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Menufactory

Menufactory is a Kotlin DSL (Domain Specific Language) for building USSD applications as state machines.

With the outburst of USSD apps; especially in Africa, there still is little guidance about best practices for developing these apps. On the surface, these apps are no different from regular web apps or microservices exposed over REST. The tricky part becomes apparent with handling navigations. This is the crux of USSD apps.

For every user input, the app must make a decision on the next menu to display

The annoyingly popular way of building these apps is using conditionals. Here's a snippet from Africa's talking documentation.

...
if ($text == "1") {
    $response = "CON Choose account information you want to view \n";
    $response .= "1. Account number \n";
    $response .= "2. Account balance";
} else if ($text == "2") {
    $response = "END Your phone number is ".$phoneNumber;
}
...
echo $response;

Here are some issues with this approach:

  • As the menu gets deeper, these conditionals can quickly grow into a gigantic mess
  • We would have to write a conditional to match every possible user input
  • It isn't modular. One could have conditionals from menu screen 2 mixed up with conditionals from menu screen 10.

All in all, using these conditionals for handling navigation is untenable.

FSM according to Wikipedia;

It is an abstract machine that can be in exactly one of a finite number of states at any given time. The FSM can change from one state to another in response to some inputs; the change from one state to another is called a transition. An FSM is defined by a list of its states, its initial state, and the inputs that trigger each transition

Conceptually, we may think of every menu in the USSD app as a state in the state machine. The first menu that is displayed when a user dials the ussd code is the FSM's start state. Each state is tagged with a unique identifier. For any given input, the current state determines the next state of the FSM. The transitions employs the target state's unique identifier. We use the sessionId to keep track of the FSM's current state.

In the past, I'd wired up some USSD apps as FSMs in a messy way. While they were messy, they were more manageable than using gigantic conditionals.

Then I came across this really nice JS library for building USSD apps as FSMs. Menufactory is highly inspired by that library.

This library is still in experimental state, however feel free to try it out.

Installation:

At the moment, it is hosted on Github Package Registry. Hence, install by adding this to your build.gradle.

repositories {
    maven {
        url "https://maven.pkg.github.com/shobande-femi/menufactory"
        credentials {
            username "shobande-femi"
            password "*********"
        }
    }
}

dependencies {
    compile("com.shobande:menufactory:1.1.0")
}

Find the repo password here

Instantiation

There are 2 ways to build start building your menu.

Create a singleton Menu, then register its states:

menu = Menu("Mystery App", HubTel, InMemorySession())

menu.startState {
    run {
        end("You can reach me on Twitter @shobande_femi")
    }
}

OR use the builder pattern:

Menu.menu("Mystery App", HubTel, InMemorySession()) {
    startState {
        run {
            end("You can reach me on Twitter @shobande_femi")
        }
    }
}

In the snippets above, we created a menu:

  • called "Mystery App"
  • used Hubtel as our gateway of choice
  • chose to store session data in memory
  • registered the start state that displays some text when visited.

Gateway

A USSD gateway is a service that routes traffic from a USSD to your app.

Currently supported gateways include:

  • Africas Talking (Default)
  • Hubtel
  • Nsano

The major differences between these gateways message format they pass to our application, and the response format they expect. There are also slight naming differences. While one gateway may say "phoneNumber", another may say "msisdn".

You can add support for any gateway by implementing the Gateway interface and passing that object as a param while instantiating the USSD Menu.

The supported gateways implement that common interface. Looking through their implementation may provide useful insight should you decide to add support for another gateway.

State Runner

The run method describes what this state displays to the user. con responds to the user without ending the USSD session end responds and ends the USSD session

These 2 methods accept strings. If you need user display prettified, simply use triple quotes and trimMargin() as seen in the example below.

Menu.menu("Mystery App") {
    startState {
        run {
            println(it.phoneNumber)

            con("""Welcome to ${this@menu.name}
                |1. Check Balance
                |2. Buy Airtime
                |3. Contact Us
            """.trimMargin())
        }
    }
}

Within the run method, you have access to the full request that comes from the gateway.

Here's the full request object:

data class Request(
    val phoneNumber: String,
    val sessionId: String,
    val ussdCode: String?,
    val message: String,
    val operator: String?
)

To get the user's phone number simply do it.phoneNumber.

Transition Definitions

Transitions map the user input to the name of the next state. The mapped value (next state) may be a string or a lambda that returns a string. When using a lambda, the request object is accessible; it.phoneNumber.

