Conflict Resolution

GUN uses a state-based CRDT (Conflict-free Replicated Data Type) approach to ensure eventual consistency across all peers, even in the presence of network partitions, concurrent updates, and offline operations.

State-Based CRDT

GUN’s conflict resolution is built on the concept of state timestamps - each property update includes a monotonically increasing timestamp that determines which value wins.

How It Works

Every update in GUN includes metadata:

{
  _: {
    '#': 'user/alice',           // Soul (unique ID)
    '>': {                        // State vector
      name: 1678901234567.001,    // Timestamp for 'name' property
      age: 1678901234567.002      // Timestamp for 'age' property
    }
  },
  name: 'Alice',
  age: 30
}

Source: ~/workspace/source/src/state.js:18-27

State Vector Clocks

State Generation

GUN generates unique, monotonically increasing timestamps:

function State(){
  var t = +new Date;
  if(last < t){
    return N = 0, last = t + State.drift;
  }
  return last = t + ((N += 1) / D) + State.drift;
}

Source: ~/workspace/source/src/state.js:4-10

Components

Base timestamp: Milliseconds since epoch
Sub-millisecond counter: Allows multiple updates within the same millisecond
Drift compensation: Accounts for clock skew across machines

// 999 updates per millisecond are possible
var D = 999;

// Initialize counter
var N = 0;

// Last timestamp generated
var last = -Infinity;

Source: ~/workspace/source/src/state.js:12

Getting State Values

// Get the state timestamp for a property
var timestamp = State.is(node, 'propertyName');

if(timestamp > 0){
  console.log('Property last updated at:', timestamp);
}

Source: ~/workspace/source/src/state.js:13-17

Setting State Values

// Create a node with state metadata
var node = State.ify(
  null,           // existing node or null
  'name',         // property key
  1678901234567,  // state timestamp
  'Alice',        // value
  'user/alice'    // soul (optional)
);

// Result:
// {
//   _: {
//     '#': 'user/alice',
//     '>': { name: 1678901234567 }
//   },
//   name: 'Alice'
// }

Source: ~/workspace/source/src/state.js:18-27

Last-Write-Wins (LWW)

GUN uses a Last-Write-Wins strategy at the property level, not the document level.

Property-Level Resolution

// Peer A writes at time 1000
gun.get('user').put({name: 'Alice', age: 25});

// Peer B writes at time 2000
gun.get('user').put({name: 'Bob'});

// Result: {name: 'Bob', age: 25}
// 'Bob' wins for 'name', 'age' is unchanged

Why Property-Level?

This approach provides better merge semantics:

// Two peers update different properties concurrently

// Peer A
gun.get('user').get('name').put('Alice');

// Peer B (at nearly the same time)
gun.get('user').get('age').put(30);

// Both updates are preserved!
// Result: {name: 'Alice', age: 30}

Handling Conflicts

Automatic Merging

GUN automatically merges concurrent updates:

// Offline peer A
gun.get('doc').put({title: 'Hello', count: 1});

// Offline peer B (concurrent)
gun.get('doc').put({subtitle: 'World', count: 2});

// When peers reconnect and sync:
// The update with the higher timestamp wins per-property
// Result: {title: 'Hello', subtitle: 'World', count: 2 or 1}

Conflict Detection

You can detect conflicts by tracking state changes:

let previousState = {};

gun.get('doc').on(function(data){
  const currentState = data._['>'];
  
  Object.keys(currentState).forEach(key => {
    if(previousState[key] && 
       currentState[key] !== previousState[key]){
      console.log(`Conflict resolved on ${key}`);
      console.log(`Old state: ${previousState[key]}`);
      console.log(`New state: ${currentState[key]}`);
    }
  });
  
  previousState = {...currentState};
});

Dealing with Clock Skew

Drift Compensation

GUN includes a drift parameter to handle clock differences:

