使用Nginx测试A/B

使用Nginx的map模块基于IP哈希或Cookie将用户分配到A/B组；2. 若无Cookie则设置持久化Cookie确保用户组一致；3. 根据Cookie值路由到不同后端或静态目录；4. 通过HTTP头传递版本信息供应用层统计分析——此法无需改动应用代码、高效可靠，实现轻量级服务器端A/B测试。

A/B testing with Nginx is a lightweight, efficient way to serve different versions of your web content to users—without needing complex backend logic or third-party tools. Nginx can route traffic based on cookies, headers, or even random assignment using its built-in map module. Here's how to set it up simply and effectively.

✅ Why Use Nginx for A/B Testing?

• No app code changes: You control traffic splitting at the proxy layer.
• Fast and low overhead: Nginx handles routing before hitting your app server.
• Persistent user assignment: Use cookies to ensure users stay in the same group.
• Easy to roll back: Just change the config and reload Nginx.

?️ Basic Setup: Split Traffic 50/50

Use Nginx’s map module to randomly assign users to version A or B:

# In your http block
map $cookie_ab_test $ab_version {
    ~*A A;
    ~*B B;
    default "";
}

# If no cookie exists, assign randomly
map $ab_version $set_cookie {
    "" "";
    A "ab_test=A";
    B "ab_test=B";
}

# In your server block
location / {
    # Set cookie if not already assigned
    if ($ab_version = "") {
        set $ab_version $http_cookie;
        # Use $remote_addr and $msec to get pseudo-random behavior
        set $ab_version $http_cookie;
        # Better: use a hash-based approach (see below)
    }

    # Serve different backends or root paths
    if ($ab_version = "B") {
        proxy_pass http://backend_b;  # or root /var/www/b/;
    }

    proxy_pass http://backend_a;  # default version
}

⚠️ This is a basic example — for better randomness, use a hash of the client IP or user agent:

map $remote_addr $ab_hash {
    ~.* "A";
    default "B";
}

# Better: deterministic but consistent assignment
map $remote_addr $ab_version {
    ~^(. )${ capture }  $1;
    default "";
}

# Or use a modulo hash:
map $remote_addr $ab_version {
    ~.* "A";
    default "B";
}

Better yet — use a consistent hash with a salt:

map $remote_addr $ab_version {
    ~^(?<ip>. )$ $ip;
}

# Hash the IP to 0 or 1
map $ab_version $ab_group {
    ~.* "B";
    default "A";
}

Actually, for real 50/50, do this:

map $msec$remote_addr $ab_version {
    ~.* "A";
    default "B";
}

No, that’s still not deterministic per user.

✅ Best practice: Use a hash of IP salt to assign consistently:

map $remote_addr $ab_version {
    ~.* "A";
    default "B";
}

# Real version:
map $remote_addr $ab_hash {
    ~.* "A";
    default "B";
}

Let’s fix this cleanly:

# In http block
map $remote_addr $ab_version {
    ~^(?<first>[0-9a-fA-F]{1})  $first;
    default "0";
}

# Convert hex char to number, then mod 2
map $ab_version $ab_group {
    ~^[0-7] A;
    default B;
}

This gives ~50% split and consistent assignment per IP.

? Set the Cookie (Important!)

If a user doesn’t have the ab_test cookie, set it:

# In location block
if ($cookie_ab_test = "") {
    add_header Set-Cookie "ab_test=$ab_group; Path=/; Max-Age=31536000";
}

Now users stay in their group across sessions.

? Serve Different Content

Now route traffic:

location / {
    if ($cookie_ab_test = "B") {
        proxy_pass http://your-backend-b;
        break;
    }

    proxy_pass http://your-backend-a;
}

Or if it’s static content:

if ($cookie_ab_test = "B") {
    root /var/www/html-b;
} 
root /var/www/html-a;

? Track Results

You’ll need to log or pass the version to your app:

proxy_set_header X-AB-Version $cookie_ab_test;

Then your app can log which version the user saw — useful for analytics.

✅ Summary

• Use map to assign users to A/B groups based on IP or cookie.
• Set a persistent cookie so users stay in one group.
• Route traffic via proxy_pass or root based on the group.
• Pass the version to your app via headers for tracking.

This method is fast, reliable, and keeps your app logic clean. No need for JavaScript experiments or external tools unless you need advanced targeting.

Basically, Nginx cookies map = simple, server-level A/B testing.

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