How we built offline data access & sync in LoopBack
Miroslav Bajtoš
What is LoopBack?
Data access & ORM
● Own querying language inspired by MongoDB
● Connectors for SQL, NoSQL and more
● In-memory database
Todo.find({
where: { completed: false },
limit: 10
}, function(err, list) { /*...*/ });
REST API
Built-in data-access methodsTodo.find => GET /todos?filter=:filter
Todo.create => POST /todos
Todo.findById => GET /todos/:id
(and so on)
Custom methodsTodo.stats => GET /todos/stats
LoopBack on the server
Todomodel
RESTAPI
datasource
Mongo
MySQL
SOAP
The trouble with offline mode
Different tools & APIs
Online● REST/HTTP calls● full power of back-end &
database
GET /todos?where=
{"completed":false}
&limit=10
Offline● local storage/index db● key/value store
JSON.parse(
localStorage.todos
).filter(function(t){
return
!t.completed;
}).slice(0,10);
● What has been changed locally?
● What has changed on the server?
● How to detect conflicts?
Synchronization
Single API for offline & online
● LoopBack in the browser● LocalStorage as a database● REST server as a database
Synchronization
● Change replication between data-sources
The LoopBack recipe
Single API for online & offline
Node.js
Browser
Welcome to the browser
Todomodel
datasource
Localstorage
LoopBack server
Remoting metadata
Todo.find(filter, cb) { … }
Todo.remoteMethod('find', {
accepts: [{ arg: 'filter', type: 'object' }], returns: [{ arg: 'data', type: 'array',
root: true }], http: { verb: 'get', path: '/' }});
Todo.sharedClass.http = { path: '/todos' }
Remoting implementation - server
app.get('/todos', function(req, res, next) { Todo.find(
req.param['filter'], function(err, data){ if (err) next(err)
else res.send(data); }
);
}
Remoting implementation - client
Todo.find = function(filter, cb) { request.get( {
path: '/todos', query: { filter: filter } },
function(err, res) {
if (err) cb(err)
else cb(null, res.body); });
}
The result
One API to rule them all: ● server● online browser● offline browser
Todo.find({
where: { completed: false },
limit: 10
}, function(err, list) { /*...*/ });
Data synchronization
Change tracking
Change tracking record:modelName "Todo"
modelId "42"
revision "SHA of data"
checkpoint 2
● Updated immediately by loopback writes
● Manual update for external writers (scheduled at a regular interval)
The replication algorithm
1. Find local changes
2. Compare remote and local changes,detect conflicts
3. Resolve conflicts
4. Perform a bulk update
5. Create a new checkpoint
Browser
Synchronizing client
localTodo
model
datasource
Localstorage
LoopBack server
change tracking & replication
remoteTodo
model
datasource
DEMO!
LoopBackhttp://loopback.io
Slideshttp://bit.ly/sync-loopback
Get in touchhttp://twitter.com/bajtoshttp://linkedin.com/in/bajtos
THANK YOU!