State.drift = 0; // Adjust for clock differences

Source: ~/workspace/source/src/state.js:11

Best Practices

Use NTP: Keep system clocks synchronized
Trust timestamps: Don’t try to “fix” timestamps
Monotonic clocks: GUN ensures timestamps always increase locally

Advanced Conflict Strategies

Counter CRDTs

For counters, use increment operations:

// DON'T do this (race condition)
gun.get('counter').once(val => {
  gun.get('counter').put(val + 1); // WRONG!
});

// DO this instead (using sets for each increment)
gun.get('counter').get(Gun.text.random()).put(1);

// Then sum the set
gun.get('counter').map().once((val, key) => {
  count += val;
});

Set CRDTs

GUN’s .set() implements a grow-only set:

// Add items to a set
gun.get('users').set({name: 'Alice'});
gun.get('users').set({name: 'Bob'});

// Items are never removed, only marked as deleted
gun.get('users').map().once((user, id) => {
  // Each user has a unique ID
  if(user){ // null means deleted
    console.log(user);
  }
});

Custom Resolution

Implement custom conflict resolution by tracking versions:

function customPut(gun, key, value){
  const id = Gun.text.random();
  const timestamp = Gun.state();
  
  gun.get(key).get('versions').get(id).put({
    value: value,
    timestamp: timestamp,
    id: id
  });
  
  // Application resolves conflicts by choosing a version
}

Vector Clocks vs State Timestamps

GUN uses state timestamps rather than traditional vector clocks:

Vector Clocks

// Traditional vector clock (NOT GUN)
{
  versions: {
    'peer-A': 5,
    'peer-B': 3,
    'peer-C': 7
  }
}

State Timestamps (GUN)

// GUN's approach
{
  _: {
    '>': {
      'property1': 1678901234567.001,
      'property2': 1678901234567.002
    }
  }
}

Advantages:

Simpler to implement and reason about
Constant space per property (not per peer)
Works well for eventually consistent systems
Natural ordering with timestamps

Tradeoffs:

Requires reasonably synchronized clocks
Cannot detect all concurrent updates (some are arbitrarily resolved)
May lose updates if clocks are severely misaligned

Data Integrity

Hash-Based Deduplication

GUN uses content hashing to detect duplicate messages:

// Messages include hashes for deduplication
{
  '#': 'msg-id-123',
  '##': 'content-hash-456',  // Hash of message content
  put: { /* data */ }
}

Source: ~/workspace/source/src/mesh.js:73

Preventing Loops

The mesh layer prevents infinite message loops:

// Track which peers already saw this message
{
  '><': 'peer1,peer2,peer3'  // Don't send back to these peers
}

Source: ~/workspace/source/src/mesh.js:76

Convergence Guarantees

Eventual Consistency

GUN guarantees that all peers will eventually converge to the same state:

All updates are commutative: Order doesn’t matter
All updates are idempotent: Applying twice has same effect
All peers use the same merge rules: Deterministic conflict resolution

Convergence Time

The time to convergence depends on:

Network latency between peers
Number of hops in the mesh network
Message batching intervals
Peer availability

// Typical convergence: milliseconds to seconds
// Under partition: converges when network heals

Testing Conflicts

Simulating Offline Scenarios

// Peer 1 (offline)
const gun1 = Gun({peers: []});
gun1.get('doc').put({a: 1});

// Peer 2 (offline)
const gun2 = Gun({peers: []});
gun2.get('doc').put({b: 2});

// Sync the peers
gun1.opt({peers: ['http://localhost:8765/gun']});
gun2.opt({peers: ['http://localhost:8765/gun']});

// Both peers converge to: {a: 1, b: 2}

Concurrent Writes

const gun = Gun();
const ref = gun.get('test');

// Simulate concurrent writes
Promise.all([
  new Promise(r => ref.get('count').put(1, r)),
  new Promise(r => ref.get('count').put(2, r)),
  new Promise(r => ref.get('count').put(3, r))
]).then(() => {
  ref.get('count').once(val => {
    console.log('Winner:', val); // 1, 2, or 3 (deterministic)
  });
});

