Active beats passive
Short theory, recall, practice, feedback.
fiveninesBuild-your-own-X
Build your own Redis.
Leetcode for system design. No coding necessary.
First 5 tutorials free · No card needed
- 24
- tutorials
- 10
- modules
Why this exists
System design learning is stuck in 2016.
In 2026, the edge is not typing code from memory. It is understanding what AI builds, spotting what breaks, and learning concepts fast enough to keep up.
Start building freeold playbook2016
- Read
- Memorize
- Interview
new loop2026
ModelFeedbackReviseUnderstand
structured practiceAI changed coding
You still need the mental model.
Faster than ever
Use AI speed with structured practice.
Old methods fail
Whiteboard trivia does not build judgment.
How it works
Learn it. Check it. Build it.
01Learn
One short tutorial.
02Check
Quick practice questions.
03Build
Model it and get graded.
The problems
Real internals, one artifact at a time.
Key-Value Core
RESP Protocol Parser
Event Loop
Command Dispatch
Dict & Rehashing
Key Expiration
LRU / LFU Eviction
RDB Snapshotting
Failover Controller
Cluster Resharding
The journey
From GET/SET to a sharded cluster.
01Key-value core
02RESP protocol
03Event loop
04Persistence
05Replication
06Cluster sharding
The feedback
Every diagram gets graded.
submissionpartition-replication.d292 / 100
Runtime path
Produce → leader → fetch holds.
Failure mode
Leader loss covered by ISR promotion.
Invariants
Offsets monotonic per partition — stated.
Trade-offs
Name the latency cost of acks=all.
PASS
Keep the mechanism. Fix one trade-off.
Pricing