@loopback/docs/site/migration/datasources.md

Documentation files rendered at [https://loopback.io](https://loopback.io)

---
lang: en
title: 'Migrating datasources'
keywords:
  LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI, LoopBack 3, Migration
sidebar: lb4_sidebar
permalink: /doc/en/lb4/migration-datasources.html
---

{% include tip.html content="
Missing instructions for your LoopBack 3 use case? Please report a [Migration docs issue](https://github.com/loopbackio/loopback-next/issues/new?labels=question,Migration,Docs&template=Migration_docs.md) on GitHub to let us know.
" %}

## Overview

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:

   ```json
   {
     "mysqlDs": {
       "name": "mysqlDs",
       "connector": "mysql",
       "host": "demo.strongloop.com",
       "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:

   ```ts
   const config = {
     name: 'mysqlDs',
     connector: 'mysql',
     host: 'demo.strongloop.com',
     port: 3306,
     database: 'getting_started',
     username: 'demo',
     password: 'L00pBack',
   };
   ```

4. Repeat steps 1-3 for each datasource you want to migrate.

{% include note.html content="We are working on a CLI command `lb4 import-lb3-datasources` that will migrate datasources from a mounted LoopBack 3 application to a LoopBack 4 project automatically. See [GitHub issue #4346](https://github.com/loopbackio/loopback-next/issues/4346) for more details." %}

## Compatibility

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`](http://github.com/loopbackio/loopback-connector-mysql).