Error during WebSocket handshake: Unexpected response code: 502 - ruby-on-rails

Question

I have Rails app on Digital Ocean (Ununtu 18.04 + nginx + passenger + Capistrano). I have chat functionality inside my app based on Action Cable. On my local everything works without problem, but on production i get next error: Error during WebSocket handshake: Unexpected response code: 502.
As i understand something wrong with Action Cable configuration. But everything done as in tutorials so i now really confused and don't know where i should looking for evil. Please help!
production.rb file
config.action_cable.url = '/cable'
config.action_cable.allowed_request_origins = ['http://example.com']
cable.yml file
production:
adapter: redis
url: redis://redis.exmaple.com:6379
messages.coffee file
jQuery(() ->
App.messages = App.cable.subscriptions.create {channel: 'MessagesChannel', id: $('#conversation_id').val() },
received: (data) ->
jQuery('#new_message')[0].reset()
jQuery('#chat').append data.message
)
messages_channel.rb file
class MessagesChannel < ApplicationCable::Channel
def subscribed
stream_from "conversation_#{params[:id]}"
end
end
/etc/nginx/sites-enabled/myapp file
server {
listen 80;
listen [::]:80;
server_name example.com;
root /home/deploy/myapp/current/public;
passenger_enabled on;
passenger_app_env production;
location /cable {
passenger_app_group_name myapp_websocket;
passenger_force_max_concurrent_requests_per_process 0;
}
# Allow uploads up to 100MB in size
client_max_body_size 100m;
location ~ ^/(assets|packs) {
expires max;
gzip_static on;
}
}
In nginx i have a lot of such errors:
2019/07/18 07:37:47 [error] 25500#25500: *9088 upstream prematurely closed connection while reading response header from upstream, client: 117.102.40.105, server: example, request: "GET /cable HTTP/1.1", upstream: "passenger:unix:/tmp/passenger.q4FqjUT/agents.s/core:", host: "example.com"
Of course everywhere example.com is replaced with my real domain name.

Answer 1

I have found the problem. It was in redis url in configuration. In each tutorial it write to use next code:
production:
adapter: redis
url: redis://redis.example.com:6379
But looks that it is mistake and correct will be next code:
production:
adapter: redis
url: redis://localhost:6379
So instead you domain should be used localhost. Hope, this will help somebody and will save a lot of time)