[boot] needs to support environment based config / booting.

[virkt25]

Description / Steps to reproduce / Feature proposal

From #441

We need to allow DI and Boot to support Environment Based Config. Example would be to boot config for a datasource based on the environment (dev, stage, prod, etc.). Also need support for loading production credentials from an external service / environment. If not, at the very least we need to have docs on this.

Related Discussion

#441 (comment) #4

In your proposal, the configuration (loaded from JSON) is hard-coded in the generated (base) class. At the same time, we need to support different configurations for different environments (dev/test/production) and load configuration from environment variables (https://12factor.net).

It was proposed to parse process.env variables inside datasource files. I consider that as an anti-pattern, because it couples high-level datasources with low/system-level configuration code. It makes the code difficult to test, because tests would have to manipulate global process.env. Instead, we should find a way how to inject ENV variables in such way that the actual usage of process.env stays in the top-level index.ts or application.ts file only.

On a similar topic, @raymondfeng proposed a sort of a registry holding datasource configuration for all environments in the app-level Context. I don't think this is a good solution - consider the case where an application is deployed to different geo location and where each geo location requires different connection strings (to use a colocated database instance). In other words, a production config is not a single object, but a set of location-specific objects.

My conclusion is that we should decouple datasource configuration from datasource implementation class and leverage dependency injection to provide arbitrary runtime-specific configuration from the application level. IMO, this addressed both needs 1) have different datasource config depending on dev/test/prod environment 2) build datasource configuration dynamically, either from process.env or perhaps external config providers like https://www.consul.io.

A mock-up app:

    class MyApp extends RestApplication {
      async boot() {
        const dbConfig = 
          // Figure out what config to use.  @loopback/boot can provide
          // helpers to load config from ENV variables
        this.bind('datasources.db$config').to(dbConfig);
        // etc.
      }
    }

A mock-up datasource:

    // default config is used in tests
    const defaultConfig = require('./db.datasource.json');
    
    class DbDataSource extends juggler.DataSource {
      constructor(
        @inject('datasources.db$config')
        config: DataSourceOptions = defaultConfig
      ) {
        super(config);
      }
    }

Acceptance Criteria

TBD

[bajtos]

Closing as a duplicate of #1464