Microservices for the Desktop

In OOP the Easy Way, I make the argument that microservices are a rare instance of OOP done well:

Microservice adopters are able to implement different services in different technologies, to think about changes to a given service only in terms of how they satisfy the message contract, and to independently replace individual services without disrupting the whole system. This […] sounds a lot like OOP.

Microservices are an idea from service-oriented architecture (SOA) in which each application—each microservice—represents a distinct bounded context in the problem domain. If you’re a movie theatre complex, then selling tickets to people is a very different thing from showing movies in theatres, that are coupled loosely at the point that a ticket represents the right to a given seat in a given theatre at a given showing. So you might have a microservice that can tell people what showings there are at what times and where, and another microservice that can sell people tickets.

People who want to write scalable systems like microservices, because they can scale different parts of their application separately. Maybe each franchisee in the theatre chain needs one instance of one service, but another should scale as demand grows, sharing a central resource pool.

Never mind all of that. The real benefit of microservices is that they make boundary-crossing more obvious, maybe even more costly, and as a result developers think about where the boundaries should be. The “problem” with monolithic (single-process) applications was never, really, that the deployment cost too much: one corollary of scale is that you have more customers. It was that there was no real enforcement of separate parts of the problem domain. If you’ve got a thing over here that needs that data over there, it’s easy to just change its visibility modifier and grab it. Now this thing and that thing are coupled, whoops!

When this thing and that thing are in separate services, you’re going to have to expose a new endpoint to get that data out. That’s going to make it part of that thing’s public commitment: a slightly stronger signal that you’re going down a complex path.

It’s possible to take the microservices idea and use it in other contexts than “the backend”. In one Cocoa app I’m working on, I’ve taken the model (the representation in objects of the problem I’m solving) and put it into an XPC Plugin. XPC is a lot like old-style Distributed Objects or even CORBA or DCOM, with the exception that there are more safety checks, and everything is asynchronous. In my case, the model is in Objective-C in the plugin, and the application is in Swift in the host process.

“Everything is asynchronous” is a great reminder that the application and the model are communicating in an arm’s-reach fashion. My model is a program that represents the domain problem, as mentioned before. All it can do is react to events in the problem domain and represent the changes in that domain. My application is a reification of the Cocoa framework to expose a user interface. All it can do is draw stuff to the screen, and react to events in the user interface. The app and the model have to collaborate, because the stuff that gets drawn should be related to the problem, and the UI events should be related to desired changes in the domain. But they are restricted to collaborating over the published interface of the XPC service: a single protocol.

XPC was designed for factoring applications, separating the security contexts of different components and giving the host application the chance to stay alive when parts of the system fail. Those are valid and valuable benefits: the XPC service hosting the model only needs to do computation and allocate memory. Drawing (i.e. messaging the window server) is done elsewhere. So is saving and loading. And that helps enforce the contract, because if I ever find myself wanting to put drawing in the model I’m going to cross a service boundary, and I’m going to need to think long and hard about whether that is correct.

If you want to talk more about microservices, XPC services, and how they’re different or the same, and how I can help your team get the most out of them, you’re in luck! I’ve recently launched the Labrary—the intersection of the library and the laboratory—for exactly that purpose.

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