Why Incremental migration plan in Microservices? - Purpose & Use Cases

The Big Idea

What if you could upgrade your whole system without breaking a thing?

The Scenario

Imagine a company with a huge, old software system that runs everything. They want to switch to smaller, faster parts called microservices. But if they try to change everything at once, the system might break, and customers could get upset.

The Problem

Trying to rebuild the whole system in one go is risky and slow. It's like trying to change all the tires on a moving car. Mistakes can cause downtime, lost data, and frustrated users. It's hard to fix problems quickly because everything is connected tightly.

The Solution

An incremental migration plan breaks the big change into small steps. You move one part at a time to microservices, test it, and make sure it works before moving on. This way, the system keeps running smoothly, and problems are easier to find and fix.

Before vs After

✗ Before

Rewrite entire monolith at once
Deploy big update
Fix many bugs under pressure

✓ After

Extract one feature to microservice
Test and deploy safely
Repeat step-by-step

What It Enables

It lets teams upgrade complex systems safely and quickly without stopping the whole business.

Real Life Example

A bank moves its payment processing from a big old system to microservices one function at a time, so customers can keep paying bills without interruption.

Key Takeaways

Big system changes are risky if done all at once.

Incremental migration breaks changes into small, safe steps.

This approach keeps systems stable and users happy during upgrades.

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