As a senior software engineer, you are demanded a better understanding of how you solve a particular design problem, how you respond when there is more than expected traffic on your system, how you design the database of your system and many more.

All design decisions are based on below 4 factors:

  • Scalability
  • Reliability
  • Availability
  • Maintainability

Do remember: Everything is a trade-off.

The right architecture = Coming up potential solutions + Managing trade-offs

Approaching a Design Problem:

  • Breaking down the problem
  • Communicating your ideas
  • Assumptions that make sense

Approaching a System Design (Mainly according to the Software Engineering):

1. Requirements Analysis

Outline the scope of the system, and come out SRS (Software Requirements Specification).

  • Use cases (Systems exist to solve specific problems)
    • Who is going to use it?
    • How are they going to use it?
    • What does the system do?
  • Constraints
    • Mainly identify traffic and data handling constraints at scale.
    • Scale of the system such as requests per second, requests types, data written per second, data read per second)
    • Special system requirements such as multi-threading, read or write oriented.
  • Assumptions

Related Tools:

  • Data Flow Diagram (DFD)
  • Data Dictionary (DD)
  • Decision Trees
  • Decision Tables

Related Constraints/Bottleneck:

  • The average QPS of a single machine can reach 800-1000.
  • The average R/W speed of HDD is 60-80MB/s.

**2. High Level Design (HLD, known as General Design) **

Outline a high level architecture design, such as system module and technical route.

  • Sketch the system architecture
    • Components
      • High cohesion and low coupling for the independence of components
      • Divide by functions
      • Divide by domains
    • Relationship of components
      • Horizontal Relationship
      • Vertical Relationship
  • Scale the design
    • Understanding Bottlenecks
      • Too many user requests
      • The data is too huge
    • Solutions
      • Caching
        • Application Caching
        • Database Caching
        • In-memory Caches
      • Load balancer
      • Vertical scaling
        • Scale by adding more power (CPU, RAM) to your existing machine.
      • Horizontal scaling
        • Scale by adding more machines into your pool of resources.
      • Database sharding
  • Justify your ideas

3. Design components (LLD, Low Level Design, or Detail design)

Dive into details for each component, such as interface, algorithm, process, state transition, etc.

Related Tools:

  • Program Flow Chart
  • N-S Structure Diagram
  • Problem Analysis Diagram (PAD)

4. Implementation

5. Testing

Software testing is the process of executing a program in order to find bugs, so that we can verify whether the software meets various requirements. When writing a test, consider what types of defects the test needs to catch.

4 stages of V model:

  • Unit Testing
  • Integration Testing
  • System Testing
  • Acceptance Testing (Validation Testing)


  • Black-box Testing
    • Also known as functional testing, data-driven testing, or specification-based testing.
    • Mainly to test the function of the application.
    • for most software tests, such as integration testing, system testing, etc.
  • White-box Testing
    • Also known as glass box testing, structural testing, logic-driven testing or testing based on the program itself.
    • Mainly to test the internal structure or operation of the application.
    • For unit testing, integration testing. The philosophy of testing:
  • Unit Testing: To test the smallest independent piece of code to prevent regressions.
  • Integration Testing: The outside world is a messy place, things usually go wrong on the edge, as the place of interaction is usually where the unexpected happens.

Integration testing and unit testing are the cornerstones of robust software. When considering adding unit testing to existing software, there are costs and benefits to consider. If your code is working, if the code rarely needs to be modified, and if the code is not easily unit-testable, then the benefits of incorporating unit tests may not justify the cost. In these cases, integration tests can be relied upon to prevent defects in this area.

Testing Pyramid:

Testing Pyramid

Key concepts:

  • Write tests at different granularities
  • The higher the level, the fewer tests you should write