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LoopBack 3 datasources are compatible with LoopBack 4 datasources, so migrating datasources from LoopBack 3 to LoopBack 4 is simple.

In LoopBack 3, all datasources are defined in the server/datasources.json file, whereas in LoopBack 4 each datasource is defined in its own file in the src/datasources folder. Each LoopBack 4 datasource is represented as a class, the source file (e.g. mysql-ds.datasource.ts) includes both the configuration and the class definition.

Migration Steps

To migrate a datasource from LoopBack 3 to LoopBack 4, complete the following steps:

  1. In the root of your LoopBack 4 application, use the lb4 datasource command to create a new datasource and enter the same datasource name as your LoopBack 3 application’s datasource (e.g. mysqlDs):

    $ lb4 datasource
    ? Datasource name: mysqlDs
  2. For the remaining prompts from the lb4 datasource command, use the defaults (press Enter for each one) since these will be replaced in the next step:

    ? Select the connector for mysqlDs: In-memory db (supported by StrongLoop)
    ? window.localStorage key to use for persistence (browser only):
    ? Full path to file for persistence (server only):
  3. Replace content of the config object defined in the newly created src/datasources/{dataSource.dataSourceName}.datasource.ts file in your LoopBack 4 application with the datasource configuration from server/datasources.json in your LoopBack 3 application.

    For example, if your server/datasources.json file contains:

      "mysqlDs": {
        "name": "mysqlDs",
        "connector": "mysql",
        "host": "",
        "port": 3306,
        "database": "getting_started",
        "username": "demo",
        "password": "L00pBack"

    Move it to src/datasources/mysql-ds.datasource.ts, so that the config object is defined as follows:

    const config = {
      name: 'mysqlDs',
      connector: 'mysql',
      host: '',
      port: 3306,
      database: 'getting_started',
      username: 'demo',
      password: 'L00pBack',
  4. Repeat steps 1-3 for each datasource you want to migrate.


As mentioned before, LoopBack 3 datasources are compatible with LoopBack 4 datasources. In both, a datasource is a connector instance that is used by legacy-juggler-bridge. For example, both a LoopBack 3 MySQL datasource and a LoopBack 4 MySQL datasource will use loopback-connector-mysql.