README

To run the code

• Clone the repository
• Change directory to xbe cd xbe
• Install dependencies bundle install
• You need a postgres server running with username as 'root' and password as 'root' and database xbe with root getting all privileges
• Create database rake db:create
• Install db changes rake db:migrate
• In new terminal run to start sidekiq bundle exec sidekiq -e development -C config/sidekiq.yml
• In new terminal run rake database_listener

Gems used

1. gem 'hairtrigger', '~> 1.0' // For creating triggers within rails env
2. gem 'sidekiq', '~> 7.1', '>= 7.1.2' # For background processes
3. gem "rspec-rails" # For testing.

Models

1. Invoices - id (uuid), invoice_no(string), total_amount(int), timestamps
2. Invoice_items - id(uuid), invoice_id(foreign_key), quantity(int), amount_per_unit(int), total_amount(int), timestamps
   • Triggers on create, update, delete
   • Every trigger contains pg_notify which sends messages on 'trigger_changes' channel
3. Notifications - event(string), payload(json)

Rake Tasks

1. database_listener - This queues DatabaseNotifications background job which in turn listens to pg_notify()

My thought process

• Have used postgres triggers as we need the application to get notified even if changes are directly done on database
• Installed hair-trigger gem which lets me create triggers in model files itself
• 3 tables - invoice, invoice_items(references invoice table via foreign key), notifications
• In invoice_item.rb file I have created triggers when quantity, amount_per_unit changes for invoice_item. It changes total_amount in invoice_table.
• The invoice_items trigger also use pg_notify() for inter-process communication and sends message to rails application
• Created an ApplicationJob which uses Sidekiq and listens to pg_notify commands. It also writes to Notifications table whenever a prompt is received.
• Created rake task "database_listener" which queues the above job
• Have written just 1 rspec test but have could not proceed for paucity of time.