wow-core/me.ahoo.wow.infra.prepare/PrepareKey

PrepareKey

Key preparation interface for ensuring uniqueness constraints in EventSourcing architectures.

Unlike traditional databases that use UNIQUE KEY constraints, EventSourcing requires application-level uniqueness guarantees. PrepareKey provides a mechanism to "prepare" or reserve keys before they're actually used, ensuring atomicity and preventing race conditions.

Key features:

Atomic key preparation and rollback
TTL (Time-To-Live) support for temporary reservations
Reprepare operations for key changes
Transaction-like semantics with automatic rollback on failure

Common use cases:

Username/email uniqueness during user registration
Product SKU uniqueness
Resource identifier allocation
Preventing duplicate operations

Parameters

The type of value associated with prepared keys

Example usage for user registration:

@AggregateRoot
class User(private val state: UserState) {
    @OnCommand
    private fun onRegister(
        register: Register,
        passwordEncoder: PasswordEncoder,
        usernamePrepare: PrepareKey<UsernameIndexValue>,
    ): Mono<Registered> {
        val encodedPassword = passwordEncoder.encode(register.password)
        return usernamePrepare.usingPrepare(
            key = register.username,
            value = UsernameIndexValue(
                userId = state.id,
                password = encodedPassword,
            ),
        ) {
            require(it) {
                "username[${register.username}] is already registered."
            }
            Registered(username = register.username, password = encodedPassword).toMono()
        }
    }
}

Example usage for changing username:

@OnCommand
private fun onChangeUsername(
    changeUsername: ChangeUsername,
    usernamePrepare: PrepareKey<UsernameIndexValue>
): Mono<UsernameChanged> {
    val usernameIndexValue = UsernameIndexValue(
        userId = state.id,
        password = state.password,
    )
    return usernamePrepare.reprepare(
        oldKey = state.username,
        oldValue = usernameIndexValue,
        newKey = changeUsername.newUsername,
        newValue = usernameIndexValue
    ).map {
        require(it) {
            "username[${changeUsername.newUsername}] is already registered."
        }
        UsernameChanged(username = changeUsername.newUsername)
    }
}