System designMedium~16 min

Interview question

Design an activity feed where users can see recent actions from their team or project.

Design an activity feed backend

Designs an activity feed with event capture, fan-out strategy, pagination, privacy, and read performance.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

I would record activity events when important actions happen, such as created, commented, assigned, uploaded, or status changed. The feed API would return recent events for a user, team, or project with cursor pagination. Each event would include actor, action, target, timestamp, and display metadata. Authorization matters because a user can only see feed items for resources they are allowed to access.

Full answer below

Practice focus

Feed modeling, write-time versus read-time fan-out, pagination, privacy filters, caching, and operational limits.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Activity feeds combine fan-out, privacy, pagination, correction, and retention. The design must keep authorization current while deciding whether feed entries are assembled at read time or projected ahead of time.
I would record activity events when important actions happen, such as created, commented, assigned, uploaded, or status changed. The feed API would return recent events for a user, team, or project with cursor pagination. Each event would include actor, action, target, timestamp, and display metadata. Authorization matters because a user can only see feed items for resources they are allowed to access.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would choose between building the feed at read time or materializing it at write time. Read-time filtering is simpler and works for smaller teams, but it can become expensive when every request joins many tables and permission rules. Write-time fan-out or precomputed read models can make reads fast, but it adds complexity when permissions change or events need correction. I would keep the source event immutable and allow display projections to be rebuilt.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I would design retention, privacy, and replay from the start. If a user loses access to a project, the feed cannot keep showing private events just because they were precomputed earlier. I would monitor feed latency, projection lag, fan-out failures, and pagination errors. For high-volume activity, I might aggregate noisy events or provide digest views. The design balances freshness, correctness, and cost rather than assuming one feed table solves every case.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Ignoring authorization when reading feed items.
Using offset pagination on a constantly changing feed.
No plan for permission changes after precomputing feed rows.
Storing only display text and losing structured event data.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot compare read-time and write-time feed generation.
Has no cursor pagination story.
Ignores privacy and access changes.
No way to rebuild feed projections.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

When would you fan out events on write?
How would permissions affect a precomputed feed?
How would you handle very noisy event types?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Abstraction vs encapsulationConcept question / Easy / ~5 minContinue Access modifiersConcept question / Easy / ~5 minContinue ActionResult vs Typed ResultsComparison question / Medium / ~7 minContinue

System designMedium~16 min

Interview question

Design an activity feed where users can see recent actions from their team or project.

Design an activity feed backend

Designs an activity feed with event capture, fan-out strategy, pagination, privacy, and read performance.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

Feed modeling, write-time versus read-time fan-out, pagination, privacy filters, caching, and operational limits.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
Activity feeds combine fan-out, privacy, pagination, correction, and retention. The design must keep authorization current while deciding whether feed entries are assembled at read time or projected ahead of time.
I would record activity events when important actions happen, such as created, commented, assigned, uploaded, or status changed. The feed API would return recent events for a user, team, or project with cursor pagination. Each event would include actor, action, target, timestamp, and display metadata. Authorization matters because a user can only see feed items for resources they are allowed to access.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would choose between building the feed at read time or materializing it at write time. Read-time filtering is simpler and works for smaller teams, but it can become expensive when every request joins many tables and permission rules. Write-time fan-out or precomputed read models can make reads fast, but it adds complexity when permissions change or events need correction. I would keep the source event immutable and allow display projections to be rebuilt.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
In production I would design retention, privacy, and replay from the start. If a user loses access to a project, the feed cannot keep showing private events just because they were precomputed earlier. I would monitor feed latency, projection lag, fan-out failures, and pagination errors. For high-volume activity, I might aggregate noisy events or provide digest views. The design balances freshness, correctness, and cost rather than assuming one feed table solves every case.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Ignoring authorization when reading feed items.
Using offset pagination on a constantly changing feed.
No plan for permission changes after precomputing feed rows.
Storing only display text and losing structured event data.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot compare read-time and write-time feed generation.
Has no cursor pagination story.
Ignores privacy and access changes.
No way to rebuild feed projections.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

When would you fan out events on write?
How would permissions affect a precomputed feed?
How would you handle very noisy event types?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Design an activity feed where users can see recent actions from their team or project.

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice

Design an activity feed where users can see recent actions from their team or project.

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice