Feed designs trade write-time fanout against read-time assembly. Highly connected users can make one uniform approach inefficient.
Before you start
You should understand API requests, storage and basic capacity estimates. Begin with a concrete user action and its correctness requirement. Draw data flow and failure boundaries before selecting infrastructure; a technology name by itself does not explain why a design meets the requirement.
The practical goal is to reason through this situation: Ordinary authors fan out on write while high-fanout authors may be combined during reads. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Estimate write amplification
One post can imply many follower updates.
Step 2: Choose workload-specific assembly
Ordinary and high-fanout authors may benefit from different paths.
Step 3: Define unified read semantics
Ranking, freshness and deletion must work across the hybrid.
Worked scenario
Ordinary authors fan out on write while high-fanout authors may be combined during reads.
A celebrity with ten million followers makes per-follower write fanout costly. Read-time assembly can reduce that write burst but adds work to feed requests. A hybrid policy needs a coherent pagination and ranking contract, plus removal of deleted or unauthorized content from both copied and assembled portions.
Common mistake
Copying every post to every follower can amplify work dramatically.
Verify the behavior
Test a celebrity publish, normal publish, deletion and page continuation.
Interview exercise
Handle a celebrity publisher.
Answer and reasoning
Use a measured hybrid policy and explain ranking, freshness, pagination and deletion propagation.
Continue learning
Compare the scenario with the System Design interview questions and test your understanding with the System Design MCQs. For terminology and implementation details, consult the reference material.