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Front-Interop Standard Interface Package

Front-Interop provides an interoperable package of standard interfaces for front controller functionality in any execution context (HTTP, CLI, etc.). It reflects, refines, and reconciles the common practices identified within several pre-existing projects.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 (RFC 2119, RFC 8174).

This package attempts to adhere to the Package Development Standards approach to naming and versioning.

Interfaces

This package defines the following interfaces:

• FrontController affords an entry point into the outermost presentation layer in any execution context (HTTP, CLI, etc.).

• FrontTypeAliases provides custom PHPStan types to aid static analysis.

FrontController

FrontController affords an entry point into the outermost presentation layer in any execution context (HTTP, CLI, etc.).

• Directives:

  • Implementations MUST gracefully handle all Throwables.

• Notes:

  • Handle all possible exceptions. The logic calling the front controller should not have to deal with any exceptions bubbling up from it. The implementation may accomplish this by catching Throwable directly, by registering a set_exception_handler() callback, or by some other means.

FrontController Methods

• • Runs the front controller.

  • Directives:

    • Implementations MUST report success by returning an integer 0.

    • Implementations MUST report non-success by returning an integer between 1 and 254 (inclusive).

  • Notes:

    • The return value is intended as an exit status code. Exit status codes may be received initially by the in-process logic that invoked run() (bootstrap scripts, test harnesses, etc.), and may ultimately be received by a parent process (shell, supervisor, init system, CI runner, monitoring tool, or similar) via exit(). Whether or not the exit status is consumed by the calling code or parent process depends on the execution environment: php-fpm and mod_php typically have no consumer, whereas worker loops, supervised long-running processes, runtime layers, and CI harnesses do.

    • "Success" and "non-success" are context-dependent. In an HTTP context, "success" typically means that the request was processed and a response was emitted regardless of the HTTP status code, whereas "non-success" may indicate that a Throwable had to be handled by the FrontController itself. In a command line context, "success" typically means that the command completed without errors, whereas "non-success" may be one of several error conditions (cf. the sysexits.h conventions where applicable).

    • The exit status code 255 is reserved by PHP itself. Cf. exit(): "Exit codes should be in the range 0 to 254, the exit code 255 is reserved by PHP and should not be used."

FrontTypeAliases

FrontTypeAliases provides custom PHPStan types to aid static analysis.

• • An int exit status code: 0 for success, 1 to 254 for non-success. The value 255 is reserved by PHP itself.

Implementations

Implementations MAY define additional class members not defined in these interfaces.

Notes:

• Reference implementations are available at https://github.com/front-interop/impl.

Q & A

Why run()?

The researched projects use one of six verbs for the front controller's main method: dispatch(), execute(), handle(), __invoke(), run(), or start(). Of these, run() is the clear majority at 12 of 23 projects; the remaining five verbs together account for the other 11. Front-Interop follows the majority practice.

Why does run() return int?

Of the 23 researched projects:

• 11 return void or null and handle response-sending themselves;
• 7 return a response object for the calling code to send;
• 3 return some other value ($this or mixed);
• 1 (Tempest) never returns; and
• 1 (Symfony) returns an int exit code.

The majority of researched projects in an HTTP execution context have the front controller handle response-sending itself and return nothing.

However, Front-Interop observes that a front controller in a different execution context may need to return an integer exit status code to its caller or parent process. These contexts include, among others:

• command line invocations
• continuous integration runners
• long-running processes
• queue workers
• test harnesses
• worker loops

While providing two interfaces (one to return void and another to return int) would cover both cases, it leads to inconsistencies in setup and expectations.

Thus, contra the most common void or null return, Front-Interop directs that run() returns an integer exit status code. This is an unusual practice for front controllers in an HTTP execution context, but imposes only a trivial implementation burden. Doing so allows the same interface to be used across many different execution contexts, and keeps the interface machine-friendly.

Why handle all Throwables?

Of the 23 researched projects, 21 handle exceptions in some way. The specific handling location varies between projects: 4 in the bootstrap, 2 in the front controller itself, and the remainder somewhere deeper in the call stack. For that remainder, either the bootstrap or the front controller defines or registers the handling logic.

Of the 21 projects that handle exceptions, 19 handle all types of Throwable, 1 handles all types of Exception, and 1 handles only specific exception subtypes.

Front-Interop observes that a front controller invocation occurs at the outermost boundary of the presentation layer. This is the last point at which any uncaught Throwables may be handled gracefully. The choice then is whether they are handled by the bootstrap script, or by the front controller proper.

In the interest of keeping such handling within a class, Front-Interop directs that FrontController itself must act as (or delegate to) a final backstop against Throwables. There may be other handling subsystems in the logic called by the FrontController, but any Throwable that escapes them will be handled by the FrontController or its delegate.