High cohesion in Microservices - Scalability & System Analysis

Scalability Analysis - High cohesion

Growth Table: High Cohesion in Microservices

Users/Scale	100 Users	10K Users	1M Users	100M Users
Service Count	Few (3-5)	10-20	50-100	Hundreds+
Service Size	Small, focused	Small, focused	Small, focused	Small, focused
Inter-service Calls	Low	Moderate	High	Very High
Data Ownership	Clear per service	Clear per service	Clear per service	Clear per service
Deployment Frequency	Low	Moderate	High	Very High
Complexity	Low	Moderate	High	Very High

First Bottleneck

At small scale, the first bottleneck is usually the database because each microservice owns its data and queries increase with users.

As users grow, inter-service communication becomes the bottleneck due to many small services calling each other, causing latency and network overhead.

Without high cohesion, services may have overlapping responsibilities, increasing coupling and making scaling harder.

Scaling Solutions for High Cohesion

• Maintain small, focused services: Each service handles a single responsibility to reduce complexity and improve scalability.
• Database per service: Avoid shared databases to reduce contention and allow independent scaling.
• Use asynchronous communication: Message queues or event-driven patterns reduce tight coupling and improve resilience.
• API Gateway and service mesh: Manage inter-service calls efficiently and monitor traffic.
• Horizontal scaling: Add more instances of services independently based on load.
• Cache frequently accessed data: Reduce database load and improve response times.
• Automate deployments: Continuous integration and delivery help manage many small services.

Back-of-Envelope Cost Analysis

Assuming 1M users generating 100 requests per second (RPS) total:

• Each microservice handles ~100-1000 RPS depending on responsibility.
• Database instances per service handle ~5000 QPS; scaling needed if exceeded.
• Network bandwidth depends on payload size; 1 Gbps (~125 MB/s) supports many small calls.
• Storage grows with data owned by each service; archiving old data reduces cost.

Interview Tip

When discussing high cohesion in microservices, start by explaining the importance of single responsibility per service.

Describe how this reduces complexity and improves scalability.

Then, discuss bottlenecks like database load and inter-service communication.

Finally, explain scaling solutions like asynchronous messaging, caching, and horizontal scaling.

Self Check Question

Your database handles 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Add read replicas and implement caching to reduce database load before scaling vertically or sharding.

Key Result

High cohesion in microservices keeps services small and focused, which helps scale independently. The first bottleneck is usually the database or inter-service communication as users grow. Solutions include database per service, asynchronous messaging, caching, and horizontal scaling.

Practice

1. What does high cohesion mean in microservices architecture?

easy

A. Using a single database for all microservices

B. Splitting every function into separate services regardless of relation

C. Combining unrelated tasks to reduce the number of services

D. Grouping related tasks and responsibilities within a single service

Solution

1. Step 1: Understand the meaning of cohesion

   Cohesion means how closely related the tasks inside a module or service are.

2. Step 2: Apply cohesion to microservices

   High cohesion means grouping related tasks in one service to keep it focused and manageable.

3. Final Answer:

   Grouping related tasks and responsibilities within a single service -> Option D

4. Quick Check:

   High cohesion = grouping related tasks [OK]

Hint: High cohesion means related tasks stay together [OK]

Common Mistakes:

• Thinking high cohesion means splitting every function separately
• Confusing cohesion with coupling
• Believing unrelated tasks should be combined
• Assuming database design defines cohesion

2. Which of the following is the correct way to describe a microservice with high cohesion?

easy

A. A service that manages all user-related operations like profile, login, and preferences

B. A service that mixes order processing and inventory updates randomly

C. A service that handles user authentication and payment processing

D. A service that only stores data without any business logic

Solution

1. Step 1: Identify related tasks in options

   A service that manages all user-related operations like profile, login, and preferences groups user-related operations which are closely related.

2. Step 2: Check other options for unrelated tasks

   Options A and B mix unrelated tasks; D lacks business logic, so not cohesive.

3. Final Answer:

   A service that manages all user-related operations like profile, login, and preferences -> Option A

4. Quick Check:

   High cohesion = related user tasks together [OK]

Hint: Look for grouping of related tasks in one service [OK]

Common Mistakes:

• Choosing options that mix unrelated responsibilities
• Ignoring business logic in cohesion
• Confusing data storage with service responsibility
• Assuming more tasks always mean better cohesion

3. Consider a microservice design where the OrderService handles order creation, payment processing, and shipping updates. What is the likely issue with this design regarding high cohesion?

medium

A. The service has low cohesion because it mixes unrelated responsibilities

B. The service has high cohesion because all tasks relate to orders

C. The service is scalable because it handles multiple tasks

D. The service is loosely coupled with other services

Solution

1. Step 1: Analyze the tasks in OrderService

   Order creation, payment, and shipping are different domains with distinct logic.

2. Step 2: Evaluate cohesion

   Mixing payment and shipping with order creation lowers cohesion because responsibilities differ.

3. Final Answer:

   The service has low cohesion because it mixes unrelated responsibilities -> Option A

4. Quick Check:

   Mixed tasks = low cohesion [OK]

Hint: Different domains in one service reduce cohesion [OK]

Common Mistakes:

• Assuming all order-related tasks are always cohesive
• Confusing scalability with cohesion
• Ignoring domain boundaries
• Believing loosely coupled means high cohesion

4. A microservice named InventoryService currently manages stock levels and supplier payments. What is the best fix to improve high cohesion?

medium

A. Combine InventoryService with OrderService

B. Add customer order tracking to InventoryService

C. Split supplier payments into a separate PaymentService

D. Keep all tasks in InventoryService for simplicity

Solution

1. Step 1: Identify unrelated responsibilities

   Supplier payments are unrelated to stock level management.

2. Step 2: Suggest separation for high cohesion

   Moving payments to a dedicated PaymentService improves cohesion by grouping related tasks.

3. Final Answer:

   Split supplier payments into a separate PaymentService -> Option C

4. Quick Check:

   Separate unrelated tasks to improve cohesion [OK]

Hint: Separate unrelated tasks into different services [OK]

Common Mistakes:

• Adding unrelated tasks to the same service
• Combining unrelated services
• Ignoring cohesion for simplicity
• Confusing cohesion with coupling

5. You are designing a microservices system for an e-commerce platform. To ensure high cohesion, which of the following service groupings is best?

hard

A. UserService (user profiles, payments), OrderService (orders, shipping), InventoryService (stock levels, payments)

B. UserService (user profiles, authentication), OrderService (orders, payments), ShippingService (shipping updates, tracking)

C. One big service handling users, orders, payments, shipping, and inventory

D. Split every function into its own microservice regardless of relation

Solution

1. Step 1: Evaluate grouping of related tasks

   UserService (user profiles, authentication), OrderService (orders, payments), ShippingService (shipping updates, tracking) groups related tasks logically by domain, supporting high cohesion.

2. Step 2: Compare with other options

   UserService (user profiles, payments), OrderService (orders, shipping), InventoryService (stock levels, payments) mixes payments in unrelated services; C is a monolith; D over-splits causing low cohesion.

3. Final Answer:

   UserService (user profiles, authentication), OrderService (orders, payments), ShippingService (shipping updates, tracking) -> Option B

4. Quick Check:

   Group related domain tasks for high cohesion [OK]

Hint: Group by domain responsibilities for high cohesion [OK]

Common Mistakes:

• Mixing unrelated tasks in one service
• Creating too many tiny services
• Building monolithic services
• Ignoring domain boundaries