Fallback pattern in Microservices - Architecture Diagram

System Overview - Fallback pattern

The fallback pattern helps microservices handle failures gracefully by providing alternative responses when a service is down or slow. It ensures the system remains responsive and improves user experience by avoiding complete failure.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Service A (Primary Service) <-----> Cache
  |
  v
Fallback Service (Backup Service)
  |
  v
Database

Components

User

client

Initiates requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to services and applies fallback logic

Service A (Primary Service)

service

Main service handling business logic and data retrieval

Cache

cache

Stores recent responses to reduce load and latency

Fallback Service (Backup Service)

service

Provides alternative responses when primary service fails

Database

database

Stores persistent data for services

Request Flow - 12 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Cache

Cache → API Gateway

API Gateway → Service A (Primary Service)

Service A (Primary Service) → Database

Database → Service A (Primary Service)

Service A (Primary Service) → Cache

Service A (Primary Service) → API Gateway

API Gateway → User

API Gateway → Fallback Service (Backup Service)

Fallback Service (Backup Service) → API Gateway

Failure Scenario

Component Fails:Service A (Primary Service)

Impact:Primary service is unavailable or slow, causing request failures or delays

Mitigation:API Gateway detects failure or timeout and routes requests to Fallback Service to provide alternative responses, maintaining system responsiveness

Architecture Quiz - 3 Questions

Which component decides to use the fallback service when the primary service fails?

A Cache

B Load Balancer

C API Gateway

D Database

Design Principle

The fallback pattern improves system resilience by providing alternative responses when a primary service fails. It uses the API Gateway to detect failures and route requests to a backup service, ensuring users receive timely responses even during partial outages.

Practice

1. What is the main purpose of the fallback pattern in microservices?

easy

A. To provide a backup response when a service call fails

B. To increase the number of service calls

C. To replace the main service permanently

D. To log all service requests for auditing

Solution

1. Step 1: Understand the fallback pattern role

   The fallback pattern is designed to handle failures gracefully by providing an alternative response.

2. Step 2: Identify the main goal

   Its main goal is to keep the system responsive and avoid cascading failures by returning backup data or default messages.

3. Final Answer:

   To provide a backup response when a service call fails -> Option A

4. Quick Check:

   Fallback pattern = backup response [OK]

Hint: Fallback means backup response on failure [OK]

Common Mistakes:

• Thinking fallback increases service calls
• Confusing fallback with permanent service replacement
• Assuming fallback is for logging only

2. Which of the following is a correct way to implement a fallback method in a microservice?

easy

A. Ignore the failure and return an error to the user

B. Call the main service repeatedly until it succeeds

C. Return cached data or a default message when the main service fails

D. Restart the entire microservice on failure

Solution

1. Step 1: Review fallback implementation options

   Fallback should provide a quick alternative response like cached data or default messages.

2. Step 2: Eliminate incorrect options

   Repeated calls can cause delays, ignoring failure hurts user experience, and restarting service is costly and slow.

3. Final Answer:

   Return cached data or a default message when the main service fails -> Option C

4. Quick Check:

   Fallback = cached or default response [OK]

Hint: Fallback returns cached or default data on failure [OK]

Common Mistakes:

• Retrying endlessly instead of fallback
• Returning errors instead of fallback data
• Restarting services unnecessarily

3. Consider this pseudocode for a microservice call with fallback:

response = callMainService()
if response.failed:
    response = fallbackResponse()
print(response)

What will be printed if callMainService() fails?

medium

A. The fallback response

B. The original failed response

C. An error message and no response

D. Nothing, the program crashes

Solution

1. Step 1: Analyze the failure condition

   If callMainService() fails, the code assigns fallbackResponse() to response.

2. Step 2: Determine printed output

   The printed output will be the fallback response, not the failed original response or an error.

3. Final Answer:

   The fallback response -> Option A

4. Quick Check:

   Failed main call triggers fallback output [OK]

Hint: Failed call triggers fallback print [OK]

Common Mistakes:

• Assuming failed response is printed
• Expecting program crash on failure
• Confusing fallback with error message

4. This code snippet tries to implement a fallback but has a bug:

def get_data():
    try:
        return call_service()
    except:
        call_fallback()

What is the bug here?

medium

A. The code does not catch exceptions

B. The try block does not call the service

C. The except block should raise an error

D. The fallback function is not returned

Solution

1. Step 1: Check try-except behavior

   The try block returns the service call result, but except calls fallback without returning it.

2. Step 2: Identify missing return

   Without returning fallback's result, the function returns None on failure instead of fallback data.

3. Final Answer:

   The fallback function is not returned -> Option D

4. Quick Check:

   Missing return in except causes None [OK]

Hint: Always return fallback result in except block [OK]

Common Mistakes:

• Forgetting to return fallback data
• Misunderstanding try-except flow
• Assuming fallback raises error

5. You design a microservice that calls a payment gateway. To avoid delays, you want to use the fallback pattern. Which fallback strategy is best to keep the system responsive and safe?

hard

A. Return a generic error message without fallback

B. Return a cached success response immediately and update later asynchronously

C. Retry the payment gateway call 10 times before fallback

D. Restart the payment microservice on failure

Solution

1. Step 1: Understand fallback goals for payment service

   Fallback should keep system responsive and avoid blocking user with delays.

2. Step 2: Evaluate options for responsiveness and safety

   Returning cached success immediately and updating asynchronously balances responsiveness and eventual consistency.

3. Step 3: Eliminate risky or slow options

   Retries cause delays, generic errors hurt UX, restarting service is costly and slow.

4. Final Answer:

   Return a cached success response immediately and update later asynchronously -> Option B

5. Quick Check:

   Cached immediate fallback with async update = best practice [OK]

Hint: Use cached immediate fallback plus async update [OK]

Common Mistakes:

• Excessive retries causing delays
• No fallback causing poor user experience
• Restarting services on failure