{% include anchor.html edit="true" title="Sync a database" hash="sync" %} 

{% highlight js %}
const sync = PouchDB.sync(src, target, [options])
{% endhighlight %}

Sync data from `src` to `target` and `target` to `src`. This is a convenience method for bidirectional data replication.

In other words, this code:

{% highlight js %}
PouchDB.replicate('mydb', 'http://localhost:5984/mydb');
PouchDB.replicate('http://localhost:5984/mydb', 'mydb');
{% endhighlight %}


is equivalent to this code:

{% highlight js %}
PouchDB.sync('mydb', 'http://localhost:5984/mydb');
{% endhighlight %}


### Options

* `options.push` + `options.pull`: Allows you to specify separate [replication options](api.html#replication) for the individual replications.

Replication options such as `filter` passed to sync directly will be passed to both replications. Please refer to [replicate()](api.html#replication) for documentation on those options.

#### Example Usage:
{% highlight js %}
const sync = PouchDB.sync('mydb', 'http://localhost:5984/mydb', {
  live: true,
  retry: true
}).on('change', function (info) {
  // handle change
}).on('paused', function (err) {
  // replication paused (e.g. replication up to date, user went offline)
}).on('active', function () {
  // replicate resumed (e.g. new changes replicating, user went back online)
}).on('denied', function (err) {
  // a document failed to replicate (e.g. due to permissions)
}).on('complete', function (info) {
  // handle complete
}).on('error', function (err) {
  // handle error
});

sync.cancel(); // whenever you want to cancel
{% endhighlight %}

There is also a shorthand for syncing given existing PouchDB objects. This behaves the same as `PouchDB.sync()`:

{% highlight js %}
db.sync(remoteDB, [options]);
{% endhighlight %}

It is also possible to combine "one-way" replication and sync for performance reasons. 
When your PouchDB application starts up it could perform a one-off, one-way replication to completion and then initiate the two-way, continuous retryable sync:

{% highlight js %}
const url = 'http://localhost:5984/mydb';
const opts = { live: true, retry: true };

// do one way, one-off sync from the server until completion
db.replicate.from(url).on('complete', function(info) {
  // then two-way, continuous, retriable sync
  db.sync(url, opts)
    .on('change', onSyncChange)
    .on('paused', onSyncPaused)
    .on('error', onSyncError);
}).on('error', onSyncError);
{% endhighlight %}

The above technique results in fewer HTTP requests being used and better performance than just using `db.sync` on its own.

#### Example Response:

Change events in `sync` have an extra property `direction` which refers to the direction the change was going. Its value will either be `push` or `pull`.

{% highlight js %}
{ direction: 'push',
  change:
   { ok: true,
     start_time: '2015-10-21T15:26:51.151Z',
     docs_read: 1,
     docs_written: 1,
     doc_write_failures: 0,
     errors: [],
     last_seq: 1,
     docs: [ [Object] ] } }
{% endhighlight %}

For any further details, please refer to [replicate()](api.html#replication).