Backup servers in Nginx - Step-by-Step Execution

Process Flow - Backup servers

Client sends request

↓

Primary server receives request

↓

Is primary server UP?

No→Send request to backup server

|Yes↓

Primary server processes request

↓

Response sent to client

The client sends a request to the primary server. If the primary server is down, the request is sent to the backup server instead.

Execution Sample

Nginx

upstream backend {
    server primary.example.com;
    server backup.example.com backup;
}

server {
    location / { proxy_pass http://backend; }
}

This nginx config defines a primary and a backup server. Requests go to primary unless it is down, then to backup.

Process Table

Step	Event	Primary Server Status	Action Taken	Response Source
1	Client sends request	UP	Send to primary server	primary.example.com
2	Primary server responds	UP	Serve response	primary.example.com
3	Client sends request	DOWN	Send to backup server	backup.example.com
4	Backup server responds	DOWN	Serve response	backup.example.com
5	Client sends request	UP	Send to primary server	primary.example.com

💡 Requests stop or continue depending on server availability; backup used only if primary is down.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
Primary Server Status	UP	UP	UP	DOWN	DOWN	UP
Request Routed To	primary.example.com	primary.example.com	primary.example.com	backup.example.com	backup.example.com	primary.example.com
Response Source	primary.example.com	primary.example.com	primary.example.com	backup.example.com	backup.example.com	primary.example.com

Key Moments - 2 Insights

Why does nginx send requests to the backup server only when the primary is down?

Because the backup server is marked with the 'backup' keyword, nginx uses it only if the primary server is unavailable, as shown in execution_table rows 3 and 4.

What happens if the primary server comes back up after being down?

Nginx automatically routes requests back to the primary server once it is detected as UP again, as seen in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution table, at step 3, which server receives the client request?

A primary.example.com

B backup.example.com

C Both servers

D No server

💡 Hint

Check the 'Action Taken' and 'Response Source' columns at step 3.

At which step does nginx switch back to the primary server after downtime?

A Step 5

B Step 3

C Step 2

D Step 4

💡 Hint

Look for when 'Primary Server Status' changes back to UP and 'Request Routed To' is primary.

If the backup server was not marked as 'backup' in the config, what would happen?

A Nginx would never use the backup server

B Nginx would only use the backup server

C Nginx would load balance between both servers always

D Nginx would fail to start

💡 Hint

Consider how nginx treats servers without the 'backup' keyword in upstream blocks.

Concept Snapshot

nginx backup servers:
- Define upstream with primary and backup servers
- Use 'backup' keyword for fallback server
- Requests go to primary if UP
- If primary DOWN, requests go to backup
- Automatic failover and recovery

Full Transcript

This visual execution shows how nginx handles backup servers. The client sends requests to the primary server first. If the primary server is up, nginx routes requests there and serves responses from it. If the primary server goes down, nginx automatically sends requests to the backup server marked with the 'backup' keyword. When the primary server recovers, nginx switches back to it. The execution table tracks each step, showing server status, routing decisions, and response sources. This setup ensures high availability by providing a fallback server without manual intervention.