Consider how to support load balancing

[korydraughn]

This issue is about how to share private session information across multiple servers.

[korydraughn]

Based on the following ChatGPT conversation, perhaps we don't need to adjust anything code-wise. We only need to prove that proxy applications can do what is mentioned.

ChatGPT Conversation

(Me): do http server proxies have a way to direct a client request to the same http server if necessary? consider the case where client-specific state is only available to the http server which generated the client state.

(ChatGPT): Yes, HTTP server proxies can handle cases where a client request needs to be directed to the same HTTP server for various reasons, such as maintaining client-specific state. This scenario is typically achieved through a process called "session affinity" or "sticky sessions."

When a client makes a request to a load balancer or proxy server, the proxy needs to decide which backend server (HTTP server) should handle the request. In a typical load balancing setup, requests from a client can be distributed across multiple backend servers for load distribution. However, in cases where client-specific state needs to be maintained on a specific backend server, this random distribution could cause issues.

To ensure that client requests are consistently directed to the same backend server that generated the client-specific state, the proxy can use one of the following methods:

1. IP-based Affinity: The proxy can use the client's IP address to determine which backend server to route the request to. This ensures that requests from the same client IP are always directed to the same backend server. However, this method may not work well in cases where clients are behind a NAT (Network Address Translation) or use dynamic IPs.

2. Cookie-based Affinity: The proxy can use cookies to identify a specific backend server for each client. When the client first connects to the server, the server can set a unique identifier (a session ID) as a cookie in the client's browser. The proxy uses this cookie to determine which backend server to route the subsequent requests to.

3. Session Hashing: The proxy can use a combination of client-specific information (e.g., IP address, user-agent, etc.) to compute a hash, which is then used to select a backend server. This approach ensures that requests from the same client end up on the same backend server.

4. Server-Side Sessions: In this approach, the proxy communicates with a shared session store, which all backend servers can access. When a client makes a request, the proxy looks up the client's session data in the shared session store and directs the request to the appropriate backend server.

The choice of method depends on the specific requirements of the application and the capabilities of the proxy server. Session affinity is important when maintaining client-specific state, such as user sessions, shopping carts, or other personalized data, on a particular backend server.

(Me): does HAProxy support these capabilities?

(ChatGPT): Yes, HAProxy is a versatile and powerful load balancer and proxy server that supports various capabilities, including session affinity methods for directing client requests to the same backend server. Here's a brief overview of how HAProxy supports these capabilities:

1. IP-Based Affinity: HAProxy can use the client's IP address to determine which backend server to route the request to. This is achieved using the "source" load balancing algorithm, which hashes the source IP to select a backend server. However, this approach has limitations when clients are behind NAT or use dynamic IPs.

2. Cookie-Based Affinity: HAProxy supports cookie-based session affinity through the use of the cookie directive in the backend configuration. You can configure HAProxy to set a specific cookie in the client's browser, and subsequent requests from the same client with the same cookie will be directed to the same backend server.

3. Session Hashing: HAProxy can use a combination of information from the client, such as the IP address, user-agent, and other custom data, to compute a hash for session-based affinity. This can be achieved using the hash-type directive in the backend configuration.

4. Server-Side Sessions: While HAProxy itself doesn't manage server-side session storage, you can integrate HAProxy with a shared session store, such as Redis or a database, to achieve server-side session persistence. This way, HAProxy can direct requests based on session information stored externally.

It's important to note that the specific implementation details may vary based on the version of HAProxy you're using and the specific use case you're trying to address. Additionally, HAProxy is highly configurable, and you can customize its behavior to match your application's requirements.

Before implementing session affinity or any other load balancing strategy, it's a good practice to thoroughly test and validate the configuration to ensure that it meets the desired behavior and performance expectations for your application.

[trel]

Very cool - we will definitely document a 'simple' use-case in our README as an example.

[korydraughn]

See http://docs.haproxy.org/2.8/intro.html#3.3.6.