Routing and load balancing in Microservices - Architecture Diagram

System Overview - Routing and load balancing

This system manages incoming user requests by distributing them evenly across multiple service instances. It ensures high availability and scalability by routing traffic through a load balancer that directs requests to healthy microservice instances.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  +-----> Service Instance 1
  |
  +-----> Service Instance 2
  |
  +-----> Service Instance 3
  |
  v
Database

Components

User

client

Sends requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly across service instances

Service Instance 1

service

Processes user requests

Service Instance 2

service

Processes user requests

Service Instance 3

service

Processes user requests

Database

database

Stores persistent data for services

Request Flow - 6 Hops

User → Load Balancer

Load Balancer → Service Instance 2

Service Instance 2 → Database

Database → Service Instance 2

Service Instance 2 → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Load Balancer

Impact:All incoming user requests fail as no routing is possible

Mitigation:Use multiple load balancers with failover or DNS-based load balancing to avoid single point of failure

Architecture Quiz - 3 Questions

What is the main role of the load balancer in this system?

A Store user data persistently

B Distribute incoming requests evenly across service instances

C Process database queries

D Send responses directly to the database

Design Principle

This architecture uses a load balancer to distribute user requests evenly across multiple service instances, improving scalability and availability. It prevents any single service instance from becoming a bottleneck and allows the system to handle failures gracefully by routing around unhealthy instances.

Practice

1. What is the main purpose of routing in a microservices architecture?

easy

A. To store data persistently across services

B. To monitor the health of microservices

C. To encrypt communication between services

D. To send requests to the correct microservice based on rules

Solution

1. Step 1: Understand routing role

   Routing directs incoming requests to the right microservice based on predefined rules like URL paths or headers.

2. Step 2: Differentiate routing from other functions

   Storing data, encrypting communication, and monitoring are separate concerns handled by databases, security layers, and monitoring tools respectively.

3. Final Answer:

   To send requests to the correct microservice based on rules -> Option D

4. Quick Check:

   Routing = directing requests [OK]

Hint: Routing directs requests to the right service [OK]

Common Mistakes:

• Confusing routing with data storage
• Mixing routing with security or monitoring
• Thinking routing balances load

2. Which of the following is a correct syntax for defining a load balancer rule that forwards requests to multiple instances evenly?

easy

A. round_robin: [instance1, instance2, instance3]

B. loadbalance = {instance1; instance2; instance3}

C. balance->instances(instance1, instance2, instance3)

D. forward: instance1 | instance2 | instance3

Solution

1. Step 1: Identify common load balancing syntax

   Round robin is a standard load balancing method cycling through instances evenly, often expressed as a list.

2. Step 2: Evaluate options for correct syntax style

   round_robin: [instance1, instance2, instance3] uses a clear list with round_robin keyword, matching common config styles. Others use invalid or uncommon syntax.

3. Final Answer:

   round_robin: [instance1, instance2, instance3] -> Option A

4. Quick Check:

   Round robin uses list syntax [OK]

Hint: Look for standard list syntax with round robin keyword [OK]

Common Mistakes:

• Using semicolons instead of commas
• Incorrect assignment operators
• Using arrows or pipes incorrectly

3. Given the following pseudo-code for a load balancer using weighted routing:

weights = {"serviceA": 3, "serviceB": 1}
requests = 8
for i in range(requests):
    target = weighted_choice(weights)
    print(target)

What is the expected number of requests routed to serviceA?

medium

A. 6

B. 8

C. 4

D. 2

Solution

1. Step 1: Understand weighted routing concept

   Weights define how many times a service should receive requests relative to others. ServiceA has weight 3, serviceB has weight 1, total weight is 4.

2. Step 2: Calculate expected requests for serviceA

   Out of 8 requests, serviceA should get (3/4)*8 = 6 requests on average.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Weighted share = 6 requests [OK]

Hint: Multiply total requests by service weight fraction [OK]

Common Mistakes:

• Ignoring weights and dividing requests equally
• Confusing total weight with individual weights
• Calculating requests for serviceB instead

4. A load balancer is configured with the following rule:

if (instance.isHealthy()) {
  forwardRequest(instance)
} else {
  skipInstance(instance)
}

However, requests are still being sent to unhealthy instances. What is the most likely cause?

medium

A. Instances are overloaded but still marked healthy

B. Health check logic is not integrated with the load balancer

C. Load balancer is using round robin instead of weighted routing

D. Routing rules are missing URL path matching

Solution

1. Step 1: Analyze health check integration

   The code shows a health check condition, but if the load balancer does not actually use this logic, unhealthy instances may still receive traffic.

2. Step 2: Evaluate other options for relevance

   Round robin vs weighted routing does not affect health checks. Overload does not mark instances unhealthy. URL path matching is unrelated to health status.

3. Final Answer:

   Health check logic is not integrated with the load balancer -> Option B

4. Quick Check:

   Health check integration = key [OK]

Hint: Check if health logic is actually used by load balancer [OK]

Common Mistakes:

• Assuming routing method affects health checks
• Confusing overload with health status
• Ignoring missing integration of health logic

5. You need to design a routing and load balancing system for a microservices app that handles both user requests and background jobs. User requests must be routed based on URL paths, and load balanced evenly. Background jobs should be routed to a separate set of instances with weighted load balancing. Which architecture best fits this requirement?

hard

A. Use DNS-based routing to split traffic, then apply round robin load balancing on all instances

B. Deploy two separate load balancers, one for user requests with weighted balancing, another for jobs with round robin

C. Use a single load balancer with path-based routing directing to two target groups; one uses round robin, the other weighted balancing

D. Route all traffic to a single instance that forwards requests internally based on type

Solution

1. Step 1: Identify routing needs for user requests and jobs

   User requests require path-based routing to separate them from background jobs, which need different load balancing strategies.

2. Step 2: Choose architecture supporting both routing and load balancing rules

   A single load balancer with path-based routing can direct traffic to two target groups. One group uses round robin for user requests, the other weighted for jobs, meeting all requirements efficiently.

3. Final Answer:

   Use a single load balancer with path-based routing directing to two target groups; one uses round robin, the other weighted balancing -> Option C

4. Quick Check:

   Path-based routing + mixed balancing = Use a single load balancer with path-based routing directing to two target groups; one uses round robin, the other weighted balancing [OK]

Hint: Combine path routing with separate load balancing per target group [OK]

Common Mistakes:

• Using weighted balancing for user requests instead of round robin
• Splitting with DNS which lacks path awareness
• Routing all traffic to one instance causing bottlenecks