Best Practices

Embrace eventual consistency: Don’t rely on immediate consensus
Use property-level updates: More granular merging
Avoid read-modify-write: Use semantic operations instead
Implement tombstones: Mark deletions rather than removing data
Monitor state timestamps: Debug sync issues
Trust the CRDT: Don’t try to “fix” conflicts manually
Design for commutativity: Order-independent operations
Test offline scenarios: Ensure your app handles partitions gracefully

Common Patterns

Optimistic UI Updates

function updateUI(newValue){
  // Update UI immediately (optimistic)
  displayValue(newValue);
  
  // Write to GUN
  gun.get('data').put(newValue);
  
  // Listen for conflicts
  gun.get('data').on(actualValue => {
    if(actualValue !== newValue){
      // Conflict resolved differently
      displayValue(actualValue);
    }
  });
}

Collaborative Editing

// Each character has a unique ID and state
function insertChar(pos, char){
  const id = Gun.text.random();
  gun.get('doc').get('chars').get(id).put({
    pos: pos,
    char: char,
    state: Gun.state()
  });
}

// Reconstruct document by sorting characters
gun.get('doc').get('chars').map().once((charObj, id) => {
  // Rebuild document from character positions
});

Debugging

Inspecting State Metadata

gun.get('node').once((data, key) => {
  console.log('Soul:', data._['#']);
  console.log('State vector:', data._['>']);
  
  Object.keys(data._['>']).forEach(prop => {
    console.log(`${prop}: ${new Date(data._['>'][prop])}`);
  });
});

Monitoring Conflicts

gun.on('in', function(msg){
  if(msg.put && msg['@']){
    console.log('Incoming update:', msg);
  }
  this.to.next(msg);
});

Next Steps

Optimize Performance for high-throughput scenarios
Understand CAP Theorem tradeoffs in GUN
Learn about Networking protocols
Deploy to Production with confidence

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Documentation Index

​State-Based CRDT

​How It Works

​State Vector Clocks

​State Generation

​Components

​Getting State Values

​Setting State Values

​Last-Write-Wins (LWW)

​Property-Level Resolution

​Why Property-Level?

​Handling Conflicts

​Automatic Merging

​Conflict Detection

​Dealing with Clock Skew

​Drift Compensation

​Best Practices

​Advanced Conflict Strategies

​Counter CRDTs

​Set CRDTs

​Custom Resolution

​Vector Clocks vs State Timestamps

​Vector Clocks

​State Timestamps (GUN)

​Data Integrity

​Hash-Based Deduplication

​Preventing Loops

​Convergence Guarantees

​Eventual Consistency

​Convergence Time

​Testing Conflicts

​Simulating Offline Scenarios

​Concurrent Writes

​Best Practices

​Common Patterns

​Optimistic UI Updates

​Collaborative Editing

​Debugging

​Inspecting State Metadata

​Monitoring Conflicts

​Next Steps

Build docs developers (and LLMs) love

State-Based CRDT

How It Works

State Vector Clocks

State Generation

Components

Getting State Values

Setting State Values

Last-Write-Wins (LWW)

Property-Level Resolution

Why Property-Level?

Handling Conflicts

Automatic Merging

Conflict Detection

Dealing with Clock Skew

Drift Compensation

Best Practices

Advanced Conflict Strategies

Counter CRDTs

Set CRDTs

Custom Resolution

Vector Clocks vs State Timestamps

Vector Clocks

State Timestamps (GUN)

Data Integrity

Hash-Based Deduplication

Preventing Loops

Convergence Guarantees

Eventual Consistency

Convergence Time

Testing Conflicts

Simulating Offline Scenarios

Concurrent Writes

Best Practices

Common Patterns

Optimistic UI Updates

Collaborative Editing

Debugging

Inspecting State Metadata

Monitoring Conflicts

Next Steps