Detailed Component Design

When client calls /shorten API, request goes to LB and then to API gateway then to application server.
Application service then takes the original_url, pulls a key from KGS then encodes this key with base62. Using base62 gives us over 56.8 billion urls, as we calculated before that over a lifetime of years we get to 15.768 billion urls, thus chances of collision are near to none and they are unique. Since they are base62 encoded, they cannot be easily guessed.
In the KGS, instead of generating the short_url from the original_url, we asynchronously generate 6 letter random strings and store them in KGS's DB, these strings are the keys. Initially these are stored in the DB with their in_use field set to false. Thus, when our application service generates a url, KGS gives a "key" whose in_use is set to false, when KGS gives this key it sets in_use to true at the same time.
Once the shortened URL is generated, a record is created in application service's DB storing the original URL and the shortened version.
When the user hits /{shortened_url} api, the request goes to our application server, which then checks if the url is in the cache. If the url is in cache, we redirect to original_url. If not, we pull the original_url from the DB, then check expiration_date, if the link hasn't expired we save this to cache and redirect with code 302. If url has expired or doesn't exist, we send it to cleanup service and send a response (code 404) to client notifying the shortened_url has expired.
Now, we know our URL shortener application is very read heavy, so to combat this we create a read replica of the application service's DB, that way all the traffic isn't redirected to a single DB and prevents latency.
Also, since our DBs are NoSql , horizontal scaling is easily possible and thus availability is achieved.
We use CDN so that our application is available across the globe.
The cleanup service cleans up the Application DB by removing the records of shortened_url which have gone past expiration date. Then, in the URL DB, this shortened_url is set to in_use = false. Also, when the client hits /{shortened_url} api, this cleanup is done then as well.

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