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Domain-Specific Languages: An Introductory Example

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In this excerpt from his book, Domain-Specific Languages, Martin Fowler offers a concrete example to demonstrate the different forms a DSL can take.

When I start to write, I need to swiftly explain what it is I’m writing about; in this case, to explain what a domain-specific language (DSL) is. I like to do this by showing a concrete example and following up with a more abstract definition. So, here I’m going to start with an example to demonstrate the different forms a DSL can take. In the next chapter I’ll try to generalize the definition into something more widely applicable.

1.1 Gothic Security

I have vague but persistent childhood memories of watching cheesy adventure films on TV. Often, these films would be set in some old castle and feature secret compartments or passages. In order to find them, heroes would need to pull the candle holder at the top of stairs and tap the wall twice.

Let’s imagine a company that decides to build security systems based on this idea. They come in, set up some kind of wireless network, and install little devices that send four-character messages when interesting things happen. For example, a sensor attached to a drawer would send the message D2OP when the drawer is opened. We also have little control devices that respond to four-character command messages—so a device can unlock a door when it hears the message D1UL.

At the center of all this is some controller software that listens to event messages, figures out what to do, and sends command messages. The company bought a job lot of Java-enabled toasters during the dot-com crash and is using them as the controllers. So whenever a customer buys a gothic security system, they come in and fit the building with lots of devices and a toaster with a control program written in Java.

For this example, I’ll focus on this control program. Each customer has individual needs, but once you look at a good sampling, you will soon see common patterns. Miss Grant closes her bedroom door, opens a drawer, and turns on a light to access a secret compartment. Miss Shaw turns on a tap, then opens either of her two compartments by turning on correct light. Miss Smith has a secret compartment inside a locked closet inside her office. She has to close a door, take a picture off the wall, turn her desk light on three times, open the top drawer of her filing cabinet—and then the closet is unlocked. If she forgets to turn the desk light off before she opens the inner compartment, an alarm will sound.

Although this example is deliberately whimsical, the underlying point isn’t that unusual. What we have is a family of systems that share most components and behaviors, but have some important differences. In this case, the way the controller sends and receives messages is the same across all the customers, but the sequence of events and commands differs. We want to arrange things so that the company can install a new system with the minimum of effort, so it must be easy for them to program the sequence of actions into the controller.

Looking at all these cases, it emerges that a good way to think about the controller is as a state machine. Each sensor sends an event that can change the state of the controller. As the controller enters a state, it can send a command message out to the network.

At this point, I should confess that originally in my writing it was the other way around. A state machine makes a good example for a DSL, so I picked that first. I chose a gothic castle because I get bored of all the other state machine examples.

1.1.1 Miss Grant’s Controller

Although my mythical company has thousands of satisfied customers, we’ll focus on just one: Miss Grant, my favorite. She has a secret compartment in her bedroom that is normally locked and concealed. To open it, she has to close the door, then open the second drawer in her chest and turn her bedside light on—in either order. Once these are done, the secret panel is unlocked for her to open.

I can represent this sequence as a state diagram (Figure 1.1).

If you haven’t come across state machines yet, they are a common way of describing behavior—not universally useful but well suited to situations like this. The basic idea is that the controller can be in different states. When you’re in a particular state, certain events will transition you to another state that will have different transitions on it; thus a sequence of events leads you from state to state. In this model, actions (sending of commands) occur when you enter a state. (Other kinds of state machines perform actions in different places.)

This controller is, mostly, a simple and conventional state machine, but there is a twist. The customers’ controllers have a distinct idle state that the system spends most of its time in. Certain events can jump the system back into this idle state even if it is in the middle of the more interesting state transitions, effectively resetting the model. In Miss Grant’s case, opening the door is such a reset event.

Figure 1.1 State diagram for Miss Grant’s secret compartment

Introducing reset events means that the state machine described here doesn’t quite fit one of the classical state machine models. There are several variations of state machines that are pretty well known; this model starts with one of these but the reset events add a twist that is unique to this context.

In particular, you should note that reset events aren’t strictly necessary to express Miss Grant’s controller. As an alternative, I could just add a transition to every state, triggered by doorOpened, leading to the idle state. The notion of a reset event is useful because it simplifies the diagram.

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