31. Hybrid Fan-Out (Push and Pull)

The brief

Hybrid Fan-Out (Push and Pull). Building a timeline when someone opens the app means reading from everyone they follow and merging — slow, and slowest for the most active users. Building it when someone posts makes reads a single lookup, but one post by an account with fifty million followers becomes fifty million writes. Work from the scoping questions below. State assumptions for any unspecified load, guarantee or target, then trace your design end to end. Explain one difficult case and a credible alternative; the worked example is a reference, not a required implementation.

• Set the scope: How skewed are follower counts and posting rates? How quickly must a new post appear in a follower’s feed?
• Define the contract: Which readers are active enough to justify work before their next visit? What happens to already prepared feeds after a deletion or privacy change?
• Test the boundaries: How does an exceptionally popular author change the amount of work?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for Hybrid Fan-Out (Push and Pull). Separate stated behavior from assumptions, and identify what is outside your design.

Supported operating targets

Declare relevant volume, latency, freshness, quality or cost targets with units. Show calculations or an evaluation plan that can test them; unspecified targets are your assumptions, not hidden pass criteria.

Failure and boundary behavior

Explain how your guarantees hold in a difficult case relevant to this subject. Address: What happens to already prepared feeds after a deletion or privacy change? How does an exceptionally popular author change the amount of work?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: How skewed are follower counts and posting rates? How quickly must a new post appear in a follower’s feed?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: Which readers are active enough to justify work before their next visit? What happens to already prepared feeds after a deletion or privacy change?

3. 03

   Capacity and operating targets

   Estimate the dominant workload and resource demand with units and explicit assumptions. For a learned system, also state how quality is measured and what data is available.

4. 04

   Architecture and central flow

   Draw or describe the responsibilities needed for Hybrid Fan-Out (Push and Pull). Trace a representative request, event or job from its input to a visible result; identify durable state owners.

5. 05

   Failure and boundary walkthrough

   Walk through a difficult case step by step, including detection and recovery. Consider: How does an exceptionally popular author change the amount of work?

6. 06

   Tradeoffs and operations

   Compare a credible alternative using your chosen workload and guarantees. Explain a remaining risk, a signal to watch and when you would change the design.

Worked designs

Explore the architecture and decisions, then build on an example with Coach.

Official design

Hybrid Fan-Out (Push and Pull)

Push new items into followers' timelines at write time, except for accounts with huge followings, whose posts are pulled and merged at read time.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.