The application has been deployed to heroku https://peaceful-ravine-17683.herokuapp.com/ It uses free tiers for PostgreSQL and Redis.

Install ruby, redis, postgresql, all gems. Create database and run seeds. Use rails s for running rails server Execute sidekiq for running background jobs

Went well

• Challenging task
• All the task requirements were fullfilled

Didn't went well

• Nothing I would put here

If I had more time to work on this, I would ...

• Improve frontend part for a list of links. Using react-table would make it more flexible and robust. This could also easily help to add a pagination.
• Make better frontend side with using the most wideused React libraries
• Add ESLint
• Get advantage of using React Proptypes
• Setup nginx or CDN for fast aseets
• Setup rubocop additional extensions
• Setup CI for running tests, and other tools like (Brakeman]
• Covered it with rspec tests, according to testing pyramide
• Generate unique short digests in advance in the background(that could help us to avoid possible collision) and probably some fancy features like reserve some very short url(1-3 letters) for futher sell
• Tune Redis to be RDB and AOF persistent
• Tune redis & psql connection pool limit to handle more connections(now its limited by ree tiers)
• Make RedisStatsWorker idempotant. See its TODO inside
• Separate redis for sidekiq, application cache and counter storage
• If there would be any rare issues with redis, I would create a worker for the future redis data restoring, accepting link_id , fetching(seee SQL attached) all needed data from SQL database - that we need for redis(total num of rows and attendances, uniq attendances). After getting all data we need, I would sync it to back Redis.

SELECT countries.name as country_name, array_length(array_agg(DISTINCT ip), 1), COUNT(attendances.id) as attendance_count FROM attendances INNER JOIN countries ON countries.id = attendances.country_id WHERE link_id = #{link_id} GROUP BY countries.id;

Though, this approach will require some blockings in order to be thread-safe

• Track uniq visitors using cookie(GDPR popup required!)
• Think about scaling - alternatives DBMS like NoSQL - they are easier to scale.
• Setting up replicas for further scalability. Using some other techniques, like partitioning.
• Think about removing old links(more than X years ago and visited not more that Y times - that could help us to free a lot of space)