• I have a catalog, say at https://example.com
  • Then I have a blog, say at https://subdomain.example.com

The catalog was built time ago and the blog is new.

I have the Google Tag Manager at both sites. Then I route from there to Facebook pixel and so on and so on.

For Google Analytics (v4), say I already had a stream with this data:

  • Stream Name = My nice Catalog
  • Stream URL = https://example.com
  • Stream Id = 1111111111
  • Measurement Id = G-XX8X8XXX88

I now want to track the subdomain.

My mindset

I think I want the same stream for both sites for these reasons:

  • a) The userbase is the very same
  • b) Same GTM, routing to a single stream, avoid config potential mistakes, just fire one datastream in "all page views" with no filters.


My users are the same user-base, so they share behaviour and the content is related.

  1. Do I "have" to create another stream (not desired)? Or I may go with the same stream (desired)?
  2. Does 1 stream correlate to 1 measurement ID? Or there a 1-to-N relationship?
  3. If possible to use a single stream, do I have to configure anything special to capture the subdomain data?
  4. The GTM is shared. In the necessary case of creating another stream, should I "filter" in the GTM and say "if the URL begins exactly with xxxx then fire this analytics, it the URL is that other, fire this other analytics? Or may I fire them all and Google will take care to only track whatever is expected?
  5. Despite my mindset, even if the userbase is the same, in the case it's not "required" to separate, is there any reason to "want stream separation"?


2 Answers 2

  1. You don't have to create a new stream. Since you are dealing with a subdomain, it is perfectly reasonable to have one stream for both sites. If you are dealing with a situation where you have different site owners, then it might make sense to have separate streams for your subdomains.

  2. Each stream will have their own measurement IDs. I believe each of these will also be dimensions in the GA4 property where you can create audiences and reports for specific sites.

  3. The only thing you might need to adjust is the referral settings in the stream. you might also want to push a new parameter to the tag for the subdomain to allow for easier reporting to avoid any potential issues using the domain or hostname dimensions.

  4. With a shared GTM and leveraging two streams, you would have to create two separate Google Tags and fire those on their respective pages. You might be able to do a RegEx Lookup table and do some magic that way but it will introduce a lot of variables that could break your setup / data capture. Along with double tagging your main Google Tag, you would essentially need to double up on all your event tagging as well. So there is a real possibility that the container size will get large depending on the level of event tagging you are looking to do.

  5. I can't think of any to be honest. It just makes for more work on the admin side of things especially with a shared GTM.

  • Thanks! Straight to the point. Will consider all your comments. Thanks! Nov 15, 2023 at 0:14
  • @XaviMontero, no problem. If this answers your question remember toake it as your chosen answer. Nov 15, 2023 at 0:49

I have GA4 running on my main domain and added it to a tumblr subdomain that I run alongside it to test this. The code works and data is in the stream, but the Pages & Screens report does not show the full URL, at least not by default, but you can probably change some settings to get that to show. It may or may not be important, your title naming convention may show you which subdomain they are on also.

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