Tuesday, 29 April 2014

Upgrade your models from PHP to Java

I’ve recently had an opportunity to work with my team’s new developer, as part of the ongoing campaign to bring him over to core Java development from our soon-to-be end-of-lifed PHP tools. Since I was once in his position, only two years ago—in fact, he inherited the PHP tools from me when I was forcefully reassigned to the Java team—I feel a certain affinity toward him, and a desire to see him succeed.

Like me, he also has some limited Java experience, but nothing at the enterprise scale I now work with every day, and nothing especially recent. So, I gave him a copy of the same Java certification handbook I used to prepare for my OCA Java 7 exam, as well as any other resources I could track down that seemed to be potentially helpful. This sprint he’s physically, and semi-officially, joined our team, working on the replacement for the product he’s been maintaining since he was hired.

And just to make the learning curve a little bit steeper, this tool uses JavaServer Faces. If you’ve developed for JSF before, you’re familiar with how much of a thorough pain in the ass it can be. Apparently we’re trying to weed out the non-hackers, Gunnery Sergeant Hartman style.

So, as part of his team onboarding process, we picked up a task to begin migrating data from the old tool to the new. On investigating the requirements, and the destination data model, we discovered that one of the elements that this task expects has not yet been implemented. What a great opportunity! Not only is he essentially new to Java, he’s also new to test-driven development, so I gave him a quick walkthrough the test process while we tried to write a test for the new features we needed to implement.

As a quick sidebar, in trying to write the tests, we quickly discovered that we were planning on modifying (or at least starting with) the wrong layer. If we’d just started writing code, it probably would have taken half an hour or more to discover this. By trying to write the test first, we figured this out within ten minutes, because the integration points rapidly made no sense for what we were trying to do. Hurray!

Anyway, lunch time promptly rolled around while I was writing up the test. I’d suggested we play “TDD ping-pong”—I write a test, then he implements it, and the test was mostly set up. I said I’d finish up the test on the new service we needed, and stub out the data-access object and the backing entity so he’d at least have methods to work with. After I sent it his way, I checked in after about an hour, to see how he was doing, and he mentioned something that hadn’t occurred to me, because I had become so used to Java: he was completely unfamiliar with enterprise Java’s usual architecture of service, DAO and DTO.

And of course, why would he be? I’m not aware of any PHP frameworks that use this architecture, because it’s based on an availability of dependency injection, compiled code and persistent classes that is pretty much anathema to the entire request lifecycle of PHP. For every request, PHP loads, compiles, and executes each class anew. So pre-loading your business model with the CRUD utility methods, and operating on them as semi-proper Objects that can persist themselves to your stable storage, is practically a necessity. Fat model, skinny controller, indeed.

Java has a huge advantage here, because the service, DAO, and whole persistence layer, never leave memory between requests, just request-specific context. Classes don’t get reloaded until the servlet container gets restarted (unless you’re using JSF, in which case, they’re serialised onto disk and rehydrated when you restart, for… some reason). So you can write your code where your controller asks a service for records, and the service calls out to the DAO, which return an entity (or collection thereof), or a single field’s value for a given identifier.

This is actually a really good thing to do, from an engineering perspective.

For a long time, with Project Alchemy, I was trying to write a persistence architecture that would be as storage-agnostic as possible—objects could be persisted to database, to disk, or into shared memory, and the object itself didn’t need to care where it went. I only ever got as far as implementing it for database (and even then, specifically for MySQL), but it was a pretty valiant effort, and one that I’d still like to return to, when I get the opportunity, if for no other reason than to say I finished it. But the entity’s superclass still had the save() and find() methods that meant that persistence logic was, at some level in the class inheritance hierarchy, part of the entity. While effective for PHP, in terms of performance, this unfortunately doesn’t result in the cleanest of all possible code.

Using a service layer provides separation of concerns that the typical PHP model doesn’t allow for. The entity that moves data out of stable storage and into a bean also contains both business logic and an awareness of how it’s stored. It really doesn’t have to, and should. Talking to the database shouldn’t be your entity’s problem.

But it’s still, overall, part of the business model. The service and the entity, combined, provide the business model, and the DAO just packs it away and gets it back for you when you need it. This two classes should be viewed as part of a whole business model, within the context of a “Model-View-Controller” framework. The controller needs to be aware of both classes, certainly, but they should form two parts of a whole.

Besides, if you can pull all your persistence logic out of your entity, it should be almost trivial to change storage engines if and when the time comes. Say you needed to move from JPA and Hibernate to an XML- or JSON-based document storage system. You could probably just get away with re-annotating your entity, and writing a new DAO (which should always be pretty minimal), then adjusting how your DAO is wired into your service.

Try doing that in PHP if your entity knows how it’s stored!

One of these days, I’ll have to lay out a guide for how this architecture works best. I’d love to get involved in teaching undergrad students how large organisations try to conduct software engineering, so if I can explain it visually, then so much the better!

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