Why gradual migration reduces risk in Microservices - Architecture Impact

System Overview - Why gradual migration reduces risk

This system shows how a gradual migration from a monolithic application to microservices reduces risk. Instead of switching all at once, parts of the system are moved step-by-step. This allows testing and fixing issues in small pieces, avoiding big failures.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  +---------------------+
  |                     |
Monolith Service    New Microservice
  |                     |
Database (Shared)   Database (New)
  |
Cache

Components

User

user

End user who sends requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to either the monolith or new microservice based on migration progress

Monolith Service

service

Original application handling most requests before migration

New Microservice

service

Newly developed microservice handling migrated features

Database (Shared)

database

Database used by the monolith and partially by microservices during migration

Database (New)

database

New database dedicated to the microservice for isolated data management

Cache

cache

Speeds up data retrieval for both monolith and microservices

Request Flow - 10 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Monolith Service or New Microservice

Monolith Service or New Microservice → Cache

Cache → Monolith Service or New Microservice

Monolith Service or New Microservice → Database (Shared or New)

Database (Shared or New) → Monolith Service or New Microservice

Monolith Service or New Microservice → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:New Microservice

Impact:Requests routed to the new microservice fail, causing partial service disruption for migrated features.

Mitigation:API Gateway routes affected requests back to the monolith service as a fallback until the microservice is fixed.

Architecture Quiz - 3 Questions

Why does the API Gateway route some requests to the monolith and others to the new microservice?

A Because the monolith is faster than microservices

B To balance load evenly between services

C To gradually migrate features and reduce risk

D To avoid using the cache

Design Principle

Gradual migration reduces risk by allowing parts of the system to be moved step-by-step. This approach enables testing, monitoring, and rollback of small changes without disrupting the entire system, improving stability and reliability during transition.

Practice

1. Why is gradual migration preferred when moving from a monolithic system to microservices?

easy

A. It eliminates the need for testing after migration.

B. It speeds up the migration by doing everything at once.

C. It reduces risk by allowing small, testable changes.

D. It requires no changes to the existing system.

Solution

1. Step 1: Understand the risks of big changes

   Big changes done all at once can cause failures and downtime.

2. Step 2: See how gradual migration helps

   Breaking changes into small steps allows testing and fixing early, reducing risk.

3. Final Answer:

   It reduces risk by allowing small, testable changes. -> Option C

4. Quick Check:

   Gradual migration = smaller risk [OK]

Hint: Small steps mean fewer surprises and easier fixes [OK]

Common Mistakes:

• Thinking migration is faster if done all at once
• Believing testing is unnecessary during migration
• Assuming no system changes are needed

2. Which of the following is a correct practice during gradual migration to microservices?

easy

A. Remove the old system immediately after starting migration.

B. Deploy all microservices at once without testing.

C. Skip monitoring to save resources during migration.

D. Migrate one service at a time and test thoroughly.

Solution

1. Step 1: Identify correct migration practices

   Gradual migration means moving one part at a time with testing.

2. Step 2: Evaluate options

   Only migrating one service at a time and testing fits gradual migration best.

3. Final Answer:

   Migrate one service at a time and test thoroughly. -> Option D

4. Quick Check:

   One service + test = gradual migration [OK]

Hint: Migrate and test one service at a time [OK]

Common Mistakes:

• Deploying all services simultaneously
• Ignoring monitoring during migration
• Removing old system too early

3. Consider this migration plan code snippet:

services = ['auth', 'payment', 'order']
migrated = []
for s in services:
    migrate_service(s)
    migrated.append(s)
    if not test_service(s):
        rollback_service(s)
        break
print(migrated)

What will be the output if test_service('payment') returns False?

medium

A. ['auth', 'payment']

B. ['auth', 'payment', 'order']

C. []

D. ['auth']

Solution

1. Step 1: Trace migration and testing

   'auth' migrates, appends to migrated, tests OK. 'payment' migrates and appends to migrated.

2. Step 2: Rollback and break loop

   On test failure for 'payment', rollback happens but 'payment' was already appended, then loop breaks.

3. Final Answer:

   ['auth', 'payment'] -> Option A

4. Quick Check:

   Appends before test, so includes failed service [OK]

Hint: Stop migration on test failure, rollback last service [OK]

Common Mistakes:

• Thinking failed service is not added to migrated list
• Ignoring append before test
• Continuing migration after failure

4. A team tries to migrate microservices gradually but faces downtime during migration. What is the most likely mistake?

medium

A. They did not maintain backward compatibility during migration.

B. They migrated services one by one with testing.

C. They monitored the system during migration.

D. They rolled back failing services immediately.

Solution

1. Step 1: Understand downtime causes in gradual migration

   Downtime often occurs if new services are incompatible with old ones.

2. Step 2: Identify mistake

   Not maintaining backward compatibility breaks communication causing downtime.

3. Final Answer:

   They did not maintain backward compatibility during migration. -> Option A

4. Quick Check:

   Compatibility issues cause downtime [OK]

Hint: Keep old and new services compatible to avoid downtime [OK]

Common Mistakes:

• Assuming testing alone prevents downtime
• Ignoring backward compatibility
• Believing monitoring causes downtime

5. You are designing a gradual migration plan for a large e-commerce system. Which approach best reduces risk while ensuring continuous service?

hard

A. Migrate all payment-related services first, then all user services, without fallback.

B. Migrate one microservice at a time with automated tests and fallback mechanisms.

C. Switch completely to microservices overnight to avoid prolonged complexity.

D. Disable monitoring during migration to improve performance.

Solution

1. Step 1: Analyze migration strategies

   Migrating all services of one type at once risks big failures; overnight switch is risky.

2. Step 2: Evaluate best practice

   One service at a time with tests and fallback reduces risk and keeps system running.

3. Final Answer:

   Migrate one microservice at a time with automated tests and fallback mechanisms. -> Option B

4. Quick Check:

   Small steps + tests + fallback = low risk [OK]

Hint: One service, test, fallback = safe migration [OK]

Common Mistakes:

• Migrating large groups without fallback
• Doing full overnight switch
• Disabling monitoring during migration