Menu.menu("Mystery App") {
    startState {
        run {
            con("""Welcome to ${this@menu.name}
                |1. Check Balance
                |2. Buy Airtime
                |3. Contact Us
            """.trimMargin())
        }
        transitions {
            "1" to "checkBalance"
            "2" to {
                if (!it.operator.equals("999999")) {
                    "buyAirtime"
                } else {
                    States.CONTACT_US.name
                }
            }
            "3" to States.CONTACT_US.name
            """^[a-zA-Z]*$""" to States.CONTACT_US.name
        }
        defaultNextState(States.CONTACT_US.name)
    }
}

Note: I have used enumerations for the contact us state names to avoid errors that may arise from typos. Generally, I prefer using enums to plain strings. When using enumerations, remember to retrieve the enum's name.

You can use regular expression for matching inputs to next states. You needn't escape any characters in the regex. Simply use triple quotes around the regex as is.

When user input cannot be matched with any transition definition, the state doesn't change. You can also define a custom defaultNextState

Adding more states

Only one start state can be registered. You can add as many other state however.

When adding another state that isn't the start state, you must specify a state name. This is the state's unique identifier.

Menu.menu("Mystery App") {
    startState {
        run {  }
        transitions {  }
    }
    state("checkBalance") {
        run {
            end("You balance is ${fetchBalance()}")
        }
        transitions {  }
    }
}

State redirection

Within a state's run, it possible to redirect to another state using goTo. To redirect to the start state, you can use goToStart. Redirecting to another state doesn't break the state chain, hence, you may redirect as many times as you wish. Like redirecting to a state which redirects to another state and so on.

Menu.menu("Mystery App") {
    startState {
        run { goTo(States.CONTACT_US.name) }
    }
    state(States.CONTACT_US.name) {
        run {
            end("You can reach me on Twitter @shobande_femi")
        }
    }
}

Sessions

A Session interface exists for defining interactions with whatever storage you may use for saving session info. By default, the menu used the InMemorySession. You may define yours by implementing Session.

In your implementation, override startStateName and set to any value of your choice. This will be a reserved state name and any state definition (other than startState) that tries to use this name will throw a ReservedStateName exception.

The key methods to implement are;

  • start(sessionId: String): initializes a new session. Invoked internally before any state is run. If there is an active session for the specified sessionId, initialization is skipped
  • set(sessionId: String, key: String, value: Any): stores a key-value pair in the current user session
  • get(sessionId: String, key: String): retrieves a value from the current user session
  • end(sessionId: String): deletes a user session. Invoked internally by final states.

Sample App

Checkout the full Sample app here

suspend fun buildMenu(): Menu {
    return Menu.menu("Mystery App", HubTel, InMemorySession()) {
        startState {
            run {
                con("""Welcome to ${this@menu.name}
                    |1. Check Balance
                    |2. Buy Airtime
                    |3. Contact Us
                """.trimMargin())
            }

            transitions {
                "1" to States.CHECK_BALANCE.name
                "2" to {
                    if (!it.operator.equals("MTN")) {
                        States.BUY_AIRTIME.name
                    } else {
                        States.CONTACT_US.name
                    }
                }
                "3" to States.CONTACT_US.name
            }
        }

        state(States.CHECK_BALANCE.name) {
            run {
                if (it.phoneNumber.length != 14) {
                    goTo(States.CONTACT_US.name)
                } else {
                    end("You balance is ${fetchBalance()}")
                }
            }
        }

        state(States.BUY_AIRTIME.name) {
            run {
                con("""Enter your phone number
                    |0. Go back
                """.trimMargin())
            }
            transitions {
                "0" to session.startStateName
                """^[0-9]*$""" to {
                    session.set(it.sessionId, "phoneNumber", it.message)
                    "airtimeBought"
                }
                """^[a-zA-Z]*$""" to States.CONTACT_US.name
            }
            defaultNextState(States.CONTACT_US.name)
        }

        state(States.CONTACT_US.name) {
            run {
                end("You can reach me on Twitter @shobande_femi")
            }
        }

        state("airtimeBought") {
            run {
                end("You have successfully bought some airtime on ${session.get(it.sessionId, "phoneNumber")}")
            }
        }
    }
}

enum class States {
    CHECK_BALANCE,
    BUY_AIRTIME,
    CONTACT_US
}

suspend fun fetchBalance(): Int = 100

Serve Requests

Now we are ready to deploy our USSD app. Feel free to use what ever mechanism to serve http requests. For simplicity, I use Ktor in this example.

Checkout the full Sample app here

suspend fun main() {
    val menu = buildMenu()

    embeddedServer(Netty, 9090) {
        routing {
            install(ContentNegotiation) {
                gson {
                    setDateFormat(DateFormat.LONG)
                    setPrettyPrinting()
                }
            }

            post("/") {
                menu.handle(call.receive()) { call.respond(it) }
            }
        }
    }.start(wait = true)
}

The key part to note is menu.handle. This method takes 2 arguments

  1. The request body
  2. A lambda to be called when the menu has processed the request and determined the result to be returned. That result is fed as an argument into the lambda.