On php-fig and Shared Interfaces

This is a post I've been meaning to write for a long time, and one requested of me personally by Evert Pot during the Dutch PHP Conference in June 2012. It details some observations I have of php-fig, and hopefully will serve as a record of why I'm not directly participating any longer.

I was a founding member of the Framework Interoperability Group, now called "php-fig". I was one of around a dozen folks who sat around a table in 2009 in Chicago during php|tek and started discussions about what we could all do to make it possible to work better together between our projects, and make it simpler for users to pick and choose from our projects in order to build the solutions to their own problems.

The first "standard" that came from this was PSR-0, which promoted a standard class naming convention that uses a 1:1 relationship between the namespace and/or vendor prefix and the directory hierarchy, and the class name and the filename in which it lives. To this day, there are both those who hail this as a great step forward for cooperation, and simultaneously others who feel it's a terrible practice.

And then nothing, for years. But a little over a year ago, there was a new push by a number of folks wanting to do more. Paul Jones did a remarkable job of spearheading the next two standards, which centered around coding style. Again, just like with PSR-0, we had both those feeling it was a huge step forward, and those who loathe the direction.

What was interesting, though, was that once we started seeing some new energy and momentum, it seemed that everyone wanted a say. And we started getting dozens of folks a week asking to be voting members, and new proposal after new proposal. Whether or not somebody likes an existing standard, they want to have backing for a standard they propose.

And this is when we started seeing proposals surface for shared interfaces, first around caching, and now around logging (though the latter is the first up for vote).

Shared Interfaces

The idea around shared interfaces is simple: if we can come to a consensus on the basic interface for a common application task, libraries and frameworks can typehint on that shared interface, allowing developers to drop in the implementation of their choosing -- or even a standard, reference implementation. The goal is to prevent Not Invented Here (NIH) syndrome, as well as to make it simpler to re-use components between one library and another. As an example, if you're using Framework A, and it has a caching library, and you're consuming ORM B, you'd be able to pass the same cache object to the ORM as you use in the framework.

Great goals, really.

But I'm not sure I buy into them.

Problems

First, I agree that NIH is a problem.

Second, I also think there's space for multiple implementations of any given component. Often there are different approaches that different authors will take: one might focus on performance, another on having multiple adapters for providing different capabilities, etc. Sometimes having a different background will present different problem areas you want to resolve. As such, having multiple implementations can be a very good thing; developers can look at what each provides, and determine which solves the particular issues presented in the current project.

Because of this latter point, I have my reservations about shared interfaces.

What if a particular approach requires deviating from the shared interface in order to accomplish its goals? Additionally, in order to keep the greatest amount of compatibility between projects, shared interfaces tend to be so generic that specific implementations require developers to do a ton of manual type checking and munging of parameters, leading to more overhead, more difficulty testing and maintaining, and more difficulty documenting and understanding.

As an example, consider the following (made up) signature for a log method:


public function log($message, array $context = null);

What if your library supports an idea of priorities? Where would that information go in the above signature -- and would that differ between libraries -- would one library use the key for a completely different purpose? What about logging objects -- the signature doesn't say you can't, but how would I know if a specific implementation supports it, and won't blow up if I do pass one? Why must the $context be an array -- why won't any Traversable or ArrayAccess object work?

Basically, by being overly generic, the signature becomes a liability for those implementing the interface; it prevents meaningful interoperability and leads to splintering implementations.

(Please note: the above is completely fictional and has no bearing on current proposed or accepted standards. It is a thought exercise only.)

Furthermore, if a given project writes their own implementation of a component, and it has specialized features, why would they want to typehint on a generic, shared interface that doesn't implement those features? This would be counter-intuitive, as the project would then need to either check on additional interfaces for the specialized capabilities, duck-type, etc. -- all of which make for more maintenance and code.

In summary, my primary problem with the idea of shared interfaces is that I feel there is always room for new thinking and ideas in any given problem space, and that this thinking should not be restricted by what already exists. Secondarily, I feel that it's okay for a given project to be selective about what capabilities it requires for its internal consumption and consistency, and should not limit itself to a standardized interface.

But, but, SHARING

Remember, the first point I made was that I think NIH is a problem. How do I reconcile that with a firm stance against shared interfaces?

Easy: bridges and/or adapters.

Let's go back to that example of Framework A, its caching library, and working with ORM B.

Let's assume that ORM B defines an interface for caching, and let's say it looks like this:


interface CacheInterface
{
    public function set($key, $data);
    public function has($key);
    public function get($key);
}

Furthermore, we'll assume that the expected parameter values and return types are documented.

What we as a consumer of both Framework A and ORM B can do is build an implementation of CacheInterface that accepts a cache instance from Framework A, and proxies the various interface methods to that instance.


class FrameworkACache implements CacheInterface
{
    protected $cache;

    public function __construct(Cache $cache)
    {
        $this->cache = $cache;
    }

    public function set($key, $data)
    {
        $item = new CacheItem($key, $data);
        $this->cache->setItem($item);
    }

    public function has($key)
    {
        return $this->cache->exists($key);
    }

    public function get($key)
    {
        $item = $this->cache->getItem($key);
        return $item->getData();
    }
}

Assuming your code is well-decoupled, and you're using some sort of Inversion of Control container, you can likely create a factory for your ORM that will grab the above class, with the cache injected, and inject it into the ORM instance. Yes, it's a bit more work, but it's difficult to question the end result: shared caching between the framework and the ORM - and no need for shared interfaces, nor any need to sacrifice features within the framework or the ORM.

Sharing is good, developing solutions is better

I think the core idea of the php-fig group is sound: let's all start thinking about how we can make it easier to operate with each other. That said, my thoughts on how to accomplish that goal have changed significantly, and boil down to:

  • Use naming conventions that will reduce collisions (i.e., use per-project vendor prefixes/namespaces)
  • Use semantic versioning
  • Keep your installation packages segregated
  • Have a simple, discoverable way to autoload
  • Provide interfaces for anything that could benefit from alternate implementations
  • Don't write code that has side-effects in the global namespace (including altering PHP settings or superglobals)

Following these principals, you can play nice with each other, while still fostering innovative and differentiating solutions to shared problems.

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