Sync 3

sync3.js

function Server() {
	this.rev = 0;
	this.data = {};
	this.changes = []; // { id, rev }
}

/**
 * Finds changes after given revision. Returns array of {id, op, rev}.
 */
Server.prototype.changesAfter = function(rev) {
	var changes = [];
	
	var server = this;
	
	this.changes.forEach(function(change) {
		if (change.rev > rev) {
			changes.push({
				id: change.id,
				op: server.data[change.id] === undefined ? 'D' : 'U',
				rev: change.rev
			});
		}
	});
	
	return changes;
};

function Client() {
	this.rev = 0;
	this.data = {};
	this.changes = []; // { id, rev }
}

Client.prototype.add = function(id, data) {
	this.data[id] = data;
	this.changes.push({ id: id, rev: this.rev });
};

Client.prototype.remove = function(id) {
	delete this.data[id];
	this.changes.push({ id: id, rev: this.rev });
};

Client.prototype.update = function(id, data) {
	this.data[id] = data;
	this.changes.push({ id: id, rev: this.rev });
};

/**
 * Finds current changes. Returns array of {id, op, rev}.
 */
Client.prototype.currentChanges = function() {
	var changes = [];
	
	var client = this;
	
	this.changes.forEach(function(change) {
		changes.push({
			id: change.id,
			op: client.data[change.id] === undefined ? 'D' : 'U',
			rev: change.rev
		});
	});
	
	return changes;
};

Client.prototype.sync = function(server) {
	var scs = server.changesAfter(this.rev);
	var ccs = this.currentChanges();
	
	var conflicting = {}; // id -> true
	
	var client = this;
	
	// Find and resolve conflicting changes.
	
	scs.forEach(function(sc) {
		ccs.forEach(function(cc) {
			if (sc.id === cc.id) {
				console.log('Conflicting for id ' + sc.id);
				conflicting[sc.id] = true;
				
				// Current resolution: prefer master
				// Which changes to send to master (below) also depend on it.
				
				if (sc.op === 'U') {
					client.data[sc.id] = server.data[sc.id];
				} else if (sc.op === 'D') {
					delete client.data[sc.id];
				}
			}
		});
	});
	
	// Find and apply non-conflicting server changes.
	
	scs.forEach(function(sc) {
		if (conflicting[sc.id] === undefined) {
			if (sc.op === 'U') {
				client.data[sc.id] = server.data[sc.id];
			} else if (sc.op === 'D') {
				delete client.data[sc.id];
			}
		}
	});
	
	// Find and apply non-conflicting client changes.
	
	ccs.forEach(function(cc) {
		if (conflicting[cc.id] === undefined) {
			if (cc.op === 'U') {
				server.data[cc.id] = client.data[cc.id];
			} else if (cc.op === 'D') {
				delete server.data[cc.id];
			}
		}
	});
	
	// Push rev.
	
	server.rev += 1;
	this.rev = server.rev;
	
	// Transfer client changes with increased revision number.
	// Conflicting changes are not sent since we chose to prefer master.
	
	ccs.forEach(function(cc) {
		if (conflicting[cc.id] === undefined) {
			server.changes.push({ id: cc.id, rev: client.rev });
		}
	});
	
	// Cleanup.
	
	this.changes = [];
};

function report(party, title) {
	console.log(title);
	
	console.log('\tRevision: ' + party.rev);
	
	console.log('\tData:');
	for (id in party.data) {
		console.log('\t\t' + id + ': ' + party.data[id]);
	}
	
	console.log('\tChanges:');
	party.changes.forEach(function(change) {
		console.log('\t\tr' + change.rev + ' ' + change.id);
	});
};

var s = new Server();

var c1 = new Client();

c1.add(1, 'd1-1.0');
c1.add(2, 'd2-2.0');

c1.sync(s);

report(s, 'Server data after 1st sync');

var c2 = new Client();
c2.add(1, 'd1-1.a');

// Now c2-server has conflict for id = 1.

c2.sync(s);

report(s, 'Server data after 2nd sync'); // must be unchanged
report(c2, 'Client 2 data after 2nd sync'); // override by server

/*
report(c1, 'Client 1 data before 1st sync');

c1.sync(s);

report(c1, 'Client 1 data after 1st sync');
report(s, 'Server data after 1st sync');

var c2 = new Client();

c2.sync(s);

report(c2, 'Client 2 data after 2nd sync');

c2.remove(1);
c2.add(3, 'd3-1.0');

report(c2, 'Client 2 data before 3rd sync');

c2.sync(s);

report(c2, 'Client 2 data after 3rd sync');
report(s, 'Server data after 3rd sync');

c1.sync(s);
report(c1, 'Client 1 data after 4th sync');

var c3 = new Client();
c3.sync(s);

report(c3, 'Client 3 data after 5th sync');
report(s, 'Server data after 5th sync');
*/