The Tool Desk
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What the Single Responsibility Principle means
Robert C. Martin’s 2014 explanation puts SRP in terms of actors: “A module should be responsible to one, and only one, actor.” An actor is a stakeholder, or group of stakeholders, whose needs can cause a module to change. His companion formulation is: “Gather together the things that change for the same reasons. Separate those things that change for different reasons.” (Robert C. Martin, “The Single Responsibility Principle”.)
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The familiar phrase “one reason to change” is about coherent ownership of change, not a literal limit of one method or operation per class. A class can have several methods and still serve one coherent purpose. Conversely, a class with only a few methods may combine concerns owned by different stakeholders.
How to tell whether a class has too many responsibilities
In a review, ask: Which stakeholder or policy change would make us edit this class? Consider a class that handles HTTP request mapping, enforces a business rule, and formats an external report. If those areas change for unrelated reasons or at the request of different stakeholders, the class may be combining responsibilities.
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Use these questions to decide whether a boundary would help:
- Change driver: Do these behaviors change for the same actor or policy, or for different ones?
- Cohesion: Do the operations form one understandable responsibility in the domain?
- Independent complexity: Has one operation become complex enough to understand or evolve on its own?
- Refactor cost: Would extraction make changes and testing clearer, or merely add indirection?
These are practical prompts derived from SRP, not a Laravel checklist. Extract a boundary when it improves comprehension or lets concerns change independently. Do not extract code solely to reduce a method count.
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Applying SRP to Laravel controllers
Laravel describes controllers as a way to organize request handling. A controller may group methods for related requests, and resource controllers provide conventional actions for creating, reading, updating, and deleting a resource. These are framework-supported patterns, not automatic SRP violations. Judge whether the actions concern the same resource and change for related reasons. Laravel also notes that a particularly complex action may merit an entire single-action controller (Laravel 12.x controller documentation).
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A measured refactor: a crowded OrderController
Imagine an OrderController that validates an HTTP request, applies pricing rules, saves an order, generates an invoice PDF, and sends a customer email. This is an illustrative design example, not a report of tested code. The useful question is not how many jobs appear in the list, but whether they have different change owners: request requirements may change with the API or form, pricing with business policy, invoice layout with presentation needs, and email behavior with notification policy.
- Keep request coordination in the controller. Let it receive and validate the request, coordinate the use case, and return an appropriate response. Avoid putting unrelated document-formatting or notification details there just because the request begins there.
- Move pricing only when it is a meaningful boundary. If pricing rules form a business policy that changes independently, give them a focused policy or service. If extraction would only wrap a trivial operation and obscure the flow, it may not help.
- Give invoice output its own owner when appropriate. A dedicated invoice component can own PDF generation and presentation details if those concerns evolve separately from order handling.
- Separate notification policy when it merits independence. A notification component can own decisions and behavior about customer email without making the controller responsible for the email’s full implementation.
These are design choices, not required Laravel classes or proof that every controller should be thin. The right split is the one that makes the flow and the likely changes easier to understand without adding needless indirection.
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SRP and PHP file standards are different ideas
PHP-FIG’s PSR-1 is a coding standard, not a definition of SRP. It says files “SHOULD either declare symbols (classes, functions, constants, etc.) or cause side-effects (e.g. generate output, change .ini settings, etc.) but SHOULD NOT do both.” It also provides class and method naming conventions. Those guidelines can support clear PHP file organization; they do not require one responsibility per class (PHP-FIG PSR-1: Basic Coding Standard).
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Start with the likely change, not a target number of classes. Identify who or what policy would drive the change, then see whether the affected behavior has a clear, cohesive home. Keep related Laravel resource actions together when they share a purpose; consider a dedicated action or collaborator when complexity or independent change makes the boundary useful. SRP is a tool for reasoning about change ownership, not a mandate to fragment code.
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