Sync

Channels/Data Routing

In the Access Control lesson we discussed how the Couchbase Sync Gateway supports Authorization and Access Control functions. In this lesson we discuss how it can be used for Data Synchronization and Routing.

A Sync Gateway configuration file determines the runtime behavior of Sync Gateway, including server configuration and the database or set of databases with which a Sync Gateway instance can interact.

Sync Gateway uses channels to make it easy to share a database between a large number of users and control access to the database. Conceptually, a channel could be viewed as a tag. Every document in the database belongs to a set of channels, and a user is granted appropriate access a set of channels. A channel is used to:

Partition the data set.
Authorize users to access documents.
Minimize the amount of data synced down to devices.

In the Sync Gateway installation section, we walked you through the steps to launch Sync Gateway with a specific config file.

Open the sync-gateway-config-travelsample.json file located at https://github.com/couchbaselabs/mobile-travel-sample/blob/master/sync-gateway-config-travelsample.json. It includes the sync function which is a JavaScript function whose source code is stored in the Sync Gateway’s database configuration file.

/* Routing */
// Add doc to the user's channel.
channel("channel." + username);

Shared Bucket Access

Before you begin this lesson, confirm that you have Sync Gateway up and running by following the instructions in the Sync Gateway installation section.

Sync Gateway and Couchbase Server mobile and server/web applications now have the ability to read and write to the same bucket. It is an opt-in feature that can be enabled in the Sync Gateway configuration file.

The sync metadata used by the Sync Gateway for replication with mobile clients is stored in the Extended Attributes or XAttrs associated with the document.

The capability can be enabled through a configuration setting in the sync gateway config file. It is to be noted that if you are using Enterprise Edition of Sync Gateway ^[1], then the "import_docs" flag is optional. Every node with "enable_shared_bucket_access" set to "true" will automatically import document mutations from the server bucket.

Open the sync-gateway-config-travelsample.json file located at https://github.com/couchbaselabs/mobile-travel-sample/blob/master/sync-gateway-config-travelsample.json

"import_docs": "continuous",
"enable_shared_bucket_access": true

You can specify the Couchbase Server documents that need to be imported and processed by the Sync Gateway by defining an import filter function. In our demo, we will only be synchronizing the "user" document. So every other document type is ignored.

function(doc) {
  /* Just ignore all the static travel-sample files */
  if (doc._deleted == true ) {
    return true;
   }
  if (doc.type == "landmark" || doc.type == "hotel" || doc.type == "airport" || doc.type =="airline" || doc.type == "route") {
    return false;
  }

  return true;
}

Replication

Replication is the process by which clients running Couchbase Lite synchronize database changes with the remote (server) database.

Pull Replication is the process by which clients running Couchbase Lite download database changes from the remote (server) source database to the local target database.
Push Replication is the process by which clients running Couchbase Lite upload database changes from the local source database to the remote (server) target database.

Couchbase Mobile replication protocol is implemented as a messaging protocol layered over WebSocket.

The replication process can be “continuous” or "`one shot"`.

In “Continuous” replication mode, the changes are continually synchronized between the client and Sync Gateway.
In “One shot” mode, the changes are synchronized once and the connection between the client and server disconnects. When any future changes need to be pushed up or pulled down, the client must start a new replication.

Open the file ReplicatorManager.java. We will review the method void start(@Nonnull String username, @Nonnull char[] password).

ReplicatorManager.java

  @Nonnull
  void start(@Nonnull String username, @Nonnull char[] password)
      throws CouchbaseLiteException, IOException, AuthenticationException, URISyntaxException {
    ...
  }

First, you will configure the replication. The ReplicatorConfiguration is initialized with the local database and URL of the target DB on Sync Gateway. The replicatorType in the Replicator Config specifies the type of replication. It is set to pushAndPull indicating that both push and pull replication is enabled. The continuous mode is set to true in the Travel app.

  final ReplicatorConfiguration config
      = new ReplicatorConfiguration(database, new URLEndpoint(new URI(SGW_ENDPOINT)));

  // !!! copying the password into the string is unsecure.
  config.setAuthenticator(new BasicAuthenticator(username, new String(password)));

  config.setReplicatorType(ReplicatorConfiguration.ReplicatorType.PUSH_AND_PULL);
  config.setContinuous(true);

The Replicator is configured with relevant authentication credentials. The list of users that are permitted access is configured in the Sync Gateway configuration file as discussed in the Access Control section.

  // !!! copying the password into the string is unsecure.
  config.setAuthenticator(new BasicAuthenticator(username, new String(password)));

The replicator is configured with a Push filter to prevent the Travel sample documents that’s bundled with the app from being pushed up.

  config.setPushFilter((document, flags) ->
      !(Hotel.DOC_TYPE.equals(document.getString(DbManager.PROP_DOC_TYPE))
          || DbManager.DOC_TYPE_AIRLINE.equals(document.getString(DbManager.PROP_DOC_TYPE))
          || DbManager.DOC_TYPE_AIRPORT.equals(document.getString(DbManager.PROP_DOC_TYPE))
          || DbManager.DOC_TYPE_ROUTE.equals(document.getString(DbManager.PROP_DOC_TYPE))
          || DbManager.DOC_TYPE_LANDMARK.equals(document.getString(DbManager.PROP_DOC_TYPE))));

The Replicator is initialized with the specified configuration

final Replicator repl = new Replicator(config);

A change listener callback block is registered to listen for replication changes. Every time, there is a push or pull change, the callback is invoked.

  final ReplicationStartListener listener = new ReplicationStartListener(repl);
  final ListenerToken token = repl.addChangeListener(listener);

Replication is started

  repl.start();

Try Push Replication

Try it out (Mobile App)

Log into the Travel Sample Mobile app as “demo” user and password as “password”
Tap the "airline" button to make a flight reservation. Both the "From" and "To" airports and flight dates are already set.
Tap the "lookup" button
From list of flights, select the first flight listing. This automatically confirms the booking — see: Figure 1

Figure 1. Push Replication

Try it out (Web App)

Access the Travel Sample Python Web app. The URL would be http://localhost:8080. If you did cloud based install, please replace localhost in the URL with the IP Address of the cloud instance of the web app.
Log into the web app as “demo” user with password as “password”
Use the "Booked" tab to navigate to the list of booked flights
Confirm that you see the flight that you reserved via the mobile app in your list of flights in the web app — see: Figure 2

Figure 2. Check Booked Flights

Try Pull Replication

Try it out (Web App)

Access the Travel Sample Python Web app. The URL would be http://localhost:8080. If you did cloud based install, please replace localhost in the URL with the IP Address of the cloud instance of the web app.
Log into the web app as “demo” user with password as “password”
Make a flight reservation by clicking the "Flights" tab
Enter “From” airport as "Seattle" and select the airport from drop down menu.
Enter “To” airport as "San Francisco" and select the airport from drop down menu.
Enter From and Return Travel Dates
Click on "Search" button
From list of flights, select the first flight listing by clicking on the corresponding "Add to Basket" button
Confirm the booking by clicking on the "Basket" tab to view the flight selections and then click on the “Buy” button
The "Booked" tab should show the confirmed flight reservations — see: Figure 3
Log into the Travel Sample Mobile app as “demo” user and password as “password”
Confirm that you see the flight that you reserved via the web app in your list of flights in the mobile app

Figure 3. Pull Replication

1. 2.7