I have a website and mobile applications Android & IOS, all platforms are connected to the same database, what we are trying to do is to enhance the user experience by showing him specific products based on his selections however right now we can do this a cross all platforms only if the user is logged in using his user ID, now my question is if there a way to identify the guest users without signing-in? maybe if the user is signed in with the same google email on his phone and his browser I can get his token? or something similar even if its 3rd party tools. Right now I'm not even sure where to start or what to search for.

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  • Are you asking how to identify a user by some third party credentials without the user explicitly being made aware of and consenting to this? That's a big no no.
    – Flater
    Jun 19 at 22:49

1 Answer 1


No, you will not be able to get any reliable identifier to link users across devices unless they log in.

While smartphones do hold a lot of such information, the operating systems and browsers will not make this information available to you for privacy reasons.

You can use unreliable probabilistic fingerprints to show similar recommendations to similar users, but this can be quite problematic. For example, if you show similar recommendations to different devices on the same network (e.g. to members of the same household), this could leak information about one user's preferences to other people. Nevertheless, such cross-device linking is a big concern in the advertisement industry, especially for the problem of attributing conversions on one device to ads on another device.

If you want a reliable identifier, you'll have to ask the user. For example, to get the user's Google account, ask them to “log in with Google”. Such OAuth log in is often the lowest-friction way to create a user account. But if you make a log-in mandatory, note that this is still friction (and a privacy problem) that might turn some users away, potentially decreasing the value of this approach. Beyond some point, trying to wring out more conversions and more revenue from a user base will backfire.

My recommendation: focus on what is best for the user. If the users want to sync their recommendations between devices, give them tools to achieve that. If the users don't want this, don't force it.

  • but how come when i search for something on my web browser sometimes i see ads about it on my phone, there must be something linked if im signed in the same email on my phone and my web browser, maybe there is away to request from google such info ? or atleast to validate if its the same user only, i dont need anything else except id or token Jun 19 at 13:18
  • any suggestions Jun 19 at 13:24
  • @HossamHassan That's the probabilistic linking I mentioned. The advertisers likely see that both devices are on the same network/IP address, so they probably belong to the same household. So the advertiser infers that the user of the second device might share some interests with the user of the first device. But this linking is not possible with absolute certainty unless the user voluntarily provides some identifier.
    – amon
    Jun 19 at 15:10
  • @HossamHassan Assuming Google for a moment, Google decided what ads to show you, not the website itself. Websites just make an "ad space" and let Google fill it in as it pleases. The website doesn't identify your user credentials, the Google ad loader does. The decision which ads to show based on your Google identity is Google's own decision, not that of some third party website like your app.
    – Flater
    Jun 19 at 22:53

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