---
title: "Distributed Systems overview"
description: "Watch consistent hashing place keys, two-phase commit coordinate one atomic transaction, and Raft commit a replicated log — three different distributed-systems jobs, animated beat by beat."
source: https://vizlearn.app/cs/distributed
---

# Distributed Systems Visualizations

Distributed systems run into three different questions as soon as one machine is no longer enough. **Where should each key live?** **Can several resources apply one transaction together?** **Can several replicas preserve one log through failures?** Consistent hashing, two-phase commit, and Raft answer those questions. Their diagrams may all contain nodes and messages, but their guarantees are not interchangeable.

A cache ring needs placement that stays mostly stable when capacity changes. A bank transfer needs A, B, and its audit log to change together or not at all. A replicated control plane needs one committed history even when messages arrive late or the network splits. Start by naming the job; then choose the mechanism.

## Compare the lessons

| Lesson | Main question | Deciding rule | Failure boundary |
|---|---|---|---|
| Consistent Hashing | Which physical node owns a key when membership changes? | first active token clockwise from the fixed key token | only one adjacent interval should change owner; balance is improved, not guaranteed |
| Two-Phase Commit | When may A, B, and the log all change? | every participant votes YES, then all receive the same decision | one NO aborts everyone; a crashed coordinator can leave prepared locks held |
| Raft Consensus | When may a log entry become committed history? | a majority elects the leader and stores its current-term entry | a minority may store an entry but cannot commit it |

The most common category error is to borrow a rule from the wrong row. Raft's three-of-five majority cannot make a three-resource transaction atomic. Two-phase commit's unanimity would make two delayed replicas freeze a five-server log. Consistent hashing decides placement; it does not replicate data or settle conflicting writes.

## A Recommended Order

Start with **Consistent Hashing** on Node joins. Follow K11 across token zero, then watch N4@220 claim `(140, 220]`. K5, K6, and K7 change owner; nine keys do not. Switch to Node leaves to see one departed interval transfer to its clockwise successor. Finish on Virtual nodes and compare the fixed-key loads `3/2/7 -> 4/5/3`.

Continue with **Two-Phase Commit** on All yes → commit. Pause at two YES votes, at the coordinator's COMMIT decision, and at the blocking window where P1 is still PREPARED and A is still 100. Load One no → abort to watch a YES voter roll back, then Coordinator crash to leave the locks visibly held.

Finish with **Raft Consensus** on Election + commit. Three votes make a leader; three stored copies make a current-term entry committable. Then run Minority partition. The two-server side can store `set y=7` and still stall at `2 / 5`, while the three-server side elects a higher-term leader and commits a different entry at the same index.

That order moves from placement, to atomic coordination, to replicated agreement. It also makes the vocabulary harder to blur: an **owner** is not a **participant**, a prepared value is not a committed log entry, and a hash-ring token is not a quorum vote.

## What to Watch

- In consistent hashing, key dots never move when membership changes. Only their clockwise successor may change.
- A virtual node is another token position for the same physical machine, not another machine.
- In 2PC, a purple proposed value is locked but not applied. Two YES votes never cross `need ALL YES`.
- In Raft, two stored copies never cross `majority = 3`. Storage and commitment are separate beats.
- A coordinator crash can block prepared 2PC participants. A Raft partition blocks only the side without a majority.
- None of the three lessons supplies the guarantees of the other two. Real systems often combine them—for example, a hash ring for placement and Raft inside each shard for replication.

Pick a lesson below and keep your eye on its deciding evidence: the clockwise successor, the unanimous vote, or the overlapping majority.