design.puml

@startuml

title URL Shortening Service Sequence Diagram

actor User as "User"

entity URLShortenerService as "URL Shortening Service"
database RedisDatabase as "Redis Database"

== User Submits a Long URL and Receives a Short URL in Response ==
User -> URLShortenerService: POST /shorten { "url": "long_url" }
activate URLShortenerService
URLShortenerService -> RedisDatabase: GET url_sha1_base62
activate RedisDatabase
RedisDatabase --> URLShortenerService: short_code (if exists)
deactivate RedisDatabase
alt short_code does not exist
    URLShortenerService -> RedisDatabase: SET {short_code}, {long_url} and SET {url_sha1_base62}, {short_code} with TTL=24hrs
    activate RedisDatabase
    RedisDatabase --> URLShortenerService: OK
    deactivate RedisDatabase
end
URLShortenerService --> User: { "url": "short_url" }
deactivate URLShortenerService

== User Visits the Short URL and Gets Redirected to the Original Long URL ==
User -> URLShortenerService: GET /{short_code}
activate URLShortenerService
URLShortenerService -> RedisDatabase: GET {short_code}
activate RedisDatabase
RedisDatabase --> URLShortenerService: long_url
deactivate RedisDatabase
URLShortenerService --> User: 302 Redirect to long_url
deactivate URLShortenerService

== User Attempts to Access an Expired Short URL and Receives an Appropriate Response ==
User -> URLShortenerService: GET /{expired_short_code}
activate URLShortenerService
URLShortenerService -> RedisDatabase: GET {expired_short_code}
activate RedisDatabase
RedisDatabase --> URLShortenerService: nil
deactivate RedisDatabase
URLShortenerService --> User: 404 Error HTML Page
deactivate URLShortenerService

== User Submits an Invalid Long URL and Gets an Error Response ==
User -> URLShortenerService: POST /shorten { "url": "invalid_long_url" }
activate URLShortenerService
alt invalid_long_url
    URLShortenerService --> User: 400 Bad Request
end
deactivate URLShortenerService

== User Submits the Same Long URL Multiple Times and Receives the Same Short URL Within the 24-hour Duration ==
User -> URLShortenerService: POST /shorten { "url": "same_long_url" }
activate URLShortenerService
URLShortenerService -> RedisDatabase: GET url_sha1_base62
activate RedisDatabase
RedisDatabase --> URLShortenerService: short_code
deactivate RedisDatabase
URLShortenerService --> User: { "url": "same_short_url" }
deactivate URLShortenerService

@enduml