How does Marketing Cloud ID Service maintain consistent MCIDs across domains?

The article details how the Marketing Cloud ID Service maintains consistent MCIDs across domains.

Description

Environment

Adobe Experience Cloud

Issue/Symptoms

How does the Marketing Cloud ID Service maintain a consistent Marketing Cloud ID across domains in a browser that accepts third-party cookies?

Resolution

Steps to follow:

1. The user visits a website owned by My Company Inc. for the first time.
2. The VisitorAPI.js file loads and checks to see if an AMCV cookie exists on the current domain (SiteA.com). No AMCV cookie exists on the current domain, so VisitorAPI.js checks to see if a demdex.net cookie exists. It does not.
3. Since neither cookie exists, a network request is made to dpm.demdex.net with your Marketing Cloud Organization ID (also known as an IMS org ID). In this case, 1234@AdobeOrg is sent with the request.
4. The demdex.net server receives the request and generates a UUID. Next, it uses the UUID and IMS org ID (1234) to generate the MID value.
5. The server responds to the request from the page with the MID and UUID.
6. The VisitorAPI.js library writes the MID to an AMCV cookie in the browser. As part of the response, the demdex server also writes the UUID to a cookie. The AMCV cookie is written on the current domain (SiteA.com) and the demdex cookie is written on the demdex.net domain.
7. Page code executes and Adobe solution beacons are fired.
8. The user browses to SiteB.com.
9. VisitorAPI.js checks to see if an AMCV cookie exists on the current domain (SiteB.com). No AMCV cookie exists on this domain, so VisitorAPI.js checks to see if a demdex.net cookie exists. It does exist, so the value is retrieved.
10. A network request is made to dpm.demdex.net with the IMS org ID (1234) and the UUID value (ABCDE).
11. The demdex server generates a MID value using the UUID (ABCDE) and IMS org ID (1234), which were passed in the request.
12. The demdex server responds to the request with the MID value (ABC1234).
13. The VisitorAPI.js library writes the MID (ABC1234) to an AMCV cookie in the browser. The AMCV cookie is written on the current domain (SiteB.com).
14. Page code executes and Adobe solution beacons are fired.