supervisor' got {'EXIT',<0.338.0>,
{shutdown,
{failed_to_start_child,
'Elixir.Phoenix.PubSub.PG2',
{already_started,<0.237.0>}}}}
*DBG* 'Elixir.Test.Supervisor' new state {state,
{local,'Elixir.Test.Supervisor'},
one_for_one,
{['Elixir.TestWeb.Endpoint'],
#{'Elixir.TestWeb.Endpoint' =>
{child,
{restarting,<0.235.0>},
'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}}},
undefined,3,5,
[-576460567,-576460567,-576460567],
0,'Elixir.Supervisor.Default',
{ok,
{#{intensity => 3,period => 5,
strategy => one_for_one},
[{'Elixir.TestWeb.Endpoint',
{'Elixir.TestWeb.Endpoint',
start_link,[]},
permanent,infinity,supervisor,
['Elixir.TestWeb.Endpoint']}]}}}
*DBG* 'Elixir.Test.Supervisor' got cast {try_again_restart,
'Elixir.TestWeb.Endpoint'}
[info] Application test exited: shutdown
Seems like Phoenix.PubSub.PG2 is causing the problem. It cannot be started, because it is already running. What’s wrong here? I believe the app should have survived a process crash like the killing I do here. Or am I wong?
Phoenix.PubSub.PG2 is supervised by the Endpoint, and for some reason it just does not appear in observer under the Endpoint in the supervision tree.
The Phoenix version I use is 1.7.4.
Tried with OTP 21.2.1 and 21.2.2, Elixir 1.7.4 and also with 1.8.0-rc.1. The same happened.
Where Next?
Trending in Questions
Other Trending Topics
Latest Phoenix Threads
Latest on Elixir Forum
Sponsor Spotlight
Phoenix, LiveView, multimedia pipelines, distributed systems. 400 engineers, Elixir since 2017. Available for hire.
Categories:
Sub Categories:
Forums
Popular Tags
- #ecto
- #liveview
- #troubleshooting
- #learning-elixir
- #library
- #deployment
- #erlang
- #testing
- #genserver
- #mix
- #absinthe
- #remote-other
- #otp
- #plug
- #how-to-question
- #macros
- #postgres
- #elixirconf
- #channels
- #exunit
- #discussion
- #code-sync
- #podcasts
- #javascript
- #onsite
- #dialyzer
- #docker
- #authentication
- #umbrella
- #full-time-contract
- #podcasts-by-brainlid
- #ecto-query
- #elixirconf-us
- #blog-post
- #ai
- #elixir-ls
- #phoenix_html
- #iex
- #graphql
- #genstage
- #websockets
- #supervisor
- #advent-of-code
- #distillery
- #processes
- #api
- #forms
- #hex
- #security
- #metaprogramming
Our Sponsors
Build Elixir applications with speed and confidence.
Supporting innovation across the BEAM ecosystem.
Catch errors, track performance, monitor hosts and more.
Error tracking, logs, uptime, and Just Enough APM. Built for developers.
Elixir, Erlang & BEAM conferences from Code Sync - join us in 2026!
Practical resources that improve the lives of professional developers.
Master modern tools, write cleaner code, and tackle real-world challenges.
Bringing learning, work and compliance together since 1987.
The team behind Membrane, Popcorn, LiveDebugger. Available for hire.
Courses that'll move you from confusion to "Aha, now I get it!"
Showing Posts 1 to 3- Show Best Posts
- Show All (oldest first)
- Show All (newest first)
uniquenospaceshort
I’m facing the same exact problem, did you solved?
adamu
This has just been asked on Stack Overflow too.
josevalim
This is more of an issue of using
Process.exit(pid, :kill)on a supervisor.:killis a last resort message, which causes the supervisor to immediately terminate, without notifying its children to terminate properly. So the supervisor exits, the children now have to notice their parent is dead, clean up, and terminate.Meanwhile, the application is restarting part of the tree at the same time, which ends up conflicting with the old one still shutting down, causing another failure. This eventually triggers max_restarts and the application shuts down.
Overall:
Just use
:killas a last resort, when the process did not respond to any other exit signal. This especially applies to supervisors, as their only job is to ensure processes start and terminate accordingly, and sending a:killvoids thatIf you want to simulate stopping a supervisor,
Supervisor.stopwill at least go through the usual flowRun
iex --logger-sasl-reports true -S mix phx.serverto get precise logging from supervisorsOne question that may arise from this is: could my supervisor fail in a way that triggers the same behaviour as the
:killsignal? Supervisors trap exits, which means no other process (linked or not) could cause them to crash. Therefore, this can only happen if there is a bug in the supervisor. And if there is a bug in the supervisor, then indeed they can’t guarantee their fault tolerant properties anyway. That’s why supervisors rarely change, they have been strongly tested for decades and are an essential piece of your application.