Config-driven transaction pipeline (DI + fluent chains)

Goal Build a checkout pipeline where what runs and in what order comes from configuration, while the execution remains allocation-lean and testable.

This demo layers a small configuration model over the same primitives used in the mediated pipeline:

Action chains for branchless discounts → tax and rounding
Tender handling via a first-match router (see the mediated pipeline doc)
DI registration that composes a single immutable PatternKit.Examples.Chain.TransactionPipeline at startup

Quick start

Add configuration (order matters):

// appsettings.json
{
  "Payment": {
    "Pipeline": {
      "DiscountRules": [ "discount:cash-2pc", "discount:loyalty-5pc", "discount:bundle-1off" ],
      "Rounding": [ "round:charity", "round:nickel-cash-only" ],
      "TenderOrder": [ "tender:cash", "tender:card" ] // informational
    }
  }
}

Register the pipeline in DI:

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using PatternKit.Examples.Chain.ConfigDriven;

var services = new ServiceCollection();
services.AddPaymentPipeline(configuration);              // builds a TransactionPipeline from config
var provider = services.BuildServiceProvider();

var pipe = provider.GetRequiredService<ConfigDrivenPipelineDemo.PaymentPipeline>();
var (result, ctx) = pipe.Run(new TransactionContext {
    Customer = new Customer(LoyaltyId: "LOYAL-123", AgeYears: 25),
    Items = [ new LineItem("SKU-1", 22.97m) ],
    Tenders = [ new Tender(PaymentKind.Cash, CashGiven: 20m),
                new Tender(PaymentKind.Card, CardAuthType.Contactless, CardVendor.Visa) ]
});

Done — the runtime pipeline is immutable and safe to reuse concurrently.

What’s in the box

Configuration model

PatternKit.Examples.Chain.ConfigDriven.PipelineOptions drives ordering:

DiscountRules: keys of discount rules to apply in order
Rounding: keys of rounding strategies to apply in order
TenderOrder: optional, informational (e.g., control UI ordering)

Unknown keys are ignored (we map by key and skip missing entries).

Strategies provided

Discount rules (keys):

discount:cash-2pc → PatternKit.Examples.Chain.ConfigDriven.Cash2Pct First tender is cash → 2% off Subtotal
discount:loyalty-5pc → PatternKit.Examples.Chain.ConfigDriven.Loyalty5Pct Loyalty present → 5% off Subtotal
discount:bundle-1off → PatternKit.Examples.Chain.ConfigDriven.Bundle1OffEach Any BundleKey with total Qty ≥ 2 → $1 off per item in those bundles

Rounding (keys):

round:charity → PatternKit.Examples.Chain.ConfigDriven.CharityRoundUp If any CHARITY:* SKU is present → round up to next dollar
round:nickel-cash-only → PatternKit.Examples.Chain.ConfigDriven.NickelCashOnly Cash-only transactions → round to nearest $0.05 (logs “skipped (not cash-only)” otherwise)

Tender handlers (DI-registered):

PatternKit.Examples.Chain.ConfigDriven.CashTender (tender:cash)
PatternKit.Examples.Chain.ConfigDriven.CardTender (tender:card)

The router itself is assembled by the mediated pipeline pieces; we simply supply handlers via DI and the builder wires them into the tender stage.

How it composes

Discounts & tax (config-driven)

PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineBuilderExtensions.AddConfigDrivenDiscountsAndTax\*:

Recomputes Subtotal
Iterates opts.Value.DiscountRules and applies each rule that exists in the DI map
Computes tax at 8.75% of (Subtotal − DiscountTotal) and logs pre-round total

b.AddConfigDrivenDiscountsAndTax(opts, discountRules);

Rounding (config-driven)

PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineBuilderExtensions.AddConfigDrivenRounding\*:

Iterates opts.Value.Rounding and calls each strategy in order
Each strategy decides to apply or log “skipped”
Logs final total

b.AddConfigDrivenRounding(opts, rounding);

DI registration

PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineDemo.AddPaymentPipeline\*:

Binds Payment:Pipeline to PatternKit.Examples.Chain.ConfigDriven.PipelineOptions
Registers:
- Infra: PatternKit.Examples.Chain.IDeviceBus, PatternKit.Examples.Chain.CardProcessors
- Discounts: Cash2Pct, Loyalty5Pct, Bundle1OffEach
- Rounding: CharityRoundUp, NickelCashOnly
- Tenders: CashTender, CardTender
Builds a shared PatternKit.Examples.Chain.TransactionPipeline:

TransactionPipelineBuilder.New()
    .WithDeviceBus(devices)
    .AddPreauth()
    .AddConfigDrivenDiscountsAndTax(opts, discountRules)
    .AddConfigDrivenRounding(opts, rounding)
    .WithTenderHandlers(tenderHandlers)
    .AddTenderHandling()
    .AddFinalize()
    .Build();

Consumers receive a thin wrapper: PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineDemo.PaymentPipeline with Run(ctx).

