This article was written by Educative's Co-founder and CEO, Fahim ul Haq.

When I made it to the design interview round at Facebook, I was surprised when they asked me if I would prefer a System Design interview (SDI) or a Product Design interview.

At Facebook, they refer to the SDI as “Pirate”, while Product Design — also known across the industry as API Design — is referred to as “Pirate X.” But since I was given a choice, I had a dilemma on my hands: which Pirate round interview would give me the best chance to get hired?

A general rule of thumb is if you want to work on the functional building blocks of a software system, you should take the SDI. If you want to leverage those building blocks to create products and features, you should opt for the Product Design/API Design interview instead.

Since I knew I wanted to work on distributed storage at Facebook, I determined that the System Design interview was right for me. But nowadays, most software developers are not actively working on the key building blocks. That means the better fit for the majority of devs is the Product/API Design Interview. This is why we've seen the API Design interview become increasingly important across the industry — and why it’s important for every developer to learn some API Design fundamentals.

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• Grokking the API Design Interview
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Junior and senior-level developers need to become familiar with API Design and System Design Interviews (SDI) if they want to remain competitive in the job market. Securing a position at a top-tier company isn't easy, given the stiff competition. That's why being interview-ready is critical.

In this newsletter, I'll break down the key differences between the API Design Interview and the System Design Interview. Then, I'll help you decide which interview you should prioritize based on your career goals.

API Design Interviews vs. System Design Interviews

Design interviews are so important to interviewers because they focus on assessing a candidate's problem-solving skills, technical knowledge, and ability to produce well-crafted solutions to design challenges.

That’s why these interviews also serve as a primary tool for determining a candidate’s starting level. Fumbling the design interview can result in you getting down-leveled, which means you miss out on potential earnings. For example, instead of being hired at Meta at E4 (Software Engineer III), you might get offered a position at E3 (Software Engineer II). It can take anywhere from 1-3 years to be promoted to the next seniority level. During that time, you'd be losing out on anywhere from 10-20% of your earning potential — all because of a 30-45 minute interview.

At most top companies, you will be given a choice between the API interview and the SDI. If this happens, how do you know which to pick?

The concepts of API Design and System Design are representative of the larger workflow of software development teams collaborating to build functional, scalable web applications. As a result, the API Design and System Design Interviews are designed to test a candidate's aptitude for design and problem solving in two key disciplines:

1. API Design: Leveraging the Existing Software System
2. System Design: Building Blocks of Software Systems

1) API Design: Leveraging the Existing Software System

The API design interview is a recent addition to the design interview family. Interviewers seek to understand how you ask questions and communicate, design individual components, and provide reasoning for your choices.

To effectively design an API, you'll need working knowledge of the current software system. For example, if the existing distributed store is unable to meet the speed requirements of the API, you will need to call the caching layer instead. In a successful API Design interview, the candidate will creatively navigate the constraints of the software system to create fast and functional APIs.

When designing an API there are multiple factors that need defining. It is important to begin clarifying these criteria at the beginning of the interview.

• User: Is this an internal tool, or is it a product that other parties may need?
• Problem: What does the API aim to solve?
• Response type: Endpoints and successful or unsuccessful calls must be clearly defined.
  • Some standard failed HTTP request status codes to know are: 301, 403, 500
• Use-case: How will the API be implemented? This is essential for testing and debugging.
  • Common API architectures include: REST, GraphQL, and RPC
  • Common data formats are: JSON, XML, binary
• Scalability: Plan for an increase in demand. The API should be resilient and expandable.
• Documentation: What documentation is required for other developers to successfully integrate the API? This includes structure, behaviors, and parameters to be defined.

API Design interviews are a rapidly growing trend in the software industry.

More and more companies are transitioning their design interviews to this smaller scope, API-focused approach. Front-end devs, back-end devs, full-stack devs and product managers need to be prepared for an API Design interview. But if you are a senior developer that is largely unaware of the API design process, you could be drastically limiting your career growth.

Even if you aren't currently seeking a new job, learning API design can greatly aid your existing design and communication skills as well as your overall product awareness in your organization.

However, there are some careers and pathways in software engineering where System Design mastery lends an undeniable edge.

