Apps using [EventSource](/dotnet/api/system.diagnostics.tracing.eventsource) can measure the startup time to understand and optimize startup performance. The [`ServerReady`](https://source.dot.net/#Microsoft.AspNetCore.Hosting/Internal/HostingEventSource.cs,76) event in <xref:Microsoft.AspNetCore.Hosting?displayProperty=fullName> represents the point where the server is ready to respond to requests.
For more information on application startup, see <xref:fundamentals/index>.
* To configure middleware at the beginning or end of an app's middleware pipeline without an explicit call to `Use{Middleware}`. Use `IStartupFilter` to add defaults to the beginning of the pipeline without explicitly registering the default middleware. `IStartupFilter` allows a different component to call `Use{Middleware}` on behalf of the app author.
* To create a pipeline of `Configure` methods. [IStartupFilter.Configure](xref:Microsoft.AspNetCore.Hosting.IStartupFilter.Configure%2A) can set a middleware to run before or after middleware added by libraries.
`IStartupFilter` implements <xref:Microsoft.AspNetCore.Hosting.StartupBase.Configure%2A>, which receives and returns an `Action<IApplicationBuilder>`. An <xref:Microsoft.AspNetCore.Builder.IApplicationBuilder> defines a class to configure an app's request pipeline. For more information, see [Create a middleware pipeline with IApplicationBuilder](xref:fundamentals/middleware/index#create-a-middleware-pipeline-with-iapplicationbuilder).
Each `IStartupFilter` can add one or more middlewares in the request pipeline. The filters are invoked in the order they were added to the service container. Filters may add middleware before or after passing control to the next filter, thus they append to the beginning or end of the app pipeline.
The following example demonstrates how to register a middleware with `IStartupFilter`. The `RequestSetOptionsMiddleware` middleware sets an options value from a query string parameter:
When a query string parameter for `option` is provided, the middleware processes the value assignment before the ASP.NET Core middleware renders the response:
Middleware execution order is set by the order of `IStartupFilter` registrations:
* Multiple `IStartupFilter` implementations may interact with the same objects. If ordering is important, order their `IStartupFilter` service registrations to match the order that their middlewares should run.
* Libraries may add middleware with one or more `IStartupFilter` implementations that run before or after other app middleware registered with `IStartupFilter`. To invoke an `IStartupFilter` middleware before a middleware added by a library's `IStartupFilter`:
* Position the service registration before the library is added to the service container.
* To invoke afterward, position the service registration after the library is added.
Note: You can't extend the ASP.NET Core app when you override `Configure`. For more informaton, see [this GitHub issue](https://github.com/dotnet/aspnetcore/issues/45372).
An <xref:Microsoft.AspNetCore.Hosting.IHostingStartup> implementation allows adding enhancements to an app at startup from an external assembly outside of the app's `Program.cs` file. For more information, see <xref:fundamentals/configuration/platform-specific-configuration>.
For information on using the <xref:Microsoft.AspNetCore.Hosting.StartupBase.ConfigureServices%2A> and <xref:Microsoft.AspNetCore.Hosting.StartupBase.Configure%2A> methods with the minimal hosting model, see:
* [Use Startup with the minimal hosting model](xref:migration/50-to-60#smhm)
* The [ASP.NET Core 5.0 version of this article](?view=aspnetcore-5.0&preserve-view=true#the-startup-class):
