Why does Action Cable need Redis in production?

Question

Ruby on Rails

In the generated rails application version 5.2.0, if you look in the Gemfile, there are commented lines
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 4.0'
Can explain why Action Cable needs Redis in production and how it is they use?

Answer 1

7.1.1 Adapter Configuration
Below is a list of the subscription adapters available for end users.
7.1.1.1 Async Adapter
The async adapter is intended for development/testing and should not be used in production.
7.1.1.2 Redis Adapter
The Redis adapter requires users to provide a URL pointing to the Redis server. Additionally, a channel_prefix may be provided to avoid channel name collisions when using the same Redis server for multiple applications. See the Redis PubSub documentation for more details.
7.1.1.3 PostgreSQL Adapter
The PostgreSQL adapter uses Active Record's connection pool, and thus the application's config/database.yml database configuration, for its connection. This may change in the future. #27214