Cross-zone load balancing in AWS - Step-by-Step Execution

Process Flow - Cross-zone load balancing

Client sends request

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Load Balancer receives request

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Check cross-zone load balancing setting

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Distribute evenly

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Forward request

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Target instances process request

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Response sent back

The load balancer receives a request and decides how to distribute it based on cross-zone load balancing setting. If enabled, it spreads requests evenly across all zones; if disabled, it sends requests only to targets in the same zone as the client.

Execution Sample

AWS

Client -> Load Balancer
Load Balancer: cross-zone enabled = true
Targets in Zone A: 3
Targets in Zone B: 2
Distribute requests evenly across zones

This setup shows a load balancer with cross-zone load balancing enabled distributing client requests evenly across targets in multiple zones.

Process Table

Step	Action	Cross-zone Enabled?	Targets in Zone A	Targets in Zone B	Request Distribution	Result
1	Client sends request	true	3	2	N/A	Request received by load balancer
2	Load balancer checks setting	true	3	2	Enabled	Will distribute evenly across zones
3	Distribute request 1	true	3	2	Evenly	Sent to Zone A target 1
4	Distribute request 2	true	3	2	Evenly	Sent to Zone B target 1
5	Distribute request 3	true	3	2	Evenly	Sent to Zone A target 2
6	Distribute request 4	true	3	2	Evenly	Sent to Zone B target 2
7	Distribute request 5	true	3	2	Evenly	Sent to Zone A target 3
8	All requests distributed	true	3	2	Evenly	Requests balanced across zones
9	Client sends request	false	3	2	N/A	Request received by load balancer
10	Load balancer checks setting	false	3	2	Disabled	Will distribute only to targets in client's zone
11	Client in Zone A sends request	false	3	2	Zone A only	Sent to Zone A target 1
12	Client in Zone B sends request	false	3	2	Zone B only	Sent to Zone B target 1
13	Requests distributed per zone	false	3	2	Zone-specific	Requests not balanced across zones

💡 Execution stops after requests are distributed according to cross-zone load balancing setting.

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	Final
Cross-zone Enabled	true	true	true	true	true	true	false
Requests sent to Zone A targets	0	1	1	2	2	3	3
Requests sent to Zone B targets	0	0	1	1	2	2	2

Key Moments - 2 Insights

Why does the load balancer send requests only to targets in the client's zone when cross-zone load balancing is disabled?

When cross-zone load balancing is disabled (see execution_table rows 10-13), the load balancer restricts requests to targets in the same zone as the client to reduce cross-zone data transfer costs.

How does enabling cross-zone load balancing affect request distribution?

Enabling cross-zone load balancing (execution_table rows 2-8) causes the load balancer to distribute requests evenly across all healthy targets in all zones, improving load distribution.

Visual Quiz - 3 Questions

Look at the execution_table at step 5. Which target receives the request?

A Zone B target 1

B Zone A target 2

C Zone A target 1

D Zone B target 2

💡 Hint

Check the 'Request Distribution' and 'Result' columns at step 5 in execution_table.

At which step does the load balancer distribute requests only to targets in the client's zone?

A Step 2

B Step 6

C Step 10

D Step 8

💡 Hint

Look for when 'Cross-zone Enabled?' is false and distribution is zone-specific in execution_table.

If cross-zone load balancing is enabled, how many requests are sent to Zone B targets after 7 requests?

A 2

B 3

C 4

D 1

💡 Hint

Refer to variable_tracker row 'Requests sent to Zone B targets' after step 7.

Concept Snapshot

Cross-zone load balancing controls how a load balancer distributes requests across availability zones.
Enabled: requests spread evenly across all zones' targets.
Disabled: requests sent only to targets in the client's zone.
Improves fault tolerance and load distribution.
May increase cross-zone data transfer costs when enabled.

Full Transcript

Cross-zone load balancing is a setting on a load balancer that decides how client requests are distributed across multiple availability zones. When enabled, the load balancer sends requests evenly to all healthy targets in all zones, balancing the load well. When disabled, it sends requests only to targets in the same zone as the client, which can reduce cross-zone data transfer costs. The execution table shows step-by-step how requests are distributed differently based on this setting. The variable tracker shows how many requests each zone's targets receive over time. Key moments clarify why the load balancer behaves differently depending on the setting. The visual quiz tests understanding of request distribution at specific steps.