29. Cache-Aside (Lazy Loading)

The brief

Cache-Aside (Lazy Loading). A read-heavy service sends every request to a database that is slower and more expensive per read than memory, and the same few rows are asked for over and over. 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: Which reads repeat, and how skewed is their popularity? How stale may a cached value be after a write?
• Define the contract: What happens when a popular entry expires? How much traffic can the database absorb during a cache outage?
• Test the boundaries: Which values should never enter a shared cache?

Constraints

Explicit scope and guarantees

Resolve the scoping questions for Cache-Aside (Lazy Loading). 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: How much traffic can the database absorb during a cache outage? Which values should never enter a shared cache?

What to cover

1. 01

   Scope and behavior contract

   Identify users, required behavior and exclusions. Answer: Which reads repeat, and how skewed is their popularity? How stale may a cached value be after a write?

2. 02

   State and interfaces

   Define the information owned by the system and the inputs, outputs and errors at its boundaries. Resolve: What happens when a popular entry expires? How much traffic can the database absorb during a cache outage?

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 Cache-Aside (Lazy Loading). 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: Which values should never enter a shared cache?

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

Cache-Aside (Lazy Loading)

The service reads the cache first and fills it on a miss, so the cache holds only what is actually asked for and an outage degrades to slow rather than down.

Explore the design

Review rubric

AI feedback uses these criteria. Scores are practice feedback.