Should I use base62 encoding or hashing for the short code?

Base62 over a unique ID is the cleaner default: zero collisions by construction and no retry logic. Choose hashing only if deduplicating identical long URLs matters to you, and then implement collision handling as an atomic insert-or-compare, not a check-then-write, because the race between two concurrent inserts is the classic bug in this design.

Why not just use a UUID as the short code?

A UUID is 36 characters, which defeats the purpose of a short link. The whole problem is compressing an identifier into 6 to 8 characters, and that means either encoding a compact sequential ID in base62 or truncating a hash and handling collisions. UUIDs buy uniqueness by spending exactly the length budget you cannot afford here.

How does the shortener stay fast for users on other continents?

Replicate the read path, not the write path. Run redirect servers and Redis caches in each region, backed by a replicated store, and route users with GeoDNS or anycast. A link created in Virginia may take a second to become servable from Singapore, which nobody notices, while every click gets a local sub-100 ms redirect.

Common Designschapter 1 of 8 · 4 lessons

URL Shortener

Map a short code to a long URL. Read-heavy, globally cached.

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You paste a link into a message and it comes out seven characters long. Somebody clicks it, and about 40 milliseconds later they are on the original page.

Between those two moments a server took those seven characters, found the address they stand for among six billion others, and sent the browser onward. It did that 40,000 times that second, because links get clicked far more often than they get made.

That ratio is the whole design. Roughly forty new links a second, forty thousand redirects a second, off the same table.

Interviewers keep asking it because it is small enough to finish and still contains generating unique identifiers, caching strategy, and the redirect decision that quietly determines whether you have a business.

Lessons

4 in this chapter

Requirements and EstimationPin down the read-write ratio first, because it decides the whole architecture.2 min
Short Codes and the Data ModelBase62 over a unique ID gives 3.5 trillion codes in 7 characters, no collisions ever.3 min
The Read PathA redirect is one cache lookup; the database should almost never hear about it.2 min
301 vs 302 and the Analytics PipelineOne HTTP status code decides whether you ever see the click again.2 min