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HackerRank CodePair HLD: Two People, One File, No Conflicts

A real-time collaborative editor design (CodePair / Google Docs-style): a sequence CRDT with fractional positions, converging concurrent edits over WebSockets, presence, and per-room scaling.

By fiveyearsdevJune 14, 202623 min read

Twenty minutes into a phone screen, you and the interviewer are in the same shared editor. You fix a typo on line 4 while she drops a test case on line 12 — and for half a second your cursor jumps, her test lands inside your function name, and the file on her screen is no longer the file on yours. Somebody says "refresh?", the round loses two minutes it didn't have, and you spend the rest of the hour typing carefully, in turns, like two people sharing one pen. That half-second is the entire system design. When both of you type "in the same place" at the same instant, whose character goes first…

What’s inside

Let's start nowhere near a computer

Where this exact shape shows up

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Nouns, the data model, and where each one lives

Step 4 — Verbs become APIs

Step 5 — The model: fractional positions

Step 6 — Convergence: concurrent edits merge

Step 7 — The delete that resurrects (and the tombstone that fixes it)

Step 8 — The anomaly the tiebreak can't fix

Step 9 — Transport: ops over a WebSocket

Step 10 — The Run button: executing a stranger's code

Step 11 — Trade-offs (each one keeping an NFR)

The complete implementation

Step 12 — Only now, the boxes

Step 13 — Scaling the design, one bottleneck at a time

Step 14 — When a piece fails: designing for failure

The interview corner

Where to go from here

Read this one free

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