9. Design Search Autocomplete

The brief

Design autocomplete for a public search box that returns ten suggestions after each typed prefix. Explain candidate lookup, popularity ranking and how a new index becomes visible. Handle corrections, language normalization and removed suggestions without exposing raw individual search histories.

• Serve 100,000 prefix lookups/second at peak over 100 million known phrases.
• Popularity updates hourly, while an operator may require an entry to disappear within five minutes.
• Users type, backspace and change language; a response for an older prefix may arrive last.

Constraints

Suggestion latency≤ 50 milliseconds

Declare p95 backend lookup latency in a healthy region.

Removal precedence

Explain how a blocked phrase remains absent across stale caches and index versions.

Aggregation privacy

Use aggregate signals and a stated retention policy rather than publishing rare personal queries.

What to cover

1. 01

   Lookup and ranking

   Define normalized keys, prefix matching, top-k selection and tie-breaking.

2. 02

   Index lifecycle

   Show aggregation, index building, validation, version switching and rollback.

3. 03

   Capacity and caches

   Estimate index memory and hot-prefix traffic using stated phrase-size assumptions.

4. 04

   Client and removal walkthrough

   Trace out-of-order responses and an urgent removal; compare two prefix-index choices.

Worked designs

No worked design has been published for this brief yet. You can start an attempt and share your approach in the discussion.

Review rubric

AI feedback uses these criteria. Scores are practice feedback.