Can I access a database during startup in ASP.NET Core?

Yes, you can access the database! Code that runs in the Configure method can access any services that are added in the ConfigureServices method, including things like database contexts.

For example, if you have a simple Entity Framework context:

using Microsoft.EntityFrameworkCore;
using SimpleTokenProvider.Test.Models;

namespace SimpleTokenProvider.Test
{
    public class SimpleContext : DbContext
    {
        public SimpleContext(DbContextOptions<SimpleContext> options)
            : base(options)
        {
        }

        public DbSet<User> Users { get; set; }
    }
}

And you add it in ConfigureServices:

services.AddDbContext<SimpleContext>(opt => opt.UseInMemoryDatabase());

Then, you can access it when you are setting up the middleware:

var context = app.ApplicationServices.GetService<SimpleContext>();

app.UseSimpleTokenProvider(new TokenProviderOptions
{
    Path = "/api/token",
    Audience = "ExampleAudience",
    Issuer = "ExampleIssuer",
    SigningCredentials = new SigningCredentials(signingKey, SecurityAlgorithms.HmacSha256),
    IdentityResolver = (username, password) => GetIdentity(context, username, password)
});

And rewrite the GetIdentity method a little:

private Task<ClaimsIdentity> GetIdentity(SimpleContext context, string username, string password)
{
    // Access the database using the context
    // Here you'd need to do things like hash the password
    // and do a lookup to see if the user + password hash exists
}

I'm the author of the original sample. Sorry it wasn't clear initially! I tried to write the IdentityResolver delegate in a way that makes it easy to provide your own functionality -- like integrating with your own database (as above), or hooking it up to ASP.NET Core Identity. Of course, you're free to throw away my code and do something better, too. :)

On .NET CORE 2.1, just pass the context as an argument to the Configure method:

public void Configure(IApplicationBuilder app, YourDbContext context, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
        //do whatever you want with the context here...
}

Adding services to the service container makes them available within the app and in the Configure method. The services are resolved via dependency injection or from ApplicationServices.