System requirements

Functional:

should handle lots of data

will support many more reads than writes.

should track number of times each link is clicked

The system should reflect updates to tiny urls with in 30 seconds with a SLA of99.999%

The system should return redirect reads with in 10 milliseconds.

Non-Functional:

List non-functional requirements for the system...

Immeiate consistencey

Capacity estimation

1 trillion shortened urls

1 million read requests per second

1000 write requests per second

API design

create alias

look up alias

delete alias

update won't be supported

Database design

Casandra

Add a table

short_to_full

shortended url -> full length url, is_visable

full_to_short

full url-> short url

High-level design

Read requests will flow from CloudFront into to casanda on cache misses. CloudFront will have a ttl of 10 minutes for cache entries. CloudFront will aggregate url hits and send them to the service in bunchs every 10 minutes. The service will save those to casandra.

This ensures that Casandra always anaytics data that's no more than 10 minutes out of date.

Casandra records have a ttl of 6 months. Updates, or analtyics dumps to the records refresh this ttl back to 6 months. So if a record is untouched for 6 months's it's delted.

On deltes, the service will reach out to cloudfront to delete the cached entry.

Url generation

tiny urls will be randomly genrated. if a new url already exists in Casandra, the system will generate a new url and try the process again. once sucessfully, it will return the new url to the user.

the url will be 11 hex charaters long. This supports over 2 trillion entries.

DB Setup

I'd deploy the app to sevral regions around the globe. Each region would own a subset of the url address space. I'd have 3 replicas for each entry. I'd do quorum writes the 3 nodes on updates, and I'd do single node reads on read reqeusts.

Trade offs/Tech choices

A no sql db is harder to work with than using a relational db, but it will scale much better. the CDN will be out of sync with Casandra for a maxof a few minteus.

Fail cases

A deleted entry will remain live in the CDN for a few minutes as the cache purge is carried out.

Future improvements

I'd partition the services globally to own a differetn range of the url space that it can give out new urls for. this would ensure that a given AZ would be able to independntyly decide if a url is free without contacting other AZs.