How to send custom dimensions, medium, source or referer with an event via Measurement Protocol V2?

Question

With v1 of the measurement protocol, you could use these parameters to add custom dimensions or change medium, source or refer for a page view:

https://ssl.google-analytics.com/collect?v=1&tid=UA-xxxxxxxx&cid=[custom-id]&t=pageview&dp=[Url of pageview]&dh=[hostname of pageview]&cm=[new-medium]&cs=[new-source]&dr=[new-referer]&cd1=[custom-dimension-1]&cd2=[custom-dimension-2]

How is it done in measurement protocol v2?

I couldn't find any documentation about the page-view-event in V2 (for example it's just not mentioned here https://developers.google.com/analytics/devguides/collection/protocol/ga4/reference/events), even the event-builder (https://ga-dev-tools.web.app/ga4/event-builder/) doesn't support a simple page-view.

So, all I got so far is this:

$data = '
{    "client_id": "'.[custom-id].'",
     "events": [
     {
         "name": "page_view",
         "params": {
                "page_location": "'.[Url of pageview].'"
         }
     }
     ]
 }
 ';

So, what are possible parameters for a page-view-event?

Answer 1

Ok, a few things here right away that you should know if you're playing with MP:

1. Measurement protocol is a poor name. It implies there's more than one protocol for data gathering. There's none. There is just only one protocol for tracking.
2. MP2 still largely MP1. Google tries to pose GA4 as a new product, but it's just our old good GA UA with a simplified backend and overengineered front-end that tries to deliver the level of quality Site Catalyst/Omniture/Adobe Analytics have been delivering for a decade. MP is largely the same. dr, cm, cs and a lot of other fields are still there. cds aren't there anymore cuz they're replaced with eps and ups, but more about that a bit later.
3. GA4 uses this big marketing claim that the new analytics is so wonderfully event-based, unlike the old one. When I dug into why they keep claiming it everywhere, I realized that the only difference is that pageviews are now events. Not much difference really. But yes, a pageview is just an event named page_view. We'll talk about it a bit more later.
4. Custom dimensions are no more. Now they're called event properties and user properties. The same thing really, Google just tries to make it less obvious that there are no more session level custom dimensions. Or product-level CDs. Though the product level is seemingly on their roadmap.
5. Make sure you're using the correct measurement id. They made it a lot harder to find it in GA4. It's no longer just the property id visible in the property list, unfortunately.
6. GA's real-time reports don't include all dimensions, especially if those dimensions are involved in advanced metrics/dimensions calculations. Do not use real time reports for inspecting the content of your events. It's not meant for debugging. It's a vanity report. Still helpful to check the volume of events when you're sending a bunch and expect to see them in GA. Google even has a warning here:

Like the DebugView report, the Realtime report performs limited attribution analysis to ensure responsive reporting. We recommend that you refer to the Acquisition reports for the most accurate attribution information.

Finally, what I often do instead of reading the so-still-unfinished-and-not-really-helpful documentation on MP2, is either use a library like this.

Or, since 1 is the case, I would just implement a moniker tracking in my test GTM, then see what and how it sends to where in the Network debugger and simply reimplement it on my side exactly how GTM does it. No magic involved. Here is how my GTM tag would look like:

With a trigger on any click or any page load. After all is done, I publish the lib. Then I would inject this GTM's code in a local site, or in my test site, or however else you want to test it. And trigger the tag that you need to mimic with MP.

I use this wonderful extension to show all events that fire and their details right in my console.

Now this is how the above tag looks on my test site through the extension:

It's pretty useful.

How do I know that page_referrer is used as dr instead of ep in GTM? Here is the list of the fields that will never be seen as ep. But Google doesn't care enough to map them properly to what these fields are called in MP, so you either have to test, or know, or google it elsewhere.

Finally, here is how the network request looks like:

I published the tag to prod (I keep a test site in prod), so you can go and look at it. Or just find a site that uses GA4 and see its network requests. How does google know that this is a pageview? by the event name: en=page_view

Of course, you do the same with medium and source. Judging from the documentation I've linked to above, the medium and source look like campaign_source and campaign_medium in GTM. GTM maps them accordingly to cs and cm fields. And that's how you know these are the correct mp fields. Give GA time to process these and check on them in a few days.

Good, now this is applicable to the enhanced ecommerce hits too, it's just that they have more variables and data structures in them typically.

Finally, if you want to simulate batch events, you can just make a few tags fire in rapid succession and GTM will neatly pack them in one network request if they fit. You can then digest how the packing is done through the same methods as we do here and simulate.