Flix is a web application that stores information and ratings about movies.
Things you may want to cover:
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Ruby version
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System dependencies
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Configuration
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Database creation
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Database initialization
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How to run the test suite
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Services (job queues, cache servers, search engines, etc.)
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Deployment instructions
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...
Callbacks are a good choice when they're focused on manipulating and keeping the model data consistent. Callbacks are the wrong choice when they're used to trigger behavior that may have unintended side effects or out-of-band processes, such as sending emails.