Multi-tenant .Net Core Web API - asp.net-core

Question

I have a requirement to build a Web API for an existing system. There are various agencies throughout the country, and each agency has its own database. All databases are on one single server. All databases are identical in structure. All databases have their own username and password. An agency has one or more users. A user can belong to one or more agencies. There is also one special database which contains a table of all users, a table of all agencies, and user-agencies bridge table.
Currently they are using a traditional Windows desktop application. When a user sets up this Windows program, they log in with a username and password. The system then displays for them a list of all the agencies that they belong to (normally just one, but some "power users" can belong to a few). They pick an agency, and then the program connects to the correct database. For the remainder of the session, everything that the user does will be done on that database.
The client wants to create a web app to eventually replace the Windows program (and the two will be running side by side for a while). One developer is creating the front end in Angular 5, and I am developing the API in ASP .Net Core 2.1.
So the web app will function in a similar manner to the Windows app. A user logs in to the web app. The web app, which consumes my Web API, tells the API which user just logged in. The API then checks which agency(s) this user belongs to from that database that stores that data. The API returns the list of agencies the user belongs to to the web app. There, the user picks an agency. From this point on, the web app will include this Agency ID in the header of all API calls. The API, when it receives a request from the web app, will know which database to use, based on the Agency ID in the header of the request.
Hope that makes sense...
Obviously this means that I will have to change the connection string of the DbContext on the fly, depending on which database the API must talk to. I've been looking at this, firstly by doing it on the controller itself, which worked but would involve a lot of copy-and-paste anti-patterns in all my controllers. So I am trying to move this to the DbContext's OnConfiguring event. I was thinking it'd be best to create a DbContext Factory to create the DbContexts, using the appropriate connection string. I'm just a bit lost though. You see, when the web app calls an end point on the web api (let's say an HTTP GET request to get a list of accounts), this will fire the HttpGet handler in the Accounts controller. This action method then reads the Agency ID header. But this is all happening on the controller.... If I call the DbContext Factory from the DbContext's OnConfiguring() event, it would have to send the Agency ID (which was read in the controller) to the factory so that the factory knows which connection string to create. I'm trying not to use global variables to keep my classes loosely coupled.
Unless I have some service running in the pipeline that intercepts all requests, reads the Agency ID header, and this somehow gets injected into the DbContext constructor? No idea how I would go about doing this...
In summary, I'm a bit lost. I'm not even sure if this is the correct approach. I've looked at some "multi-tenant" examples, but to be honest, I've found them a bit hard to understand, and I was hoping I could do something a bit simpler for now, and with time, as my knowledge of .Net Core improves, I can look at improving the code correspondingly.

Answer 1

I am working on something similar you describe here. As I am also quite at the start, I have no silver bullet yet. There is one thing where could help you with your approach though:
firstly by doing it on the controller itself, which worked but would involve a lot of copy-and-paste anti-patterns in all my controllers.
I took the approach of having a middleware being in charge of swapping the dbconnection string. Something like this:
public class TenantIdentifier
{
private readonly RequestDelegate _next;
public TenantIdentifier(RequestDelegate next)
{
_next = next;
}
public async Task Invoke(HttpContext httpContext, GlobalDbContext dbContext)
{
var tenantGuid = httpContext.Request.Headers["X-Tenant-Guid"].FirstOrDefault();
if (!string.IsNullOrEmpty(tenantGuid))
{
var tenant = dbContext.Tenants.FirstOrDefault(t => t.Guid.ToString() == tenantGuid);
httpContext.Items["TENANT"] = tenant;
}
await _next.Invoke(httpContext);
}
}
public static class TenantIdentifierExtension
{
public static IApplicationBuilder UseTenantIdentifier(this IApplicationBuilder app)
{
app.UseMiddleware<TenantIdentifier>();
return app;
}
}
Here I am using a self-created http-header called X-Tenant-Guid to identify the tenants GUID. Then I make a request to the global Database, where I do get the connection string of this tenants db.
I made the example public here. https://github.com/riscie/ASP.NET-Core-Multi-Tenant-multi-db-Example (it's not yet updated to asp net core 2.1 but it should not be a problem to do so quickly)