Incremental migration plan in Microservices - Architecture Diagram

System Overview - Incremental migration plan

This system shows how to gradually move from a monolithic application to microservices. The goal is to keep the system working during migration, reduce risks, and improve scalability step-by-step.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  +-------------------+-------------------+
  |                   |                   |
Monolith Service 1  Microservice A     Microservice B
  |                   |                   |
Database (Monolith)  Database A         Database B
  |                   |                   |
Cache (Shared)     Cache A             Cache B

Components

User

client

End user sending requests

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to monolith or microservices based on migration status

Monolith Service 1

service

Original monolithic application handling some features

Microservice A

service

New microservice handling a migrated feature

Microservice B

service

Another new microservice handling another migrated feature

Database (Monolith)

database

Original monolith database storing legacy data

Database A

database

Database for Microservice A

Database B

database

Database for Microservice B

Cache (Shared)

cache

Shared cache used by monolith and microservices for fast data access

Cache A

cache

Cache dedicated to Microservice A

Cache B

cache

Cache dedicated to Microservice B

Request Flow - 12 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Monolith Service 1

API Gateway → Microservice A

Microservice A → Cache A

Microservice A → Database A

Monolith Service 1 → Cache (Shared)

Monolith Service 1 → Database (Monolith)

Microservice B → Cache B

Microservice B → Database B

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Database A

Impact:Microservice A cannot read or write data, causing feature unavailability. Cache may serve stale data but updates fail.

Mitigation:Use database replication and failover to standby database. Implement circuit breaker in microservice to degrade gracefully.

Architecture Quiz - 3 Questions

Which component decides whether to send a request to the monolith or a microservice?

A Cache

B API Gateway

C Load Balancer

D Database

Design Principle

This architecture shows how to migrate a monolith to microservices step-by-step by routing requests through an API Gateway. It uses caching to reduce database load and isolates databases per microservice to improve scalability and fault tolerance.

Practice

1. What is the main goal of an incremental migration plan in microservices?

easy

A. To avoid testing during migration

B. To rewrite the entire system at once

C. To remove all old services immediately

D. To move functionality step-by-step to reduce risk

Solution

1. Step 1: Understand migration goals

   Incremental migration aims to reduce risk by breaking changes into small steps.

2. Step 2: Compare options

   Options B, C, and D involve big changes or skipping testing, which increase risk.

3. Final Answer:

   To move functionality step-by-step to reduce risk -> Option D

4. Quick Check:

   Incremental migration = step-by-step safe moves [OK]

Hint: Think small safe steps, not big risky jumps [OK]

Common Mistakes:

• Assuming migration happens all at once
• Ignoring the need for testing
• Believing old services must be removed immediately

2. Which of the following is a correct step in an incremental migration plan?

easy

A. Deploy all new microservices simultaneously without routing changes

B. Use feature flags or routing to direct some traffic to new services

C. Stop the old system before starting migration

D. Skip monitoring during migration to save resources

Solution

1. Step 1: Identify safe deployment practices

   Using feature flags or routing allows gradual traffic shift to new services safely.

2. Step 2: Eliminate unsafe options

   Deploying all at once, stopping old system early, or skipping monitoring are risky.

3. Final Answer:

   Use feature flags or routing to direct some traffic to new services -> Option B

4. Quick Check:

   Routing traffic gradually = safe migration [OK]

Hint: Use routing or flags to control traffic flow [OK]

Common Mistakes:

• Deploying everything at once
• Stopping old system too early
• Ignoring monitoring during migration

3. Consider this migration step code snippet for routing traffic:

if (user.isBetaTester) {
  routeToNewService();
} else {
  routeToOldService();
}

What will happen if a user is not a beta tester?

medium

A. User traffic is dropped

B. User traffic goes to the new service

C. User traffic goes to the old service

D. User traffic causes an error

Solution

1. Step 1: Analyze the condition

   If user.isBetaTester is false, the else branch runs.

2. Step 2: Determine routing for else branch

   The else branch calls routeToOldService(), so traffic goes to old service.

3. Final Answer:

   User traffic goes to the old service -> Option C

4. Quick Check:

   Non-beta users = old service routing [OK]

Hint: False condition triggers else branch routing [OK]

Common Mistakes:

• Assuming all users go to new service
• Thinking traffic is dropped or errors occur
• Ignoring the else branch logic

4. A team started migrating a service incrementally but suddenly disabled monitoring. What is the likely problem?

medium

A. They lose visibility into errors and performance

B. They can detect issues faster

C. Migration speed increases without risks

D. Old services automatically update

Solution

1. Step 1: Understand monitoring role

   Monitoring helps detect errors and performance issues during migration.

2. Step 2: Assess impact of disabling monitoring

   Without monitoring, the team loses visibility into problems, increasing risk.

3. Final Answer:

   They lose visibility into errors and performance -> Option A

4. Quick Check:

   No monitoring = no error visibility [OK]

Hint: Never disable monitoring during migration [OK]

Common Mistakes:

• Assuming disabling monitoring improves speed
• Thinking old services update automatically
• Believing issues are easier to detect without monitoring

5. You plan to migrate a monolith to microservices incrementally. Which approach best ensures minimal downtime and rollback capability?

hard

A. Deploy new microservices behind a feature flag and route a small % of traffic gradually

B. Replace the monolith entirely in one deployment window

C. Migrate database schema all at once without backward compatibility

D. Disable old services immediately after deploying new ones

Solution

1. Step 1: Evaluate migration strategies

   Deploying behind feature flags and routing small traffic allows gradual testing and rollback.

2. Step 2: Compare risks of other options

   Replacing all at once or disabling old services causes downtime; schema changes without compatibility break systems.

3. Final Answer:

   Deploy new microservices behind a feature flag and route a small % of traffic gradually -> Option A

4. Quick Check:

   Feature flags + gradual traffic = safe migration [OK]

Hint: Use feature flags and gradual traffic shift for safety [OK]

Common Mistakes:

• Trying big-bang replacement causing downtime
• Ignoring backward compatibility in database changes
• Disabling old services too early