TH6 ‐ Session persistence vs IP Hash

Session Persistence vs IP Hash

Overview

When multiple backend servers are behind an OCI Load Balancer, requests are distributed across backend servers.

For some applications, we need backend affinity, which means keeping a user's requests connected to the same backend server.

Two common methods are:

• Session Persistence
• IP Hash

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1. Session Persistence

Session Persistence keeps a user's requests directed to the same backend server using a persistence cookie.

Example

OCI Load Balancer has two backend servers:

OCI Load Balancer
       |
-----------------
|               |
Backend-1     Backend-2

First request:

User → Load Balancer → Backend-1

The Load Balancer establishes a persistence cookie.

Subsequent requests:

User + Cookie → Load Balancer → Backend-1
User + Cookie → Load Balancer → Backend-1

So the user's session continues with Backend-1.

Real-World Example

For an e-commerce application:

Login → Add Product to Cart → Checkout

If the user's session is stored on Backend-1, session persistence helps keep the user's requests going to Backend-1.

Memory Trick

Session Persistence = Cookie → Same Session → Same Backend

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2. IP Hash

IP Hash uses the client's source IP address to determine which backend server should receive the request.

The Load Balancer calculates a hash based on the client IP.

Example

Client IP: 192.168.1.10
         ↓
    Hash Function
         ↓
     Backend-1

Subsequent requests from the same IP can continue going to Backend-1:

192.168.1.10 → Backend-1
192.168.1.10 → Backend-1
192.168.1.10 → Backend-1

Memory Trick

IP Hash = Source IP → Hash → Backend

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NAT Example

Suppose multiple users are behind the same office public IP:

User A ─┐
User B ─┤
User C ─┼──→ Public IP: 203.0.113.10
User D ─┘
                ↓
          OCI Load Balancer
                ↓
            Backend-1

Because IP Hash uses the source IP, multiple users may be mapped to the same backend.

This can cause uneven load distribution.

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Mobile Hotspot Example

With IP Hash:

Old IP: 49.36.10.20
        ↓
      Hash
        ↓
    Backend-1

If the mobile network changes the public IP:

New IP: 49.36.20.50
        ↓
      Hash
        ↓
    Backend-2

The user may move from Backend-1 to Backend-2.

If the session was stored only on Backend-1, the user may lose the session.

With Session Persistence:

User + Persistence Cookie
          ↓
    Load Balancer
          ↓
       Backend-1

Even if the IP changes, the cookie can continue to maintain the backend association, subject to the persistence configuration and backend availability.

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Key Differences

Feature	Session Persistence	IP Hash
Mechanism	Persistence Cookie	Source IP
Affinity	Session/Client based	IP based
Cookie required	Yes	No
IP change	Usually does not affect cookie-based affinity	Can change backend mapping
Users behind same NAT	Can distinguish users using cookies	May map users to same backend
Main use	Stateful/session-based applications	Stable IP-based environments

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When to Use Session Persistence

Use Session Persistence when:

• The application maintains user sessions on a backend.
• User-specific state needs backend affinity.
• Cookie-based persistence is acceptable.
• The application is stateful or legacy.

Examples

• Shopping cart
• Login session
• User-specific application state
• Legacy stateful applications

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When to Use IP Hash

Use IP Hash when:

• Client IPs are relatively stable.
• IP-based affinity is sufficient.
• Cookie-based persistence is not required.
• The environment has a specific IP-based affinity requirement.

Examples

• Controlled corporate networks
• Stable client IP environments
• Applications requiring IP-based affinity

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Interview Answer

Session Persistence maintains backend affinity using a persistence cookie, while IP Hash uses the client's source IP address to select the backend. For example, if a user is accessing an application through a mobile hotspot and the IP changes, IP Hash can map the user to a different backend. With Session Persistence, the browser can continue sending the persistence cookie, so the session can remain associated with the same backend. Also, multiple users behind the same NAT can be mapped to the same backend with IP Hash, which may cause uneven load distribution.

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One-Line Difference

Session Persistence = Cookie-based affinity, whereas IP Hash = Source-IP-based affinity.