Enforcing the maximum withdrawal across commands

We’ve configured the maximum withdrawal, but multiple “withdraw <maximum>” commands can be executed back-to-back. What if we wanted to enforce that all withdraw commands never exceed the maximum? Furthermore, depositing money to the ATM should increase the maximum withdrawal for that session.

We can add a stateful object, WithdrawalLimiter to keep track of the initial maximum and all changes to the balance within a session:

final class WithdrawalLimiter {
  private BigDecimal remainingWithdrawalLimit;

  @Inject
  WithdrawalLimiter(@MaximumWithdrawal BigDecimal maximumWithdrawal) {
    this.remainingWithdrawalLimit = maximumWithdrawal;
  }

  void recordDeposit(BigDecimal amount) { ... }
  void recordWithdrawal(BigDecimal amount) { ... }
}

We can add the WithdrawalLimiter to the @Inject constructors of DepositCommand and WithdrawCommand, calling recordWithdrawal() and recordDeposit() appropriately after a successful command.

// in WithdrawCommand
BigDecimal remainingWithdrawalLimit = withdrawalLimiter.remainingWithdrawalLimit();
if (amount.compareTo(remainingWithdrawalLimit) > 0) {
  outputter.output(
      String.format(
          "you may not withdraw %s; you may withdraw %s more in this session",
          amount, remainingWithdrawalLimit));
}

...

account.withdraw(amount);
withdrawalLimiter.recordWithdrawal(amount);
// in DepositCommand
account.deposit(amount);
withdrawalLimiter.recordDeposit(amount);

If you try that, you’ll notice one small issue: the WithdrawalLimiter used by each command is not the same; updates from DepositCommand will not propagate to WithdrawCommand.

We previously solved a similar issue by annotating Database with @Singleton. In this case, @Singleton isn’t applicable because we want one WithdrawalLimiter per login. Instead, we can define a new annotation:

@Scope
@Documented
@Retention(RUNTIME)
@interface PerSession {}

Annotating both WithdrawalLimiter and UserCommandsRouter with @PerSession indicates to Dagger that a single WithdrawalLimiter should be created for every instance of UserCommandsRouter.

@PerSession
final class WithdrawalLimiter { ... }

@PerSession
@Subcomponent(...)
interface UserCommandsRouter { ... }

Rerun the application to see that deposits can now correctly update the maximum withdrawal for the current session. Logout and log back in to see that for each new session, a new maximum withdrawal exists.

Note the difference between the @Singleton Database and the @PerSession WithdrawalLimiter. There is one Database instance shared among all the objects in the single CommandProcessorFactory instance, but there is a separate instance of WithdrawalLimiter for each instance of UserCommandsRouter.

CONCEPTS

  • A @Scope annotation instructs Dagger to provide one shared instance for all the requests for that type within an instance of the (sub)component that shares the same annotation.
    • Note that @Singleton, which we described previously, is really just another scope annotation!
  • The lifetime of a scoped instance is directly related to the lifetime of the component with that scope.
  • Note that the name of the scope is meaningless; even multiple instances of a @Singleton-annotated type can be created if multiple @Singleton-annotated @Components are instantiated in a single JVM.

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