## Overview

You can proxy Kevel impression events through your own server. This allows you to:

- Control the domains used for ad serving
- Customize the format of impression tracking pixels
- Minimize reporting discrepancies due to Ad Blockers

## Understanding the Impression Pixel

An Kevel impression pixel consists of:

- `i.gif`: The dimensionless pixel that counts the impression when fired.
- `e=`: A base64 encoded shim unique to the impression.
- `s=`: A signature value to verify that the encoded shim is legitimate.

A complete pixel URL looks like:

```
https://e-1234.adzerk.net/i.gif?e=eyJhdiI6NDM3LCJhdCI6NSwiY&s=ujZhktar1g1emAP
```

In a Decision API response, the impression pixel URL is returned as the `impressionUrl` parameter.

To record an impression, a browser or script must GET the impression URL.

```json
{
  "adId": 111,
  "creativeId": 222,
  "flightId": 333,
  "campaignId": 444,
  "clickUrl": "https://e-1234.adzerk.net/r?...",
  "impressionUrl": "https://e-1234.adzerk.net/i.gif?...",
  "events": [
        { "id": 12,
          "url": "https://e-1234.adzerk.net/e.gif?..." 
        },
        { "id": 13,
          "url": "https://e-1234.adzerk.net/e.gif?..." 
        },
        { "id": 14,
          "url": "https://e-1234.adzerk.net/e.gif?..." 
        }
  ]
  ...
}
```

## Creating a Proxy Server

A server for proxying impressions must be able to:

- Ingest the `ImpressionUrl` from a Decision API response
- Create a new impression URL using the domain of your web property
- GET a Kevel impression URL using shim and signature data from the "native" URL

The server that renders an ad on a page then renders the new "native" impression URL.

Finally, when a user's browser hits the new impression URL, the proxy server will count the impression using the Kevel impression URL format.

Here's an example of how the process works:

- When a user views a page, KevelCustomer.com makes a server-to-server Decision request to get an ad
- As part of the process of rendering an ad, the proxy server ingests the `ImpressionUrl` from the Decision response and creates a new impression pixel that looks like this (for example): `http://AdzerkCustomer.com/e=value/s=value`
- The server writes the new impression URL to the page with the ad contents
- A user's browser GETs the new impression pixel
- The proxy server uses the `e=value` and `s=value` passed on the new impression pixel to create a pixel in the Kevel format
- The proxy server hits the Kevel impression pixel. Kevel counts the impression