Sys Design Question: Improving Cost on a CDN Design

Question

I was recently asked to design an autocomplete system. I suggested a solution where results by prefix are physically cached to a CDN. I was told that this solution isn't optimal because it represents an inefficient storage approach. I'm trying to understand an optimal solution.

Some alternatives I can think of:

S3 - better storage cost, too slow
Redis - about as fast, but more expensive
Dynamo - about as fast, I don't know how it compares on cost

I think the main two cost components will be storage and read cost. I'm having trouble finding useful comparative data on these. Is it commonly understood/thought that Dynamo is cheaper than a CDN at scale, or is there some other design I'm not thinking of?

Alex Chiou · Accepted Answer

Autocomplete is purely about speed (especially if you're doing live suggestions as they user types) - Wouldn't something like Redis be the fastest as it's in-memory? Physically caching to a CDN seems heavy, making it inefficient in terms of performance.  Zooming out, a good system design interview (assuming the interviewer is high-quality) is about explaining the trade-offs and discussing (often conflicting) opinions. As a senior engineer, you will inevitably run into the scenario on the job where you propose a technical design and another senior or staff engineer disagrees. At that point, you need to make a decision:  Defend your approach entirely Abandon your approach and take theirs Find a middle-ground and incorporate elements of their approach into yours (often the best)  For a senior engineer in a system design interview, I would expect them to clearly take 1 of the 3 approaches, demonstrating conviction and strong reasoning.  For me, I would state upfront that performance would be my priority (prioritize the end user) and I'm happy to sacrifice $$$ cost for that.  I asked ChatGPT as well and the response was helpful (it mentioned a Trie, which is a nice call):

Sys Design Question: Improving Cost on a CDN Design

Discussion