Domain Logic Patterns

## What a script actually does **Transaction Script** is the simplest of the three classic domain-logic patterns (alongside **Table Module** and **Domain Model**). Mechanically, you write one procedure per business transaction — a use case like `placeOrder`, `calculateShipping`, or `transferFunds` — and that single method does everything: 1. it validates the incoming request; 2. pulls whatever rows it needs from the database (often via a thin Gateway or DAO that just runs SQL); 3. applies the business rules inline as ordinary conditionals and calculations; 4. and writes the results back to the database, usually inside one transaction boundary matching the method's name. There is no object graph representing the domain; data mostly moves through as primitives, DTOs, or raw database rows. Multiple scripts are commonly grouped into a Service class per subject area (e.g. `OrderService`, `PaymentService`) purely for organizational convenience — that grouping does not imply a shared behavioral model, since each method inside is still self-contained. ## Why the pattern exists The pattern exists because a large fraction of real business logic is genuinely simple: - a handful of validations, - a calculation or two, - a database read and write. Building a full object model with associations, identity, and polymorphism for that kind of logic adds real cost — mapping layers, more classes and interfaces, more indirection to read through — for no behavioral payoff. Transaction Script maps directly onto how most engineers already think procedurally, has almost no learning curve, and maps almost one-to-one onto database operations, which makes it fast to write and fast for a new team member to trace end to end. ## The trade-off — duplication versus simplicity The trade-off is duplication versus simplicity. **On the plus side:** - each script is easy to understand in isolation, - there is minimal object-relational mapping overhead since you're usually working close to raw rows or DTOs, - and performance is often good because there's little indirection between the request and the database call. **On the minus side**, similar business steps get repeated across scripts unless someone deliberately factors them into shared subroutines, and even with subroutines the discipline to keep reusing them (instead of re-implementing 'close enough' logic) tends to erode over time. Testing at the level of a whole script often forces either a real database or heavy mocking of the gateway, since business logic and persistence aren't cleanly separated. As the number of business rules grows and the rules start interacting — pricing that depends on customer tier, order type, and an active promotion, for instance — scripts accumulate deeply nested conditionals and become 'long methods' that are expensive and risky to change. ## The failure mode in production In production, the classic failure mode is **duplication drift**: a new business rule (say, a new discount type or a new tax jurisdiction) has to be added to every script that touches pricing, and inevitably one or two scripts get missed or implemented slightly differently, producing inconsistent behavior — a customer gets charged the old tax rate through the batch-invoice script while the checkout script applies the new one correctly. A related symptom is business logic leaking sideways into UI controllers or database stored procedures because there's no natural single home for a piece of shared computation, so the same rule ends up implemented in two or three places with subtly different edge-case handling, which shows up as production bugs that are hard to trace because 'the logic' isn't in one place. Over time, complexity concentrates in a handful of scripts that become the ones nobody wants to touch: - high defect rate, - slow code review, - and a de facto bus-factor risk since only one or two engineers fully understand them. ## Where it shows up in practice In practice, Transaction Script is how a great deal of real transaction-processing software is actually built: - simple CRUD SaaS features, - batch billing/invoicing jobs (a `processMonthlyInvoice` procedure), - request handlers in early-stage APIs, - and simpler ledger/funds-transfer processing. Many 'fat controller, thin model' Rails or Django codebases are effectively Transaction Script wrapped in an MVC skin even though they look object-oriented from the outside, because the controller action (or a thin service it calls) is doing all the orchestration procedurally rather than delegating to behavior-rich domain objects. Recognizing that is useful: the pattern isn't a mistake in those codebases, it's an appropriate match to genuinely simple logic — the mistake would be sticking with it once the rules interact enough that duplication and long-method risk start dominating, which is the signal to consider Domain Model instead.