This topic is a video lesson inside the Caching Everywhere module. The goal is not to memorize the title: reconstruct the problem it solves, the mechanism that makes it work, and the signal that tells you when to use it.
Remember the shape, not the sentence
“The fastest query is the one you never run.”
What problem does this solve?
Start from the engineering problem rather than the definition. Eviction policies: LRU, LFU, TTL exists because a system needs a reliable way to handle a recurring constraint inside Caching Everywhere. Ask what would become slow, ambiguous, unsafe, or impossible without it.
How the mechanism works
Break the idea into observable steps. The useful mental model is: input → transformation → decision or state change → output. Keep each step small enough that you can redraw it from memory. The surrounding module is the context: The fastest query is the one you never run.
When to reach for it
Use nearby concepts as contrasts. In this module, compare Eviction policies: LRU, LFU, TTL with Cache-aside vs write-through vs write-behind, CDNs and edge caching, Interactive: cache stampede & how to stop it. The distinction matters because good engineering choices depend on the constraints, not on memorizing isolated definitions.
See the mechanism before you memorize it
One diagram, one causal story, one reconstruction task.
Eviction policies: LRU, LFU, TTL — causal map
Frame
State the problem and the constraints before naming the technique.
Transform
Apply the core mechanism of Eviction policies: LRU, LFU, TTL and track what state changes.
Observe
Inspect the artifact, output, invariant, or behavior produced by the mechanism.
Transfer
Explain where the same idea appears elsewhere in System Design Fundamentals.
Draw the idea yourself
Rebuild the core flow on a real canvas before moving to the worked walkthrough.
Walk the system
Frame
State the problem and the constraints before naming the technique.
Transform
Apply the core mechanism of Eviction policies: LRU, LFU, TTL and track what state changes.
Observe
Inspect the artifact, output, invariant, or behavior produced by the mechanism.
Transfer
Explain where the same idea appears elsewhere in System Design Fundamentals.
Reconstruction card
Use the card as a generative note, not a summary to copy. Fill it from memory after the explanation.
TOPIC: Eviction policies: LRU, LFU, TTL
MODULE: Caching Everywhere
TYPE: video lesson
PROMPT
What problem does it solve?
What changes step by step?
What evidence proves it worked?
When would you choose something else?What usually goes wrong
Close the tab. Rebuild it from memory.
Read the question, answer aloud, then reveal.
What problem is Eviction policies: LRU, LFU, TTL designed to solve?Reveal answer
It addresses a recurring constraint inside Caching Everywhere. A strong answer names the input, the core mechanism, and the result rather than repeating the title.
What are the four stages of the mental model?Reveal answer
Frame the problem → transform the input/state → observe the evidence → transfer the idea to a new scenario.
What is a cache stampede and the standard fix?Reveal answer
A hot key expires and thousands of requests hit the DB simultaneously; fix with a per-key mutex/single-flight so only one request refills the cache.
Make this idea travel.
Connect it next to: Vertical vs horizontal scaling • Latency, throughput, bandwidth — with real numbers • Interactive: numbers every engineer should know • Availability, nines, and single points of failure.