Building Domain-Specific “Russian-Doll” Pipelines in .NET 9 – a Functional Approach with ProblemOr

Move the elegance of ASP.NET Core middleware into any complex back-end workflow – without HttpContext, without OOP builders, and without losing DI, cancellation or early-exit behaviour.

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Why a pipeline?

Business workflows such as Get GPS Logs often involve many steps:

1. Classify the request (Site vs Council, Commercial vs Domestic).
2. Fetch way-points from a repository.
3. Convert timestamps to local time.
4. Constrain results to a bounding-box.
5. Materialise the response.

Each step should be independent, replaceable, testable and able to short-circuit on error or empty data – the classic “Russian-doll” pattern that ASP.NET Core middleware delivers for web traffic.

Key design choices

Decision	Rationale
Immutable context record (GpsLogContext)	No hidden state; easy to with-clone for functional purity.
Discriminated-union result (ProblemOr<T>)	One return type covers success or one-or-many errors; consumers call Match.
Pure functions + static “compose” helper	No builder classes or interfaces; the pipeline itself is a first-class function.
DI scope passed in the context	Middleware still resolve scoped services, but the composer stays DI-agnostic.

The core primitives

using ProblemOr;

public sealed record GpsLogContext(
    RequestModel      Request,
    ResponseModel?    Response,
    IServiceScope     Scope,
    CancellationToken Ct);

// Pipeline delegate (mirrors aspnetcore RequestDelegate)
public delegate ValueTask<ProblemOr<GpsLogContext>> GpsLogDelegate(GpsLogContext ctx);

A functional static builder

public static class DomainPipeline
{
    private static readonly GpsLogDelegate Terminal =
        ctx => ValueTask.FromResult<ProblemOr<GpsLogContext>>(ctx);

    public static GpsLogDelegate Compose(
        params Func<GpsLogDelegate, GpsLogDelegate>[] parts)
    {
        var app = Terminal;
        for (var i = parts.Length - 1; i >= 0; i--)
            app = parts[i](app);   // Russian-doll wrap
        return app;
    }
}

Compose returns one GpsLogDelegate; there is no stateful builder instance to maintain or mock.

Middleware components as pure functions

// Request classifier
static Func<GpsLogDelegate, GpsLogDelegate> RequestClassifier =>
    next => async ctx =>
    {
        if (!ctx.Request.TryDetermineFlags())
            return Error.Validation(code: "BadType", description: "Unknown request");

        return await next(ctx);
    };

// Way-point fetcher (shows early error exit)
static Func<GpsLogDelegate, GpsLogDelegate> WaypointFetcher =>
    next => async ctx =>
    {
        var repo = ctx.Scope.ServiceProvider.GetRequiredService<IWaypointRepository>();
        var points = await repo.GetAsync(ctx.Request.SiteNumber, ctx.Ct);

        if (points.Count == 0)
            return Error.NotFound(description: "No way-points");

        var updated = ctx with { Request = ctx.Request with { Waypoints = points } };
        return await next(updated);
    };

// Time-zone resolver
static Func<GpsLogDelegate, GpsLogDelegate> TimezoneResolver =>
    next => async ctx =>
    {
        var tz = ctx.Scope.ServiceProvider.GetRequiredService<ITimezoneService>();
        var local = await tz.ToLocalAsync(ctx.Request.UtcTime, ctx.Ct);

        var updated = ctx with { Request = ctx.Request with { LocalTime = local } };
        return await next(updated);
    };

// Bounding-box filter
static Func<GpsLogDelegate, GpsLogDelegate> BoundingBoxFilter =>
    next => async ctx =>
    {
        var updated = ctx with { Request = ctx.Request.FilterToBoundingBox() };
        return await next(updated);
    };

// Response builder – terminal step, always succeeds
static Func<GpsLogDelegate, GpsLogDelegate> ResponseBuilder =>
    _ => ctx =>
    {
        var response = ResponseModel.Create(ctx.Request);
        return ValueTask.FromResult<ProblemOr<GpsLogContext>>(ctx with { Response = response });
    };

Add or remove steps at will; order is controlled exclusively by the Compose call.

Wiring the pipeline in Program.cs (minimal-API style)

using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.DependencyInjection;
using ProblemOr;

using static DomainPipeline;

var builder = WebApplication.CreateBuilder(args);

builder.Services
       .AddScoped<IWaypointRepository, WaypointRepository>()
       .AddScoped<ITimezoneService, TimezoneService>();

// Build the single function at start-up
GpsLogDelegate gpsPipeline = Compose(
    RequestClassifier,
    WaypointFetcher,
    TimezoneResolver,
    BoundingBoxFilter,
    ResponseBuilder);

var app = builder.Build();

app.MapPost("/gpslogs", async (RequestModel req, IServiceProvider sp, CancellationToken ct) =>
{
    await using var scope = sp.CreateAsyncScope();
    var seed = new GpsLogContext(req, null, scope, ct);

    var result = await gpsPipeline(seed);

    return result.Match(
        ok     => Results.Ok(ok.Response),
        errors => Results.Problem(title: "GPS log error",
                                  statusCode: 400,
                                  detail: string.Join(" | ", errors.Select(e => e.Description))));
});

await app.RunAsync();

• DI scope is created per request, keeping repository and service lifetimes correct.
• All middleware run in-memory; no HttpContext, no Kestrel overhead.
• Early failures propagate as a single 400 response with an aggregated error message.

Unit-testing a component (xUnit + NSubstitute)

[Fact]
public async Task WaypointFetcher_returns_NotFound_when_empty()
{
    // Arrange
    var repo = Substitute.For<IWaypointRepository>();
    repo.GetAsync("05", Arg.Any<CancellationToken>()).Returns([]);

    var services = new ServiceCollection().AddSingleton(repo).BuildServiceProvider();
    await using var scope = services.CreateAsyncScope();

    var ctx  = new GpsLogContext(new("05"), null, scope, default);

    // Act
    var result = await WaypointFetcher(_ => throw new Exception("Next should not run"))(ctx);

    // Assert
    Assert.True(result.IsError);
    Assert.Equal("No way-points", result.FirstError.Description);
}

Because each middleware is a pure function, testing entails no test-server fixtures.

Performance and scaling notes

• No reflection – the pipeline is a pre-composed delegate chain.
• Zero allocations per step – except when with creates a new record instance (which is unavoidable for immutability).
• Parallel runs – the delegate is thread-safe; every invocation receives its own GpsLogContext.

If you need to reuse singleton configuration inside a component, capture it when you declare the lambda:

var staticOptions = builder.Configuration.GetSection("Gps").Get<GpsOptions>();
Func<GpsLogDelegate, GpsLogDelegate> ConfigInjector =
    next => ctx => next(ctx with { Request = ctx.Request with { Options = staticOptions } });

Take-aways

• Functional composition is enough – no builder objects, no interfaces.
• ProblemOr gives strongly-typed early exits; no boolean flags or exceptions are required for control-flow.
• DI remains first-class because each middleware receives the current scope from the context.
• The pattern mirrors ASP.NET Core middleware closely, so new team-members recognise the mental model instantly.

Ready-to-paste code

All code above fits into three small files:

File	Contents
Pipeline.cs	Static Pipeline.Compose and the GpsLogDelegate alias.
GpsLogContext.cs	Immutable record + supporting models.
Middleware.cs	The five lambda components shown.

Drop them into any .NET 9 project and enjoy web-grade pipeline composability inside your domain logic — no custom frameworks, no accidental complexity.