How to integrate external data sources in a SOLID Principle

Question

I've been using the SOLID Design Principles, in C# with Entity Framework for a while, and works great. Now I have a situation where I need to get data from an external Web API call. What's the best way or what approach should I take to integrate an external data source into my solution. I have the model objects I need, but not sure how to modify the Persistence layer to call and get the external data. Thanks for the help and comments.

Answer 1

I would define an interface IExternalDataService (although you should find a more suitable name for it, for example, if it fetches users from an external API I would call it IExternalUsersService) that my PersistenceLayer depends on. A concrete implementation will be injected in the PersistenceLayer on instantiation.

interface IExternalDataService
{
   Task<IEnumerable<Data>> FetchAsync();
}

class ExternalDataService: IExternalDataService
{
   // here comes the implementation
}

class PersistenceLayer 
{
   // ...other properties and methods
   private readonly IExternalDataService _externalService;

   constructor PersistenceLayer(IExternalDataService service) 
   {
      _externalService = service;
   }

   public async Task<IEnumerable<Data>> GetAll() 
   {
      return await _externalService.FetchAsync();
   }
}


// somewhere else
var persistenceLayer = new PersistenceLayer(new ExternalDataService());