Parallel running in Microservices - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is parallel running in system design?

Parallel running is a method where two systems operate simultaneously to compare results and ensure the new system works correctly before fully replacing the old one.

beginner

Why is parallel running important in microservices migration?

It helps verify that the new microservices behave as expected without disrupting the existing system, reducing risk during transition.

intermediate

Name a key challenge when implementing parallel running.

Synchronizing data and requests between old and new systems to ensure consistent results can be complex and resource-intensive.

intermediate

How does parallel running differ from a blue-green deployment?

Parallel running runs both systems fully and compares outputs, while blue-green switches traffic between two environments without running both fully in production.

beginner

What is a common indicator that parallel running is successful?

When outputs from both old and new systems match consistently over a period, showing the new system is reliable.

What is the main goal of parallel running?

A To replace the old system immediately

B To reduce system resources by shutting down old services

C To run two systems simultaneously to verify correctness

D To deploy only one system at a time

Parallel running involves running both old and new systems together to compare results and ensure the new system works correctly.

Which of the following is a challenge in parallel running?

A Ensuring data synchronization between systems

B Ignoring old system outputs

C Deploying only one microservice

D Running systems at different times

Synchronizing data and requests between old and new systems is a key challenge in parallel running.

Parallel running is most useful during which phase?

A Initial system design

B System migration or upgrade

C System decommissioning

D Performance tuning

Parallel running is used during migration or upgrade to validate the new system before full cutover.

How does parallel running reduce risk?

A By allowing comparison of outputs before full switch

B By immediately shutting down the old system

C By ignoring errors in the new system

D By running only the new system

It reduces risk by running both systems and comparing outputs to catch issues early.

Which deployment strategy is closest to parallel running?

A Canary deployment

B Blue-green deployment

C Rolling update

D Shadow testing

Shadow testing runs new code alongside old without affecting users, similar to parallel running's simultaneous operation.

Explain the concept of parallel running and why it is used in microservices migration.

Think about running two versions side by side to compare results.

Describe the main challenges faced when implementing parallel running in a microservices environment.

Consider what makes running two systems at once difficult.

Practice

1. What is the main purpose of parallel running in microservices?

easy

A. To run old and new systems together to ensure smooth transition

B. To replace the old system immediately without testing

C. To run only the new system and discard the old one

D. To run multiple unrelated services in parallel

Solution

1. Step 1: Understand the concept of parallel running

   Parallel running means running old and new systems side by side to compare their outputs and ensure the new system works correctly.

2. Step 2: Identify the purpose in microservices

   This approach helps catch errors and ensures a smooth transition before fully switching to the new system.

3. Final Answer:

   To run old and new systems together to ensure smooth transition -> Option A

4. Quick Check:

   Parallel running = run old and new systems together [OK]

Hint: Parallel running means running old and new systems side by side [OK]

Common Mistakes:

• Thinking parallel running means immediate replacement
• Confusing parallel running with running unrelated services
• Assuming old system is discarded immediately

2. Which of the following is the correct way to implement parallel running in a microservices upgrade?

easy

A. Deploy new microservice version alongside old one and route a copy of requests to both

B. Stop old microservice and deploy new one immediately

C. Deploy new microservice and ignore old service logs

D. Run new microservice only during off-peak hours

Solution

1. Step 1: Understand deployment in parallel running

   Parallel running requires both old and new versions to run simultaneously to compare results.

2. Step 2: Identify correct routing method

   Routing a copy of requests to both versions allows output comparison without disrupting users.

3. Final Answer:

   Deploy new microservice version alongside old one and route a copy of requests to both -> Option A

4. Quick Check:

   Parallel running = deploy both and route requests to both [OK]

Hint: Route requests to both old and new services in parallel [OK]

Common Mistakes:

• Stopping old service before testing new one
• Ignoring logs from old service
• Running new service only at specific times

3. Consider a microservice system where requests are sent to both old and new versions during parallel running. If the old service returns response A and the new service returns response B, what should the system do?

medium

A. Ignore the difference and continue using the new service

B. Switch back to the old service permanently

C. Stop the old service immediately

D. Log the difference and alert engineers for investigation

Solution

1. Step 1: Understand output comparison in parallel running

   Parallel running compares outputs to detect discrepancies between old and new services.

2. Step 2: Decide action on output mismatch

   If outputs differ, the system should log the difference and alert engineers to investigate before switching fully.

3. Final Answer:

   Log the difference and alert engineers for investigation -> Option D

4. Quick Check:

   Output mismatch = log and alert [OK]

Hint: Log and alert on output differences during parallel running [OK]

Common Mistakes:

• Ignoring output differences
• Stopping old service too early
• Switching back permanently without investigation

4. A team implemented parallel running but noticed that the new service never receives any requests. What is the most likely cause?

medium

A. The new service crashed immediately after deployment

B. The routing logic is only sending requests to the old service

C. The old service is not logging requests

D. The new service is slower than the old one

Solution

1. Step 1: Analyze routing in parallel running

   For parallel running, requests must be routed to both old and new services simultaneously.

2. Step 2: Identify why new service gets no requests

   If new service never receives requests, routing likely sends all traffic only to old service.

3. Final Answer:

   The routing logic is only sending requests to the old service -> Option B

4. Quick Check:

   No requests to new service = routing issue [OK]

Hint: Check routing logic if new service gets no requests [OK]

Common Mistakes:

• Assuming new service crashed without checking logs
• Blaming old service logs
• Thinking speed affects request routing

5. You are designing a parallel running strategy for a microservices system with high traffic. Which approach best balances safety and performance?

hard

A. Route 100% of traffic to new service and keep old service idle

B. Run new service only during low traffic hours without output comparison

C. Route 10% of traffic to new service and 90% to old service, compare outputs, then gradually increase new service traffic

D. Stop old service immediately and monitor new service logs

Solution

1. Step 1: Understand gradual traffic shifting in parallel running

   Gradually increasing traffic to the new service while comparing outputs reduces risk and performance impact.

2. Step 2: Evaluate options for safety and performance

   Routing a small portion initially and increasing after validation balances safety and system load.

3. Final Answer:

   Route 10% of traffic to new service and 90% to old service, compare outputs, then gradually increase new service traffic -> Option C

4. Quick Check:

   Gradual traffic shift with output comparison = safe and performant [OK]

Hint: Start small traffic to new service, compare, then increase [OK]

Common Mistakes:

• Switching 100% traffic immediately
• Skipping output comparison
• Stopping old service too early