# A/B Testing

### What is A/B Testing and why would we want to implement it? 

Through A/B testing, we can expose users to two slightly different experiences, such as changing the color of a button in the interface. Then comes the analytical part for each variant, such as the percentage of "purchases" on the website, and we can decide which variant introduces a real improvement. [This link](https://en.wikipedia.org/wiki/A/B_testing) provides a comprehensive explanation of the process. 

Okay, this sounds good, but what about the performance and security offered by the CDN? Can we implement an A/B testing model without sacrificing it? 

The CDN and its caching system (Varnish) cache the responses returned by the origin server completely, so most client requests do not reach the origin server. If the A/B testing system relies on logic located at the origin server, even if a client assigned to variant A requests the website, they could receive the cached opposite variant instead (and vice versa). This is not what we want, and any information obtained in the A/B testing experiment would be invalid.

### Solution 

In reality, there are multiple ways to solve this problem using the tools at our disposal. Let's see a simple example where we separate clients based on their IP address.

```c
sub vcl_recv {  
    if (req.url ~ "^/carrito"){   
        set req.http.abtesting = "A";  
        if (req.http.True-Client-IP ~ "[0-2]$") {  
            set req.http.abtesting = "B";  
        }
        set req.http.X-Vary-TCDN = req.http.X-Vary-TCDN + ",abtest:" + req.http.abtesting;
    }  
}
```

In the previous example, we see how an `abtesting` header is set to distinguish between different variants. Then, based on the client's IP, we assign them a different variant. This could be done based on any other header or request parameter. In this case, we are balancing the variants with a split of 30% for "B" and 70% for "A", as the IPs ending in 0, 1, or 2 will be assigned to variant "B," and the rest to variant "A."

Lastly, we set the `X-Vary-TCDN` header, which is a special internal header that manages cache variants. It includes the value of `abtesting` and stores a different version of the object in the cache for each variant. This allows us to benefit from both the CDN and A/B testing.

As for the origin server, it only needs to serve the appropriate variant based on the `abtesting` header when a cache miss occurs.