Custom Router for Maintenance - asp.net-core

Question

I would like my ASP.NET Core MVC site to route all requests to a particular controller/action method whenever a particular file exists. The idea is that I can display a "site down for maintenance" page on an as-needed basis, just by creating a particular empty file.
After playing around with writing some custom middleware, it dawned on me that I should be able to create a custom IRouter which does this. Every route request should map to the "maintenance" route when that particular file exists. But I can't get it to work.
Here's the definition of the custom router:
public class OfflineRouteHandler : IRouter
{
private readonly string _path;
private readonly IPAddress _remote;
public OfflineRouteHandler( string offlinePath, string remoteIP )
{
_path = offlinePath;
_remote = IPAddress.Parse( remoteIP );
}
public VirtualPathData GetVirtualPath( VirtualPathContext context )
{
// this returns a value...but RouteAsync is never called
StringBuilder path = new StringBuilder();
if( context.AmbientValues.ContainsKey( "controller" ) )
path.Append( context.AmbientValues["controller"] );
if( context.AmbientValues.ContainsKey("action") )
{
if( path.Length > 0 ) path.Append( "/" );
path.Append( context.AmbientValues["action"] );
}
return new VirtualPathData( this, "/" + path.ToString() );
}
public async Task RouteAsync( RouteContext context )
{
bool authorized = ( context.HttpContext.Connection.RemoteIpAddress == _remote );
if( File.Exists(_path) && authorized )
{
// just for testing... but the method never gets called
var i = 9;
i++;
}
context.IsHandled = true;
}
}
I invoke it in Configure() in Startup.cs as follows:
string offlinePath = Path.Combine( Directory.GetParent( env.WebRootPath ).FullName, "offline.txt" );
app.UseMvc(routes =>
{
routes.Routes.Add( new TemplateRoute( new OfflineRouteHandler( offlinePath, "50.3.3.2" ), "home/offline", new DefaultInlineConstraintResolver(routeOptions)) );
routes.MapRoute(
name: "default",
template: "{controller=Home}/{action=Index}/{id?}");
});
routeOptions gets passed into the call to Configure();
Any help would be greatly appreciated!

Answer 1

This is untested, but I went for the middleware approach.
It uses a similar approach to the UseStatusCodePagesWithReExecute function, in that it modifies the context.Request.Path with the path. I've loaded the settings for the OfflineMiddleware using the Options pattern.
Essentially, when the file exists, the middleware modifies the path to the path you want "home/offline", and continues execution up the mvc pipeline.
As we've added UseStaticFiles() before the middleware, any static files on your offline page will be served - only requests that make it through to the MVC pipeline will be intercepted, and all of these requests will hit home/offline.
public class Startup
{
//Partial Starup class
public void ConfigureServices(IServiceCollection services)
{
services.Configure<OfflineMiddlewareSettings>(settings =>
{
settings.OfflinePath = "thePath";
settings.RemoteIP = "50.3.3.2";
});
// Add framework services.
services.AddMvc();
}
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, IOptionsMonitor<MyValues> monitor)
{
loggerFactory.AddConsole(Configuration.GetSection("Logging"));
loggerFactory.AddDebug();
app. UseStaticFiles();
app.UseMiddleware<OfflineMiddleware>();
app.UseMvc();
}
}
public class OfflineMiddleware
{
private readonly RequestDelegate _next;
private readonly string _path;
private readonly IPAddress _remote;
public OfflineMiddleware(RequestDelegate next, IOptions<OfflineMiddlewareSettings> settings)
{
_next = next;
_path = settings.Value.OfflinePath;
_remote = IPAddress.Parse(settings.Value.OfflinePath);
}
public async Task Invoke(HttpContext context)
{
bool authorized = (context.Connection.RemoteIpAddress == _remote);
if (File.Exists(_path) && authorized)
{
context.Request.Path = "home/offline";
}
await _next.Invoke(context);
}
}
public class OfflineMiddlewareSettings
{
public string OfflinePath { get; set; }
public string RemoteIP { get; set; }
}