Reddit · senior product engineering
Your worked rehearsal: paginate a changing feed without repeating posts
A community feed displays two posts at a time. While a reader scrolls, other people add posts, vote, block authors, and remove content. A page cursor must describe which ordering the reader is continuing, while current visibility rules still decide what they may receive.
Implement a small feed-page contract and draw the stored ordering behind its cursor. Then handle score changes and moderation. You are building the original practice design below, not reconstructing Reddit’s internal architecture.
Starting point: these drills are build briefs. Implement in a local file or draw the requested architecture. The worked answer is an explanation, not a supplied end-to-end application. Choose one drill per session, then change one requirement after the baseline works.
Reddit's public product centers communities, posts, comments, votes, and conversation. We could not verify a recent, role-matched senior product SWE candidate account that established a stable interview loop. Difficulty, exact rounds, and repeat-question frequency are unknown. All prompts here are original product-inspired simulations, not reported Reddit questions.
The room · a community is not a single feed
Practice pressure: a change to ranking or moderation affects millions of independent communities, live writers, readers, and trust decisions. Ask whether “fresh” means strictly current, eventually refreshed, or stable while a user pages. This is a rehearsal mindset, not a claim about what interviewers actually ask.
Ask first: Personalized or community feed? What does a moderator control? What can be hidden or deleted? Which consistency matters to voters versus feed readers? What is the abuse model?
Choose a coding exercise · eight original drills
| Drill and exact ask | Example to settle before coding | Senior stretch |
|---|---|---|
01 · Keyset page of posts sorted by (score,id) descending |
scores 9:a,9:b,7:c, cursor (9,b) → 9:a,7:c |
Inserts, score changes, snapshot semantics |
| 02 · Flatten threaded comments depth-first with a max visible depth | a→[b,c], b→[d] → a,b,d,c |
Large fan-out and deleted-parent placeholders |
03 · Idempotent vote transition per (user,post) |
up, up, down → post delta −1 from baseline |
Concurrent opposite votes and audit log |
| 04 · Moderation queue by priority then arrival | equal priority timestamps 2,1 → 1,2 |
SLA fairness and moderator visibility |
| 05 · Near-duplicate post fingerprint from normalized title | "Hi!", " hi " → same candidate |
False positives and evasion |
| 06 · Community prefix search with top-k by activity | prefix py → matching communities, stable tie break |
Trending score lag and prefix cache |
| 07 · Hot-score evaluator with timestamp and vote count | zero votes, same timestamp → stable score | Time decay and adversarial voting |
| 08 · Comment rate limit per user/community | 3 allowed per 60s; fourth at 59s → reject | Burst fairness and spam controls |
Choose an architecture exercise · five original prompts
| Prompt | Initial product requirement | Change the requirement |
|---|---|---|
| Community feed | Rank recent posts for one community | Stable pagination as votes arrive |
| Comment tree | Read and reply to deeply nested comments | Deleted parents and moderator locks |
| Votes | Update visible counts on a post | Bot abuse and reconciliation |
| Mod queue | Escalate reported content to moderators | Appeal, audit, and urgent safety path |
| Notifications | Alert users when replied to | Fan-out spikes, muting, and consent |
Worked session · a feed that does not repeat posts
Interviewer: “Build a community feed API with limit and a cursor. A user scrolls while people submit posts and vote. Do not show the same post twice in that session. Explain what freshness you give up.”
Clarify aloud: Ranking is (rank_score DESC, post_id DESC), limit 2, unique immutable IDs, score can change. Is the contract “never duplicate,” or also “never miss a newly eligible post”? They cannot both be guaranteed with a moving ranking and a stateless cursor. Choose session snapshot or an explicitly weaker live-feed contract.
| Feed / action | Expected session page | Edge exposed |
|---|---|---|
a:9,b:8,c:7, first request |
a,b |
Baseline; cursor contains (8,b) |
Insert x:10 before second request |
c, not b,c |
Offset pagination would repeat b |
c changes score to 11 mid-session |
Snapshot returns c once in original order |
Live ranking can move across cursor |
b is deleted before page two |
Never return b again |
Visibility must be rechecked |
User blocks c before page two |
Skip c, potentially short page |
Filter after read or over-fetch |
| Empty community | Empty items, null cursor | No fake continuation |
Think in public: For the strict session contract, issue a signed cursor with snapshot/version, last sort tuple, filter identity, and expiry. Read from a versioned ranking snapshot; re-evaluate permission and deletion at serving time. Over-fetch when filtering. An unbounded per-user “seen IDs” set is not a scalable fix; an expiring snapshot is a conscious cost/freshness trade-off.
Before / after: the animation contrasts an offset sliding beneath live inserts with a cursor anchored to a particular ordering. Draw the initial boxes and label which store owns ranking, content, and permission.
Follow-up 1 (senior): “A moderator removes a post after the snapshot.” Visibility beats ordering: skip it without leaking title/snippet from stale cache; return a short page or over-fetch. Follow-up 2 (staff): “The ranking index is rebuilt and old cursors point at an expired version.” Return an explicit expiry/restart contract; preserve enough versions for a bounded session, measure expiry rate, cache cost, duplicate reports, and time to moderation hide. Decide where multi-region writes converge.
Worked extension: preserve ordering while applying current visibility
A snapshot freezes a ranking for a bounded reading session. It must not freeze permission to view a removed post. Read candidate IDs from the snapshot, then evaluate current visibility before exposing content. A cached response that bypasses that check can disclose a title even if the underlying post was deleted. Bound over-fetching and return a short page when the permitted scan budget is exhausted.
When an old snapshot is retired, its cursor cannot continue honestly. Define an expiry response and a UI action to start a fresh feed. That new session may contain posts seen in the old one, so do not claim cross-session deduplication without additional state and a retention policy.
Debrief · what a strong answer contains
Separate the contract from implementation. Offset is an okay baseline only when the list is static; a keyset cursor helps with inserts but alone does not freeze mutable scores. Show a cursor bound to filters and user context, deliberate visibility rechecks, tests for equal scores, and a plan for expiry and privacy.
Evidence: Reddit's product overview grounds the domain. The interview format and all drills remain unverified simulations, checked 22 September 2026.