using a Singleton to pass credentials in a multi-tenant application a code smell?
- by Hans Gruber
Currently working on a multi-tenant application that employs Shared DB/Shared Schema approach. IOW, we enforce tenant data segregation by defining a TenantID column on all tables. By convention, all SQL reads/writes must include a Where TenantID = '?' clause. Not an ideal solution, but hindsight is 20/20.
Anyway, since virtually every page/workflow in our app must display tenant specific data, I made the (poor) decision at the project's outset to employ a Singleton to encapsulate the current user credentials (i.e. TenantID and UserID). My thinking at the time was that I didn't want to add a TenantID parameter to each and every method signature in my Data layer.
Here's what the basic pseudo-code looks like:
public class UserIdentity
{
public UserIdentity(int tenantID, int userID)
{
TenantID = tenantID;
UserID = userID;
}
public int TenantID { get; private set; }
public int UserID { get; private set; }
}
public class AuthenticationModule : IHttpModule
{
public void Init(HttpApplication context)
{
context.AuthenticateRequest +=
new EventHandler(context_AuthenticateRequest);
}
private void context_AuthenticateRequest(object sender, EventArgs e)
{
var userIdentity = _authenticationService.AuthenticateUser(sender);
if (userIdentity == null)
{
//authentication failed, so redirect to login page, etc
}
else
{
//put the userIdentity into the HttpContext object so that
//its only valid for the lifetime of a single request
HttpContext.Current.Items["UserIdentity"] = userIdentity;
}
}
}
public static class CurrentUser
{
public static UserIdentity Instance
{
get { return HttpContext.Current.Items["UserIdentity"]; }
}
}
public class WidgetRepository: IWidgetRepository{
public IEnumerable<Widget> ListWidgets(){
var tenantId = CurrentUser.Instance.TenantID;
//call sproc with tenantId parameter
}
}
As you can see, there are several code smells here. This is a singleton, so it's already not unit test friendly. On top of that you have a very tight-coupling between CurrentUser and the HttpContext object. By extension, this also means that I have a reference to System.Web in my Data layer (shudder).
I want to pay down some technical debt this sprint by getting rid of this singleton for the reasons mentioned above. I have a few thoughts on what an better implementation might be, but if anyone has any guidance or lessons learned they could share, I would be much obliged.
