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.

Practice

1. What is the main purpose of marking a server as backup in an upstream block in nginx?

easy

A. To use it only if all main servers are unavailable

B. To prioritize it over main servers for all requests

C. To permanently disable the server

D. To balance load equally with main servers

Solution

1. Step 1: Understand the role of backup servers

   Backup servers are fallback servers used only when main servers fail.

2. Step 2: Analyze the backup keyword effect

   Marking a server as backup means it will receive traffic only if main servers are down.

3. Final Answer:

   To use it only if all main servers are unavailable -> Option A

4. Quick Check:

   Backup servers = fallback only [OK]

Hint: Backup servers handle traffic only when main servers fail [OK]

Common Mistakes:

• Thinking backup servers share load equally
• Assuming backup servers are always active
• Confusing backup with disabled servers

2. Which of the following is the correct syntax to define a backup server in an nginx upstream block?

easy

A. server 192.168.1.10;

B. server backup 192.168.1.10;

C. server 192.168.1.10 backup;

D. backup server 192.168.1.10;

Solution

1. Step 1: Recall nginx upstream server syntax

   The correct syntax is server [IP] [parameters]; with parameters like backup after IP.

2. Step 2: Identify correct placement of backup

   The backup keyword comes after the IP address in the server line.

3. Final Answer:

   server 192.168.1.10 backup; -> Option C

4. Quick Check:

   Backup keyword after IP = correct syntax [OK]

Hint: Put 'backup' after IP in server line [OK]

Common Mistakes:

• Placing 'backup' before IP address
• Using 'backup server' instead of 'server ... backup;'
• Omitting semicolon at line end

3. Given this nginx upstream block:

upstream backend {
    server 10.0.0.1;
    server 10.0.0.2 backup;
}

Which server will nginx send requests to if 10.0.0.1 is up?

medium

A. 10.0.0.1 only

B. 10.0.0.2 only

C. Both 10.0.0.1 and 10.0.0.2 equally

D. No server, error occurs

Solution

1. Step 1: Understand upstream with backup server

   Main server 10.0.0.1 handles requests first; backup 10.0.0.2 is fallback.

2. Step 2: Check server availability

   Since 10.0.0.1 is up, nginx sends all requests there, ignoring backup.

3. Final Answer:

   10.0.0.1 only -> Option A

4. Quick Check:

   Backup used only if main down [OK]

Hint: Backup server used only if main server fails [OK]

Common Mistakes:

• Assuming backup server shares load
• Thinking backup server is always active
• Believing nginx sends requests to both simultaneously

4. You have this nginx upstream configuration:

upstream backend {
    server 192.168.0.1 backup;
    server 192.168.0.2;
}

What is wrong with this configuration?

medium

A. Backup keyword is misspelled

B. No semicolon after server lines

C. IP addresses are invalid

D. Nothing is wrong with this configuration

Solution

1. Step 1: Validate syntax and parameters

   All server lines have correct syntax, valid IPs, proper semicolons, and correct backup usage.

2. Step 2: Check server order

   Position of backup servers does not matter; nginx ignores them unless all main servers fail.

3. Final Answer:

   Nothing is wrong with this configuration -> Option D

4. Quick Check:

   Backup position irrelevant [OK]

Hint: Backup servers can be listed anywhere [OK]

Common Mistakes:

• Thinking backup servers must be listed after main servers
• Believing server order causes failures
• Assuming 192.168.x.x IPs are invalid

5. You want nginx to send traffic to server1 and server2 normally, but use backup1 and backup2 only if both main servers fail. Which configuration correctly sets this up?

hard

A. upstream backend {
server backup1;
server backup2;
server server1;
server server2;
}

B. upstream backend {
server server1;
server server2;
server backup1 backup;
server backup2 backup;
}

C. upstream backend {
server server1 backup;
server server2 backup;
server backup1;
server backup2;
}

D. upstream backend {
server server1;
server backup1;
server server2;
server backup2;
}

Solution

1. Step 1: Identify main and backup servers

   Main servers are server1 and server2; backup servers are backup1 and backup2.

2. Step 2: Apply correct syntax and order

   Main servers listed first without backup, backups listed after with backup keyword.

3. Step 3: Verify options

   upstream backend { server server1; server server2; server backup1 backup; server backup2 backup; } correctly lists main servers first, backups with backup keyword after.

4. Final Answer:

   upstream backend { server server1; server server2; server backup1 backup; server backup2 backup; } -> Option B

5. Quick Check:

   Main servers first, backups last with 'backup' keyword [OK]

Hint: List main servers first, backups last with 'backup' keyword [OK]

Common Mistakes:

• Placing backup servers before main servers
• Missing 'backup' keyword on backup servers
• Marking main servers as backup by mistake