postmark-rails | Official integration library for using Rails | Email library

Ok, found the problem. Turns out that I had copied the setting config.session_store :cache_store in development.rb from a different project I had been working on. However, this setting was added as part of the StimulusReflex setup for that other project.

From the StimulusReflex docs:

Cookie-based session storage is not currently supported by StimulusReflex.

Instead, we enable caching in the development environment so that we can assign our user session data to be managed by the cache store.

The default setting for this option is cookie_store. By changing it to :cache_store without specifying a cache repo, it implements ActionDispatch::Session::CacheStore and defaults to storing it in Rails.cache, which uses the :file_store option, which dumps it in tmp/cache.

However, further down in development.rb, there is some conditional logic that assigns config.cache_store to :null_store if there is no caching-dev.txt file. This implements ActiveSupport::Cache::NullStore, which is "a cache store implementation which doesn't actually store anything."

So because I had not enabled caching with rails dev:cache for this project, the session cache was getting toasted with every request.

LESSON LEARNED: Be very careful when copying config settings from an old project to a new one!