Example scenarios (from tests)

Mixed tender (cash then card) — no nickel rounding

Config: Rounding = ["round:nickel-cash-only"]
Items: Subtotal 22.97 → Tax 2.01 → Pre-round 24.98
Tenders: $20 cash, then Visa pays the remainder

Outcome

Rounding skipped (not cash-only)
Card captures $4.98
Result: paid

See: TransactionPipelineDemoTests.MixedTender_NoNickelRounding.

Cash-only nickel rounding up to `$25.00`

Config: Rounding = ["round:nickel-cash-only"]
Pre-round total 24.98
Rounding adds +$0.02
Single cash tender $25.00 → paid

See: TransactionPipelineDemoTests.CashOnly_NickelRounding_Up.

Charity round-up

Config: Rounding = ["round:charity"]
Presence of CHARITY:RedCross SKU causes +$0.02 to next whole dollar
Paid by card

See: TransactionPipelineDemoTests.Charity_RoundUp_Works.

Preauth block (age)

Age-restricted item + underage customer → TxResult.Fail("age", ...)
Pipeline stops early

See: TransactionPipelineDemoTests.Preauth_AgeBlock.

Extending with your own rules/strategies/handlers

Implement the interface and choose a unique key:

public sealed class Employee10Pct : IDiscountRule
{
    public string Key => "discount:employee-10pc";
    public void Apply(TransactionContext ctx)
    {
        if (ctx.Customer.LoyaltyId == "EMP")
            ctx.AddDiscount(Math.Round(ctx.Subtotal * 0.10m, 2), "employee 10%");
    }
}

Register it:

services.AddSingleton<IDiscountRule, Employee10Pct>();

Enable it in config (order is important):

"Payment": { "Pipeline": { "DiscountRules": [
  "discount:employee-10pc", "discount:bundle-1off"
]}}

The same pattern holds for IRoundingStrategy and ITenderHandler.

FAQ & tips

What happens if a key is listed but not registered? It’s skipped; we only apply rules found in the DI map.
Where’s the tax rate? Inside AddConfigDrivenDiscountsAndTax we compute tax at 8.75%. Swap this with your own calculator if needed.
Thread safety? The composed PatternKit.Examples.Chain.TransactionPipeline is immutable and safe for concurrent use. Builders are not thread-safe.
Observability Every rule/strategy logs its work to ctx.Log using concise, human-readable entries suitable for unit tests and diagnostics.
Performance
- All composition happens once at startup.
- Execution uses arrays and static delegates where possible to minimize allocations.
- The config-driven action chains still short-circuit inside each component when appropriate.

Reference

Composition
- PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineBuilderExtensions
- PatternKit.Examples.Chain.TransactionPipelineBuilder
- PatternKit.Examples.Chain.TransactionPipeline
Config & DI
- PatternKit.Examples.Chain.ConfigDriven.PipelineOptions
- PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineDemo.AddPaymentPipeline\*
- PatternKit.Examples.Chain.ConfigDriven.ConfigDrivenPipelineDemo.PaymentPipeline
Strategies
- Discounts: PatternKit.Examples.Chain.ConfigDriven.Cash2Pct, PatternKit.Examples.Chain.ConfigDriven.Loyalty5Pct, PatternKit.Examples.Chain.ConfigDriven.Bundle1OffEach
- Rounding: PatternKit.Examples.Chain.ConfigDriven.CharityRoundUp, PatternKit.Examples.Chain.ConfigDriven.NickelCashOnly
- Tenders: PatternKit.Examples.Chain.ConfigDriven.CashTender, PatternKit.Examples.Chain.ConfigDriven.CardTender
Domain
- PatternKit.Examples.Chain.TransactionContext, PatternKit.Examples.Chain.TxResult, PatternKit.Examples.Chain.Tender, PatternKit.Examples.Chain.LineItem, PatternKit.Examples.Chain.Customer

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