Tutorials System Design Tutorial
Consistency Models — Complete Guide
Consistency Models — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of System Design Tutorial on Toolliyo Academy.
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System Design Tutorial · Lesson 6 of 100
Consistency Models
Basics → Scale → Interview
Basics · 1 — Building blocks · ~6 min · Module 1: System Design Foundations
What is this?
Consistency models describe when readers see writes: strong, eventual, read-your-writes, monotonic reads, and more.
Why should you care?
ShopNest users hate paying then seeing “order not found.” That is a consistency UX bug.
See it live — copy this example
Sketch the architecture on paper. These lessons focus on concepts and trade-offs.
Write order in primary (Mumbai)
Read order from replica (Delhi) 200ms later
Eventual: replica may miss the order briefly
Read-your-writes: sticky session / primary read after write
Strong: read quorum / primary only for that GETRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Eventual consistency is fine for product ratings.
- After checkout, ShopNest often reads from primary or waits for replication so the confirmation page works.
Practice next
- List three ShopNest reads and pick a model for each.
- Design order confirmation as read-your-writes.
- Note how sticky sessions or “read primary” help.
- Use a version number on cart documents.
- Sketch “session reads primary for 5 seconds after write.”
Remember
Models define stale-read rules. Match model to user expectation. Post-write reads often need stronger guarantees.
Order confirmation page
ShopNest reads new orders from primary for 30s after place-order.
Outcome: Users always see the order they just paid for.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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