Thursday, 30 November 2017

ASP.NET MVC Multitenancy, Part 2 - OpenID Connect

In part 1 we have established tenant context. Most business applications need authentication. I have decided to use OpenID Connect, luckily for me it comes with awesome middleware and it's very easy to configure. Here is how you would do it:
public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        //...
        app.UseOpenIdConnectAuthentication(new OpenIdConnectAuthenticationOptions
        {
            ClientId = "clientid",
            Authority = 'authority",
            RedirectUri = "http://mysite.com/"
        });
    }
}

However, not so fast, for each tenant you will need to configure client id, authority, url, etc. This also needs to be done at runtime as you don't want to ship new code every time new tenant registers to use your software. I have looked around for a solution and it turns out that this is a known problem, just take a look at the The Grand Auth Redesign. After a surprising amount of searching I have stumbled upon this amazing Multi-tenant middleware pipelines in ASP.NET Core blog post by Ben Foster. This solution is so damn good. Please read it. Unfortunately it did not work for me out of the box I had to port it over from ASP.NET Core to ASP.NET.

Hopefully by blogging and linking to Ben's article it will be easier for others to find it.

Example Usage

public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        //..
        app.UsePerTenant((tenantContext, appBranch) =>
        {
            appBranch.SetDefaultSignInAsAuthenticationType(CookieAuthenticationDefaults.AuthenticationType);

            appBranch.UseCookieAuthentication(new CookieAuthenticationOptions
            { 
                CookieName = $"OAuthCookie.{tenantContext.FriendlyName}"
            });

            appBranch.UseOpenIdConnectAuthentication(new OpenIdConnectAuthenticationOptions
            {
                ClientId = tenantContext.AuthClientId,
                Authority = tenantContext.AuthAuthority,
                RedirectUri = $"http://localhost:2295/{tenantContext.FriendlyName}/"
            });
        });
    }
}

ASP.NET Implementation


TenantPipelineMiddleware.cs
   
/// <summary>
/// Workaround for the known OpenID multitenancy issue https://github.com/aspnet/Security/issues/1179
/// based on http://benfoster.io/blog/aspnet-core-multi-tenant-middleware-pipelines and https://weblogs.asp.net/imranbaloch/conditional-middleware-in-aspnet-core designs 
/// </summary>
public class TenantPipelineMiddleware : OwinMiddleware
{
    readonly IAppBuilder rootApp;
    readonly Action<TenantContext, IAppBuilder> newBranchAppConfig;
    readonly ConcurrentDictionary<TenantContext, Lazy<Func<IDictionary<string, object>, Task>>> branches;

    public TenantPipelineMiddleware(OwinMiddleware next, IAppBuilder rootApp, Action<TenantContext, IAppBuilder> newBranchAppConfig)
        : base(next)
    {
        this.rootApp = rootApp;
        this.newBranchAppConfig = newBranchAppConfig;
        this.branches = new ConcurrentDictionary<TenantContext, Lazy<Func<IDictionary<string, object>, Task>>>();
    }

    public override async Task Invoke(IOwinContext context)
    {
        TenantContext tenantContext = context.GetTenantContext();

        if (tenantContext == null || tenantContext.IsEmpty())
        {
            await this.Next.Invoke(context);
            return;
        }

        Lazy<Func<IDictionary<string, object>, Task>> branch = 
            branches.GetOrAdd(tenantContext, new Lazy<Func<IDictionary<string, object>, Task>>(() =>
            {
                IAppBuilder newAppBuilderBranch = rootApp.New();
                newBranchAppConfig(tenantContext, newAppBuilderBranch);
                newAppBuilderBranch.Run((oc) => this.Next.Invoke(oc));
                return newAppBuilderBranch.Build();
            }));

        await branch.Value(context.Environment);
    }
}

OwinContextExtensions.cs
   
public static class OwinContextExtensions
{
    static string tenantContextKey = "tenantcontext";

    public static TenantContext GetTenantContext(this IOwinContext context)
    {
        if (context.Environment.ContainsKey(tenantContextKey))
        {
            return (TenantContext)context.Environment[tenantContextKey];
        }
        return null;
    }
}

AppBuilderExtensions.cs
   
public static class AppBuilderExtensions
{
    public static IAppBuilder UsePerTenant(this IAppBuilder app, Action<TenantContext, IAppBuilder> newBranchAppConfig)
    {
        return app.Use<TenantPipelineMiddleware>(app, newBranchAppConfig);
    }
}

*Note: Code in this article is not production ready and is used for prototyping purposes only. If you have suggestions or feedback please do comment.

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