March 27, 2015

...Learn TDD with Codemanship

Component-based & Microservice Architecture: Swappability Happens On The Client Side

Lunch time teleconference looms; just enough time to spew out these thoughts about distributed component architectures (you young hop folk may know them as "microservices", which is the trendy cool Dubstep name for them).

The key to distributed components is swappability - actually, that's kind of the whole point of components generally, distributed or in-process, so take this as generally applicable advice. Or don't. see if I care.

What our bearded young hipster friends often forget to mention is that the swappability in component-based design really happens on the client side of component-to-component (or service-to-service) collaborations. Not, as you may have been led to believe, on the server side.

Sure, we could make pretty much any kind of component present the same, say, REST API. But we still run the risk of binding our client to that API, and to the details of how to consume RESTful services. (Which can be, ironically, anything but to work with.)

Nope. To make that service truly swappable, we have to hide all of the details from the client.

UML 2.0, that leviathan of the component-based era, introduced the notion of components and connectors. It's a pretty neat idea: basically, we cleanly separate the logical conversation held between two components from the dirty business of the medium through which that conversation takes place.

To connect the idea with the real world, let's say I ask the prime minister if he's ever eaten three Shredded Wheat and he replied "Yes". Now, I didn't say how I asked him. Maybe I went to Number 10 and asked him to his face. maybe I emailed him. Maybe I had the words branded onto a poor person and paraded said pauper through the streets so that the PM would see the question when they reported the news on his tellybox.

What matters is that I asked, and he replied.

In component architectures, we seek to separate the logic of component interactions from the protocols through which they physically connect.

Let's say we have a Video Library web application that wants to display 3rd-party reviews of movies.

We could look for reviews on IMDB, or on Rotten Tomatoes (or on both). We want to ask the logical question: what reviews have been written for this movie (identified by the movie's title and year)?

We can codify the logic of that interact with interfaces, and package those interfaces in a component the client knows about - in this case, the VideoReviews .NET assembly, which contains the interfaces IReviewService and IReview.



The VideoTitle class that consumes these services doesn't need to know where the reviews are coming form, or how they're being marshalled. It just wants to ask the logical question. So we present it with a logical interface through which to do that.



By injecting the review service into the VideoTitle, it becomes possible to dynamically bind implementations of those interfaces that know how to connect and interact with the remote server (e.g., the Rotten Tomatoes API), and unpacks the data that comes back, translating into a form that VideoTitle can use easily.

All of that is done behind the scenes: VideoTitle knows nothing about the details. And because it knows nothing about the details, and because we're injecting it into the VideoTitle from outside - e.g., in its constructor, or as a method parameter when VideoTitle is told to get reviews - it becomes possible to swap in different connectors to get reviews from other services.



All of this can be wired together from above (e.g., we could instantiate an implementation of IReviewService when the application starts up), or with a dependency injection framework, and so on. The possibilities are many and varied for runtime hi-jinks and dynamic larks.

The component dependencies are crucial: our app logic (in the VideoLibary component) only depends on the abstractions in the VideoReviews library. It depends in no way on the external services. It does not know there are even external services involved.

All component dependencies point towards the abstractions, satisfying the Stable Abstractions package design principle.

It now becomes possible to do clever things with swappability, like pooling connectors that point to different service instances to provide basic load-balancing or fail-over, or giving the end user the choice of which service to use at runtime.

Most importantly, though, it gives us the ability to vary the logic of our applications independently of the details of how it connects to external services independently of each other. If a new movie review site came along, we would simply have to write a new connector for it, and wouldn't have to rely on them implementing the same web API as our existing providers. Because, that my friends, is beyond our control.

So, succeeding with components is about swappability, and swappability is about programming the logic of our applications against clean interfaces that we control.

The REST is details.





Posted 2 years, 8 months ago on March 27, 2015