Understanding data sources

</figure>

Overview

The diagram illustrates the relationship between LoopBack Model, DataSource, and Connector.

1. Define the model.

2. Create an instance of ModelBuilder or DataSource. DataSource extends from ModelBuilder. ModelBuilder is responsible for compiling model definitions to JavaScript constructors representing model classes. DataSource inherits that function from ModelBuilder.

3. Use ModelBuilder or DataSource to build a JavaScript constructor (i.e. the model class) from the model definition. Model classes built from ModelBuilder can be later attached to a DataSource to receive the mixin of data access functions.

4. As part of step 2, DataSource initializes the underlying Connector with a settings object which provides configurations to the connector instance.  Connector collaborates with DataSource to define the functions as DataAccessObject to be mixed into the model class. The DataAccessObject consists of a list of static and prototype methods. It can be create, retrieve, update, and delete operations or other specific functions depending on the connector’s capabilities.

The DataSource object is the unified interface for LoopBack applications to integrate with backend systems. It’s a factory for data access logic around model classes. With the ability to plug in various connectors, DataSource provides the necessary abstraction to interact with databases or services to decouple the business logic from plumbing technologies.

Creating a DataSource programmatically

The DataSource constructor accepts the following arguments:

• name: Optional name of the data source instance being created. 
• settings: An object of properties to configure the connector. Must include a connector property, specifying the connector to use. See Connecting models to data sources (Connectors).

For example:

var DataSource = require('loopback-datasource-juggler').DataSource;

var dataSource = new DataSource({
    connector: require('loopback-connector-mongodb'),
    host: 'localhost',
    port: 27017,
    database: 'mydb'
});

The connector argument passed the DataSource constructor can be one of the following:

• The connector module from require(connectorName)
• The full name of the connector module, such as ‘loopback-connector-oracle’
• The short name of the connector module, such as ‘oracle’, which will be converted to ‘loopback-connector-‘
• A local module under ./connectors/ folder

var ds1 = new DataSource('memory');
var ds2 = new DataSource('loopback-connector-mongodb'));
var ds3 = new DataSource(require('loopback-connector-oracle'));

LoopBack provides the built-in memory connector that uses in-memory store for create, retrieve, update, and delete operations. 

The settings argument configures the connector. Settings object format and defaults depends on specific connector, but common fields are:

• host: Database host
• port: Database port
• username: Username to connect to database
• password: Password to connect to database
• database: Database name
• debug: Turn on verbose mode to debug db queries and lifecycle

For more information, see  Connecting models to data sources (Data source properties). For connector-specific settings, see the connector’s documentation.

Creating a model from a data source

DataSource extends from ModelBuilder, which is a factory for plain model classes that only have properties. DataSource connects to databases and other backend systems using Connector.

var DataSource = require('loopback-datasource-juggler').DataSource;
var ds = new DataSource('memory');

var User = ds.define('User', {
  name: String,
  bio: String,
  approved: Boolean,
  joinedAt: Date,
  age: Number
});

All model classes within single data source share the same connector type and one database connection or connection pool. But it’s possible to use more than one data source to connect to different databases.

Alternatively, you can attach a plain model constructor created from ModelBuilder to a DataSource.

var ModelBuilder = require('loopback-datasource-juggler').ModelBuilder;
var builder = new ModelBuilder();

var User = builder.define('User', {
  name: String,
  bio: String,
  approved: Boolean,
  joinedAt: Date,
  age: Number
});

var DataSource = require('loopback-datasource-juggler').DataSource;
var ds = new DataSource('memory');

User.attachTo(ds); // The create, retrieve, update, and delete
                   // methods will be mixed into the User constructor

Creating a data source for a connector

Application code does not directly use a connector. Rather, you create a DataSource to interact with the connector.

The simplest example is for the in-memory connector:

var memory = loopback.createDataSource({
  connector: loopback.Memory
});

Here is another example, this time for the Oracle connector:

var DataSource = require('loopback-datasource-juggler').DataSource;
var oracleConnector = require('loopback-connector-oracle');

var ds = new DataSource(oracleConnector, {
  host: 'localhost',
  database: 'XE',
  username: 'username',
  password: 'password',
  debug: true
});

The connector argument passed the DataSource constructor can be one of the following:

• The connector module from require('connectorName')
• The full name of the connector module, such as 'loopback-connector-oracle'.
• The short name of the connector module, such as 'oracle', that LoopBack converts to 'loopback-connector-oracle' (for example).
• A local module in the /connectors folder

Initializing a connector

The connector module can export an initialize function to be called by the owning DataSource instance.

exports.initialize = function (dataSource, postInit) {

  var settings = dataSource.settings || {};   // The settings is passed in from the dataSource

  var connector = new MyConnector(settings);  // Construct the connector instance
  dataSource.connector = connector;           // Attach connector to dataSource
  connector.dataSource = dataSource;          // Hold a reference to dataSource
  // ...
};

The DataSource calls the initialize method with itself and an optional postInit callback function. The connector receives the settings from the dataSource argument and use it to configure connections to backend systems.

Please note connector and dataSource set up a reference to each other.

Upon initialization, the connector might connect to database automatically. Once connection established dataSource object emit ‘connected’ event, and set connected flag to true, but it is not necessary to wait for ‘connected’ event because all queries cached and executed when dataSource emit ‘connected’ event.

To disconnect from database server call dataSource.disconnect method. This call is forwarded to the connector if the connector have ability to connect/disconnect.