Roles requiring System Design knowledge

Certain software engineering roles demand a strong understanding of System Design principles:

• Senior Software Engineers: Work on complex systems, requiring System Design knowledge for efficiency and scalability.
• Software Architects: Create high-level design and architecture, needing deep understanding of System Design for informed decision-making.
• DevOps Engineers: Streamline development, deployment, and operation of software systems, using System Design knowledge to optimize infrastructure and processes.
• Site Reliability Engineers (SREs): Focus on reliability, scalability, and performance, requiring in-depth System Design knowledge for effective strategies and monitoring.

These are just some of the roles where System Design expertise is highly sought after. For those looking to advance into these roles, it's imperative to ramp up your System Design interview skills as soon as possible.

2) System Design: Building Blocks of Software Systems

Software engineers at all experience levels should understand System Design fundamentals. That said, most engineers won't actually be working on large distributed systems. Instead, most engineers will likely spend most of their days building applications that leverage those core infrastructure systems.

However, there are some careers and pathways in software engineering where System Design mastery lends an undeniable edge.

I will say that I have seen a consistent pattern over the years: candidates with System Design knowledge and experience consistently outperform their underprepared peers during technical interviews. This starts with strategic SDI preparation.

Understanding the System Design Interview

The System Design Interview (SDI) evaluates a candidate's ability to design and architect large-scale, complex software systems.

Here is how a typical SDI breaks down:

• Design Challenge:
  Candidates have 30-45 minutes to tackle a high-level design task involving creating, optimizing, or scaling a software system.

• Clarification and Analysis: Next, candidates delve deeper into the problem's requirements, constraints, and goals by asking relevant questions.

• Solution Presentation: Candidates present their solution, considering scalability, performance, reliability, and security.

• Discussion and Review: If there's enough time, the interviewer may provide feedback, and the candidate can refine their design.

Why you should level up your System Design knowledge

A candidate without System Design knowledge may exhibit some of the following characteristics during an interview, which can make them appear less prepared or experienced:

• They have a limited understanding of trade-offs. Candidates without System Design knowledge tend to struggle to identify or discuss trade-offs, which often results in suboptimal solutions or flawed designs.
• They have tunnel vision when it comes to implementation details. Some candidates focus too heavily on low-level implementation details, such as algorithms or data structures, without considering the overall architecture.
• They struggle to address scalability and performance issues. Without a background in System Design, some candidates may not even bring up how their proposed solutions will scale.

Lastly, candidates without System Design knowledge often have an unstructured approach to solving problems. This might be the most noticeable shortcoming I've seen during interviews, even for experienced software engineers. Luckily it's also one of the easiest to fix.

System Design principles force you to think about solutions in a more organized and methodical manner, accounting for limitations and tradeoffs.

System Design as an accelerant for career progression

A strong foundation in System Design can accelerate your career growth too. Engineers who excel at System Design are more likely to be promoted to senior or leadership positions within their organizations.

Here's a general idea of how your career might progress, just to illustrate the impact of System Design.

Without System Design experience:

• Entry-Level Software Engineer (0-2 years)
• Mid-Level Software Engineer (2-5 years)
• Senior Software Engineer (4-6 years)
• Technical Lead or Engineering Manager (8+ years)

It's possible to become an EM without System Design experience, but it's pretty much a certainty that it will be more challenging and take up more time to reach that position.

With System Design experience:

• Entry-Level Software Engineer (0-2 years)
• Mid-Level Software Engineer (1-3 years)
• Senior Software Engineer (4-5 years)
• Technical Lead or Engineering Manager (6+ years)
• Principal Engineer or Software Architect (8+ years)

People bring all sorts of unique talents and skills to the table, so none of this is set in stone. However, it is true that when you're working in the tech industry, System Design can definitely speed things up because it enables you to make more valuable contributions to a business early on.

Level up your design interview prep today

Both API Design and System Design Interviews are essential tools that hiring managers use to assess a software engineer's aptitude for problem-solving, design, and communication.

API Design Interviews are particularly relevant for front-end developers, back-end developers, and product managers to understand.

At the same time, System Design Interviews are more relevant to senior software engineers, software architects, and roles focused on infrastructure.

By understanding the differences between these interviews and the specific roles they cater to, you can prioritize your interview prep work. The demand for experienced software engineers is growing and the landscape of technology is continuing to evolve. Therefore, strategically choosing the right design interview to prepare for, based on your career goals, is a wise investment in your professional future that will set you up for success in the interview and beyond.

As always, happy learning!