Relational database strengths in HLD - Architecture Diagram

System Overview - Relational database strengths

This system uses a relational database to store and manage structured data with clear relationships. It supports complex queries, ensures data accuracy, and maintains consistency through transactions.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Application Service
  |
  v
Cache
  |
  v
Relational Database

Components

User

client

Initiates requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly to prevent overload

API Gateway

api_gateway

Handles request routing, authentication, and rate limiting

Application Service

service

Processes business logic and interacts with the database

Cache

cache

Stores frequently accessed data to reduce database load and improve response time

Relational Database

database

Stores structured data with relationships, supports ACID transactions

Request Flow - 11 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Application Service

Application Service → Cache

Cache → Application Service

Application Service → Relational Database

Relational Database → Application Service

Application Service → Cache

Application Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Relational Database

Impact:Data writes fail and fresh data cannot be retrieved; cached reads may still work but data may be stale

Mitigation:Use database replication for failover; serve reads from cache during outage; queue writes for retry after recovery

Architecture Quiz - 3 Questions

Which component ensures that user requests are evenly distributed to prevent overload?

A Load Balancer

B API Gateway

C Cache

D Relational Database

Design Principle

This architecture highlights the strength of relational databases in managing structured data with clear relationships and ensuring data consistency through ACID transactions. The use of caching improves performance by reducing database load for frequent reads.

Practice

1. Which of the following is a key strength of relational databases?

easy

A. They are best suited for storing large binary files.

B. They ensure strong data consistency across multiple users.

C. They store data in unstructured formats like documents.

D. They do not support complex queries.

Solution

1. Step 1: Understand relational database data organization

   Relational databases organize data in tables with relationships, which helps maintain consistency.

2. Step 2: Identify the strength related to multi-user access

   They support multiple users accessing and modifying data while keeping it accurate and consistent.

3. Final Answer:

   They ensure strong data consistency across multiple users. -> Option B

4. Quick Check:

   Data consistency = C [OK]

Hint: Relational = tables + strong consistency [OK]

Common Mistakes:

• Confusing relational with document databases
• Thinking relational databases store unstructured data
• Assuming relational databases don't support complex queries

2. Which SQL keyword is used to combine rows from two tables based on a related column?

easy

A. UPDATE

B. SELECT

C. INSERT

D. JOIN

Solution

1. Step 1: Recall SQL keywords for combining tables

   The JOIN keyword is used to combine rows from two or more tables based on a related column.

2. Step 2: Differentiate from other SQL commands

   SELECT retrieves data, INSERT adds data, UPDATE modifies data, but JOIN combines tables.

3. Final Answer:

   JOIN -> Option D

4. Quick Check:

   Combine tables = JOIN [OK]

Hint: JOIN combines tables on related columns [OK]

Common Mistakes:

• Confusing SELECT with JOIN
• Using INSERT or UPDATE to combine tables
• Not knowing JOIN syntax

3. Given two tables Users and Orders, what will the following SQL query return?

SELECT Users.name, Orders.id FROM Users JOIN Orders ON Users.id = Orders.user_id;

medium

A. All users with their order IDs where user IDs match

B. All users regardless of orders

C. All orders regardless of users

D. An error due to missing WHERE clause

Solution

1. Step 1: Analyze the JOIN condition

   The query joins Users and Orders where Users.id equals Orders.user_id, linking users to their orders.

2. Step 2: Understand the SELECT output

   It selects user names and order IDs only for matching pairs, so users without orders are excluded.

3. Final Answer:

   All users with their order IDs where user IDs match -> Option A

4. Quick Check:

   JOIN with ON = matched user-orders [OK]

Hint: JOIN ON filters matching rows only [OK]

Common Mistakes:

• Thinking JOIN returns all users or all orders
• Assuming WHERE clause is mandatory for JOIN
• Confusing JOIN with UNION

4. Identify the error in this SQL query that tries to fetch all customers with their orders:

SELECT Customers.name, Orders.id FROM Customers JOIN Orders;

medium

A. No error, query is correct

B. Using SELECT instead of INSERT

C. Missing ON clause to specify join condition

D. Orders table does not exist

Solution

1. Step 1: Check JOIN syntax

   JOIN requires an ON clause to specify how tables relate; missing ON causes error or unintended Cartesian product.

2. Step 2: Confirm other parts are correct

   SELECT is correct for fetching data; assuming Orders table exists; so error is missing ON clause.

3. Final Answer:

   Missing ON clause to specify join condition -> Option C

4. Quick Check:

   JOIN needs ON clause [OK]

Hint: JOIN without ON causes errors [OK]

Common Mistakes:

• Assuming JOIN works without ON
• Confusing SELECT with INSERT
• Ignoring table existence

5. A company needs a system that supports multiple users updating customer data simultaneously without losing accuracy. Which relational database feature best supports this requirement?

hard

A. ACID transactions ensuring data consistency

B. Storing data as JSON documents

C. Using key-value pairs for fast access

D. NoSQL horizontal scaling

Solution

1. Step 1: Understand multi-user data update needs

   Multiple users updating data require mechanisms to avoid conflicts and keep data accurate.

2. Step 2: Identify relational database feature for consistency

   ACID transactions ensure atomicity, consistency, isolation, and durability, preventing data loss or corruption.

3. Final Answer:

   ACID transactions ensuring data consistency -> Option A

4. Quick Check:

   Multi-user accuracy = ACID transactions [OK]

Hint: ACID = safe multi-user updates [OK]

Common Mistakes:

• Confusing NoSQL features with relational strengths
• Thinking JSON storage ensures consistency
• Ignoring transaction properties