High availability design patterns in AWS - Step-by-Step Execution

Process Flow - High availability design patterns

Start: User Request

↓

Route to Load Balancer

↓

Distribute to Multiple Instances

↓

Health Check on Instances

↓

Instance Healthy?

No→Remove from Pool

Yes↓

Serve Request

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Monitor & Auto-Recover

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End: Request Served

User requests go through a load balancer that distributes traffic to healthy instances, ensuring continuous service by monitoring and replacing unhealthy ones.

Execution Sample

AWS

User sends request
Load balancer receives request
Load balancer checks instance health
Routes request to healthy instance
Instance serves request
Monitor detects failure
Auto-recovery replaces failed instance

This flow shows how a request is handled in a high availability setup with load balancing and health checks.

Process Table

Step	Action	Condition/Check	Result	Next Step
1	User sends request	N/A	Request received by load balancer	Route to load balancer
2	Load balancer receives request	N/A	Ready to distribute	Check instance health
3	Load balancer checks instance health	Instance A healthy?	Yes	Route to Instance A
4	Load balancer checks instance health	Instance B healthy?	No	Remove Instance B from pool
5	Route request to Instance A	N/A	Instance A serves request	Monitor instances
6	Monitor detects failure	Instance B failed?	Yes	Auto-recover Instance B
7	Auto-recover Instance B	N/A	Instance B replaced and healthy	Add Instance B back to pool
8	Add Instance B back to pool	N/A	Pool restored	Ready for next request
9	End	N/A	Request served successfully	Stop

💡 Request served successfully after routing to healthy instances and recovering failed ones

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	After Step 7	Final
Instance A Health	Unknown	Healthy	Healthy	Healthy	Healthy	Healthy
Instance B Health	Unknown	Healthy	Unhealthy	Failed	Healthy	Healthy
Load Balancer Pool	Empty	Includes A and B	Includes A only	Includes A only	Includes A and B	Includes A and B

Key Moments - 3 Insights

Why does the load balancer remove an instance from the pool?

Because the health check shows the instance is unhealthy (see step 4 in execution_table), removing it prevents sending requests to a failing server.

How does auto-recovery help maintain availability?

Auto-recovery replaces failed instances automatically (step 7), restoring the pool to full capacity and ensuring requests can be served without interruption.

What happens if all instances fail health checks?

The load balancer has no healthy targets to route requests to, causing service disruption until instances recover or are replaced.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is Instance B removed from the load balancer pool?

A Step 3

B Step 6

C Step 4

D Step 7

💡 Hint

Check the 'Result' column for removal actions in execution_table row 4.

According to variable_tracker, what is the health status of Instance B after step 7?

A Healthy

B Unhealthy

C Failed

D Recovering

💡 Hint

Look at the 'Instance B Health' row under 'After Step 7' in variable_tracker.

If the load balancer did not perform health checks, what would likely happen?

A Load balancer would remove all instances

B Requests might be sent to unhealthy instances causing failures

C Auto-recovery would not be triggered

D Requests would be evenly distributed without issue

💡 Hint

Refer to the importance of health checks in execution_table steps 3 and 4.

Concept Snapshot

High availability design uses load balancers to distribute traffic
Health checks monitor instance status and remove unhealthy ones
Auto-recovery replaces failed instances automatically
This ensures continuous service despite failures
Key components: Load balancer, health checks, multiple instances, auto-recovery

Full Transcript

High availability design patterns in AWS involve routing user requests through a load balancer that distributes traffic to multiple instances. The load balancer performs health checks to ensure only healthy instances receive traffic. If an instance fails, it is removed from the pool to prevent failed requests. Monitoring detects failures and triggers auto-recovery to replace unhealthy instances. This cycle maintains continuous service availability by handling failures transparently. The process starts with a user request, routes through the load balancer, checks instance health, serves the request from a healthy instance, monitors for failures, and recovers failed instances automatically.