Serve static maintenance page with 503 when PocketBase is unreachable

Elternbeirat.Web/Program.cs

@@ -2,6 +2,8 @@ using Elternbeirat.Contracts;
 using Elternbeirat.PocketBase;
 using Elternbeirat.Web.Components;
 using Elternbeirat.Web.Features.Events;
+using Elternbeirat.Web.Shared;
+using Microsoft.AspNetCore.Components.Endpoints;
 using Microsoft.AspNetCore.HttpOverrides;
 
 var builder = WebApplication.CreateBuilder(args);
@@ -25,6 +27,10 @@ builder.Services.AddHttpClient<PocketBaseClient>(client =>
     client.Timeout = TimeSpan.FromSeconds(5);
 });
 
+// Shared by all requests, so the maintenance gate below asks PocketBase at most once
+// every few seconds rather than on every page view.
+builder.Services.AddSingleton<PocketBaseHealthCache>();
+
 var app = builder.Build();
 
 // NPM sits in front of the app and handles HTTPS. Without this line the app would
@@ -48,6 +54,37 @@ if (app.Environment.IsDevelopment() is false)
 // is down (a 503) it would wrongly show "not found" instead of our own message.
 app.UseAntiforgery();
 
+// Maintenance gate: while PocketBase is down, every page gets the static
+// maintenance.html with 503 instead of a half-empty page (the menu comes from
+// PocketBase too). Routing has already picked the endpoint at this point, so the gate
+// covers exactly the Razor component pages: /health, the calendar feeds, the static
+// assets (incl. maintenance.html and its font) are not components and answer for
+// themselves. A new endpoint is thus left alone unless it is a page. The health
+// result is cached (PocketBaseHealthCache), so the page may come back a few seconds
+// after PocketBase does. Retry-After tells well-behaved clients and crawlers when to
+// come back; no-store keeps the 503 page out of any cache.
+app.Use(async (context, next) =>
+{
+    var isPage = context.GetEndpoint()?.Metadata.GetMetadata<ComponentTypeMetadata>() is not null;
+    var services = context.RequestServices;
+    if (!isPage
+        || await services.GetRequiredService<PocketBaseHealthCache>().IsHealthyAsync(
+            services.GetRequiredService<PocketBaseClient>().IsHealthyAsync,
+            context.RequestAborted))
+    {
+        await next(context);
+        return;
+    }
+
+    context.Response.StatusCode = StatusCodes.Status503ServiceUnavailable;
+    context.Response.Headers.RetryAfter = "60";
+    context.Response.Headers.CacheControl = "no-store";
+    context.Response.ContentType = "text/html; charset=utf-8";
+    await context.Response.SendFileAsync(
+        app.Environment.WebRootFileProvider.GetFileInfo("maintenance.html"),
+        context.RequestAborted);
+});
+
 app.MapStaticAssets();
 app.MapRazorComponents<App>();