IP hash for session persistence in Nginx - Step-by-Step Execution

Process Flow - IP hash for session persistence

Client sends request

↓

Extract client IP

↓

Calculate hash from IP

↓

Map hash to backend server

↓

Forward request to selected server

↓

Server responds

↓

Client gets consistent server

The client IP is used to create a hash that decides which backend server handles the request, ensuring the same client goes to the same server.

Execution Sample

Nginx

upstream backend {
    ip_hash;
    server backend1.example.com;
    server backend2.example.com;
}

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

This config uses ip_hash to send requests from the same client IP to the same backend server for session persistence.

Process Table

Step	Client IP	Hash Calculation	Selected Server	Action
1	192.168.1.10	hash(192.168.1.10) = 1	backend1.example.com	Request forwarded to backend1
2	192.168.1.11	hash(192.168.1.11) = 2	backend2.example.com	Request forwarded to backend2
3	192.168.1.10	hash(192.168.1.10) = 1	backend1.example.com	Request forwarded to backend1 (same as step 1)
4	192.168.1.12	hash(192.168.1.12) = 1	backend1.example.com	Request forwarded to backend1
5	192.168.1.11	hash(192.168.1.11) = 2	backend2.example.com	Request forwarded to backend2 (same as step 2)
6	192.168.1.13	hash(192.168.1.13) = 2	backend2.example.com	Request forwarded to backend2
7	192.168.1.14	hash(192.168.1.14) = 1	backend1.example.com	Request forwarded to backend1
8	192.168.1.15	hash(192.168.1.15) = 2	backend2.example.com	Request forwarded to backend2
9	192.168.1.10	hash(192.168.1.10) = 1	backend1.example.com	Request forwarded to backend1 (session persistence)
10	192.168.1.16	hash(192.168.1.16) = 1	backend1.example.com	Request forwarded to backend1

💡 Requests continue to be routed based on client IP hash, ensuring session persistence.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10
Client IP	-	192.168.1.10	192.168.1.11	192.168.1.10	192.168.1.12	192.168.1.11	192.168.1.13	192.168.1.14	192.168.1.15	192.168.1.10	192.168.1.16
Hash Result	-	1	2	1	1	2	2	1	2	1	1
Selected Server	-	backend1.example.com	backend2.example.com	backend1.example.com	backend1.example.com	backend2.example.com	backend2.example.com	backend1.example.com	backend2.example.com	backend1.example.com	backend1.example.com

Key Moments - 3 Insights

Why does the same client IP always go to the same backend server?

Because the IP hash function maps the client IP to a specific backend server consistently, as shown in execution_table rows 1, 3, and 9 where the same IP 192.168.1.10 always selects backend1.

What happens if a new client IP appears?

The new IP is hashed and mapped to one of the backend servers based on the hash result, as in row 10 where 192.168.1.16 maps to backend1.

Can the distribution be uneven between servers?

Yes, because the hash depends on IP values, some servers may get more clients if IPs hash to them more often, as seen in variable_tracker where backend1 gets more selections.

Visual Quiz - 3 Questions

Look at the execution_table at step 5. Which server handles the request from IP 192.168.1.11?

A backend3.example.com

B backend1.example.com

C backend2.example.com

D No server selected

💡 Hint

Check the 'Selected Server' column at step 5 in the execution_table.

At which step does the client IP 192.168.1.10 appear for the third time?

A Step 9

B Step 3

C Step 7

D Step 10

💡 Hint

Look at the 'Client IP' column in the execution_table and count occurrences of 192.168.1.10.

If a new backend server is added, how would the hash mapping change?

A All client IPs would map to the new server only

B The hash function would distribute IPs among all servers including the new one

C The existing mapping would not change

D The IP hash would stop working

💡 Hint

Consider how hashing distributes keys among available servers as shown in the concept_flow.

Concept Snapshot

ip_hash directive in nginx upstream block
- Uses client IP to calculate hash
- Routes requests from same IP to same backend
- Helps keep session persistence
- Simple config: ip_hash; inside upstream
- Works best with stable client IPs
- May cause uneven load distribution

Full Transcript

This visual execution shows how nginx uses the ip_hash method to keep session persistence. When a client sends a request, nginx extracts the client IP and calculates a hash value from it. This hash determines which backend server will handle the request. The same client IP always maps to the same backend server, ensuring the session stays on one server. The execution table traces multiple requests from different IPs, showing consistent server selection for repeated IPs. The variable tracker shows how client IPs, hash results, and selected servers change step-by-step. Key moments clarify why the same IP maps to the same server and what happens when new IPs or servers appear. The quiz tests understanding of server selection and hash behavior. The snapshot summarizes the ip_hash directive usage and benefits.