Download the PHP package bbc/ipr-resolver without Composer

BBC\iPlayerRadio\Resolver

A simple requirement resolution system for plain-old-php objects.

• Installation
• Background
• Usage
• How this actually works
• Credits

Installation

This is a standard composer library, so usual install rules apply:

This library requires PHP 5.5 or higher as it makes use of generators!

Background

Objects often have dependencies on each other. It's a pain. To use a uniquely radio example; we sometimes put "Brand" objects on our pages (think "The Archers"). However, we have decided that the most useful thing to show to the audience is the latest episode from that brand. That doesn't come as part of the brand metadata, so we have to do a separate call for it. We end up with something like this:

This gets worse if you have a list of brands and need to loop through each of them, hydrating them with their latest episode since the work is now probably happening in series and definitely cluttering up your code:

And what about if the latest episode ALSO has to make a data call to make itself whole! This is a nightmare. And how do I even inject dependencies down that chain?!

Wouldn't it be nice if we could just make a brand object, and say; "Sort yourself out bucko!" and it does. Yeah, we thought so too.

Enter stage left; the Resolver.

Usage

To flag that an object has dependencies, you need to implement the BBC\iPlayerRadio\Resolver\HasRequirements interface which has a single method: requires().

If you've never come across coroutines before, this probably looks a bit weird! But you can think of it like a lazy promise, the yield throws up to the Resolver saying "I need this to continue, handle it!".

This is how you then grab a fully fleshed out item:

Hang on, what's that new EpisodesBackend? This is how the Resolver knows how to solve requirements. Resolver backends take requirements and generate the actual result of the requirement.

So the EpisodesBackend would look something like this:

Whilst the syntax of using the resolver is slightly nicer, how does this help performance? We're still looping through and running each query synchronously.

Well, consider that the fetchLatestEpisodes was making a cURL request. We could now batch all those up do it as a single multi-curl request:

Using the Resolver allows you to ignore the actual details of how your objects fetch their data, and instead simply define what they need and let the Resolver and ResolverBackends do the work!

Hint hint imagine coupling this with the WebserviceKit library with a ResolverBackend that looks for QueryInterface instances and then multiFetch()'s them... ;)

You can also pass flags into the Resolver to help requires() functions work out what they need to do:

Note: all requires() functions see all flags, so keep them specific to avoid problems!

If a requirement is not supported by any backend (none of the canResolve() functions return true), then a BBC\iPlayerRadio\Resolver\UnresolvableRequirementException will be thrown, contained within it the requirement that failed, which you can access with getFailedRequirement().

Resolver Backends

Resolver Backends have two functions; firstly to state whether or not they understand a requirement (via the canResolve function) and then to take a list of requirements that it can resolve and to resolve them (via doResolve)!

There are some rules for writing your own resolver backends:

• Be as specific and safe in canResolve as you can. Be sure that you can actually resolve it
• Keep backends generic. Have a "CURLResolverBackend" rather than individual "EpisodeResolver", "BrandResolver" etc. Resolver backends should be "resolution strategy" based rather than data model based. The more generic the better.
• The requirements passed in doRequire can be in any order and from multiple objects. Treat them that way!
• Always return the results in the same order as the requirements! Weird stuff will happen if you don't!

How this actually works

You might be wondering how the whole yield thing works. It's a feature in PHP called Generators. Usually when people think of Generators they think of the "cheap iterator" side of things, but there's another side to Generators in a feature called "Coroutines".

This basically involves exploiting the fact that PHP suspends execution of a function when it reaches a yield, only returning control when the loop advances. By calling a function that yield's outside of a loop, you get an object which you can manually advance, allowing you to effectively hold a function in suspended animation until you have a real value for it.

Mind melting no? Here's some really good links that helped me get my head around them:

• The original PHP RFC that details generators and coroutines
• Javascript, but explains the theory well
• JS again, but also good

Credits

This library is based off of an extremely similar technique showcased by Bastian Hofmann in his talk at the PHPUK 2015 conference. Thanks Bastian!