Getting Ready: The ATM System

Problem definition

An Automated Teller Machine (ATM) is a self-service terminal that enables bank customers to perform financial transactions without interacting with a human teller or visiting a physical bank branch. Through ATMs, users can perform essential banking operations such as depositing cash, withdrawing money, checking account balances, and transferring funds between accounts. ATMs are typically placed in accessible public locations like banks, malls, airports, and convenience stores to provide round-the-clock banking services.

In this LLD interview case study, your focus will be on:

• Enabling secure card-based access and PIN verification for users.

• Supporting core transactions: cash withdrawal, deposit, balance inquiry, and fund transfer.

• Managing cash inventory within the ATM, including withdrawal limits and handling insufficient funds.

• Ensuring a seamless and secure user experience through well-coordinated hardware and software components.

Expectations from the interviewee

During the interview, you are expected to discuss key aspects of the ATM system’s design, clarifying requirements, handling edge cases, and reasoning about hardware-software interactions. Typical areas to cover include:

ATM components

To better understand an ATM system, you may ask the interviewer the following questions:

1. What are the components of an ATM?

2. Is the ATM necessarily placed inside a room?

3. Does an ATM have a fingerprint scanner?

ATM features

Different ATMs may vary in terms of features which is why it is important to clear the following questions from the interviewer:

1. What is the withdrawal limit of an ATM?

2. Can we check our account balance using an ATM?

3. Can we set a PIN using an ATM?

ATM processing

The interviewer would expect you to ask a question regarding the processing of transactions using an ATM. Therefore, you may ask the following questions:

1. What happens when the amount entered by the user for withdrawal is greater than the user's account balance?

2. What happens when the amount entered by the user for withdrawal is greater than the ATM's cash limit?

3. What happens when the amount entered by the user exceeds the total cash present in the ATM?

4. Can the ATM be used for online transactions?

Design approach

We will design this ATM system using a bottom-up approach, following these steps:

• First, we’ll identify and model the core hardware components such as CardReader, Keypad, Screen, CashDispenser, and Printer, defining their responsibilities and interactions.

• Next, we’ll compose these components into larger entities representing the ATM’s operational state, transaction workflows, and the overall ATM machine.

• We’ll model how the ATM interacts with users for authentication, processes various transactions, manages its cash inventory, and ensures secure and reliable operations.

• This approach will allow us to address edge cases, hardware failures, and incorporate SOLID design principles for modularity and maintainability. We’ll use diagrams and code examples to illustrate the design at each step.

Design pattern 

During an interview, it is always a good practice to discuss the design patterns that the ATM system falls under. Stating the design patterns gives the interviewer a positive impression and shows that the interviewee is well-versed in the advanced concepts of object-oriented design.

Let’s explore the requirements of the ATM system in the next lesson.